Tuesday, May 28, 2019

ENSO Analysis and Outlook: Fall into Winter 2019

This post will analyze the current state of the El Nino-Southern Oscillation (ENSO) phenomenon as well as forecasts for its nature through fall 2019 and into the early winter months of 2019. It is critical to note right off the bat that this is *not* an ENSO forecast for the winter of 2019-2020, but the forecast period we will be going over will tread into November and December. Click on any image to enlarge it.

We will first define what the ENSO phenomenon is, and why we care about it.

Graphical depiction of the four different ENSO monitoring areas.
Source: Climate Prediction Center
The ENSO phenomenon, in a nutshell, is a primary driver of seasonal (and, through other shorter-term oscillations, weekly or even daily) weather patterns by way of sea surface temperature (SST) anomalies in the waters across the Equatorial Pacific. When these sea surface temperatures are above normal, we call it an 'El Nino' event. When these anomalies are below-normal, we call it a 'La Nina' event. While we monitor the entire Equatorial Pacific to analyze the ENSO phenomenon, there are four primary "zones" through which to observe. They are:

  • Nino 1+2. This is a small slice of the Pacific located between the Equator and the 10º South latitude line, extending from the far western tip of Peru to the 90º West longitude line.
  • Nino 3. This is a larger slice of the Equatorial Pacific which spans from 5º North to 5º South latitude lines, and from 90º West to 150º West longitude lines.
  • Nino 4. This is also a larger slice, and also extends between 5ºN and 5ºS on the latitude markers. For Nino 4, however, the space is spread by longitude from 150º West to about 160º East, crossing the dateline in the process.
  • Nino 3.4. This is the critical area to watch, and is typically viewed as the primary space with which to assess the state of the ENSO phenomenon. Spatially, it extends from 5ºN-5ºS latitudinally, and 120º West to 165º West longitudinally. 

Why do we break this space up into four different pieces rather than just average out the sea surface temperature anomalies and call it a day? A number of scientists with far more knowledge and research than I have come to determine that there can be more than one type of El Nino - where typically El Nino's bring warmer than normal waters to the eastern Pacific, an "El Nino Modoki" event brings warm waters to the western Pacific, and cooler waters to the eastern Pacific. This is not a trivial difference, but for our purposes here, we won't dive into that topic. For now, the key is understanding there are four different regions in which we monitor the ENSO phenomenon, with the Nino 3.4 region broadly being of most importance.

Let's view sea surface temperature anomalies over the Pacific now, with a focus on those regions that were just outlined.

Observed sea surface temperature anomalies on May 20th, centered over the Equatorial Pacific.
Source: NOAA
As of May 20th, sea surface temperature anomalies along the Equatorial Pacific were, on the whole, above normal. A solid swath of above-normal anomalies extended across Oceania to about the 130º West longitude line. From there to about the 110º West longitude line, however, SST anomalies were seen closer to zero, with a very small area of slightly below-normal temperature anomalies. We'll get in to why that's there in a little bit. By the 100º West line of longitude, however, solidly above-normal sea surface temperature anomalies return to the western tip of Peru.

Based on what was discussed earlier, this seems to point towards the presence of an El Nino event (the positive state of the ENSO phenomenon). To confirm or reject this, the Earth System Research Laboratory (ESRL) has composed a Multivariate ENSO Index (MEI) to quantify the state of the ENSO phenomenon, as shown below.

MEI, showing positive (El Nino) and negative (La Nina) changes to the ENSO phenomenon.
Source: ESRL
The MEI aims to determine if there is an El Nino in place (via positive index values), if there is a La Nina in place (via negative index values), or if there is a neutral-ENSO state (via index values equal to zero). You can identify a few extreme events on here, such as the strong El Nino in 1997 and the substantial La Nina event in 2010. Looking to the last several data points, it appears that there has been a general trend towards an El Nino event, but nothing particularly steady is in place.

I say nothing steady is in place because, as a general rule of thumb, an El Nino (La Nina) is present if sea surface temperature anomalies are above-normal by at least +0.5 degrees Celsius (below-normal by at least -0.5 degrees Celsius). If SST anomalies are positive but just barely so, it's technically a neutral-ENSO state, but clearly there's a better shot at an El Nino forming down the line. The same logic applies to SST anomalies that are negative but just barely so, with respect to a La Nina.

As of May 9th, the Climate Prediction Center continued its El Nino Advisory (click here for full briefing), which indicates that an El Nino event is ongoing. Indeed, the agency assigns a 70% probability of an El Nino continuing through the summer months, with a 55-60% chance of the El Nino persisting through the fall months. These probabilities may seem rather low given that the El Nino is actually occurring already, but as the rest of this post will show, it isn't that cut-and-dry with the ENSO phenomenon.

SST anomalies for each of the ENSO regions.
Source: CPC
Since we've already learned about the four different ENSO regions, it's time to apply that to observed data. Shown above are four panels of SST anomaly data over the past twelve months, with each panel corresponding to a different ENSO region. The top panel shows SST anomalies for the Nino 4 region; the second-from-top panel for the Nino 3.4 region; the second-from-bottom panel for the Nino 3 region; and the bottom panel for the Nino 1+2 region. In the aggregate, the data confirm that we are in an El Nino event, at least judging by sea surface temperature anomalies, with anomalies exceeding the +0.5º Celsius threshold in the Nino 4 and Nino 3.4 regions, with anomalies in the Nino 3 region right around that threshold. In contrast, the Nino 1+2 region has reversed to marginally-negative SST anomalies. This likely owes to the weak but broad area of slightly below-normal anomalies immediately southwest of the westernmost tip of Peru back on that observed SST anomaly graphic. Since it isn't a significant deviation, I don't see any major reason to raise concern over the difference in Nino 1+2 with the other three regions.

We are also able to look at sea temperature anomalies with varying depth along the Equator in the Pacific, as shown below.

Equatorial temperature anomalies (top) and observed nominal temperatures (bottom) as of May 18th.
Source: CPC
Viewing sea temperature anomalies along the Equator as a function of depth can prove massively beneficial to forecasting abilities, as it can enable the forecaster to identify an area of well-below-normal anomalies right below the surface that is eating away at above-normal SSTs on the surface. In this hypothetical, someone only viewing the surface map would think there's a solid El Nino in place, but the forecaster with the depth map as well can see that the El Nino is actually about to dissolve.

Turning back to actual data, the depth chart above shows a broad expanse of above-normal water temperatures extending from 140º East to about 120º West longitudinally, with the positive anomalies reaching a depth of almost 150 meters in the western portion of this swath. However, when reaching the 120º-100º West longitude area, well-below-normal temperature anomalies appear, and seem to be threatening the warmer anomalies located at the surface. We can view an animation of this depth map to see how these two opposing bodies of water have been interacting lately.

Sea temperature anomalies by depth, animated. Refresh the page if the animation stops looping.
Source: CPC
Indeed, when viewing the animation we are able to grasp the rather-dire situation the ongoing El Nino seems to be in. After covering almost the entire Equatorial Pacific with well-above-normal temperature anomalies from the surface to almost 150 meters down in March, cooler than normal waters have gradually grown between 150 meters and 250 meters below the surface since then and have materially weakened the formerly-stout positive temperature anomalies below the surface. Even more concerningly for the El Nino, the negative temperature anomalies appear to be growing and deepening east of the 120º West line of longitude, suggesting that the positive SST anomalies in that vicinity may be at risk of dissolving in coming weeks.

We have the ability to determine if this is likely to happen.

Equatorial Pacific upper-ocean (through 300 meters) heat anomalies over the last year.
Source of graphic: CPC
Source of annotations: Author
El Nino and La Nina events can be driven by Equatorial Kelvin Waves, and whether the wave moving eastward along the Equatorial Pacific is upwelling or downwelling. If that sentence made you raise an eyebrow, you're most likely not alone. I can assure you, though it's actually pretty simple to understand. Let's break it down.

The phrase 'Equatorial Kelvin Wave' seems intimidating, so for our purposes here all we need to know is that, from time to time, these Equatorial Kelvin waves develop in the western part of the Equatorial Pacific and gradually move eastward along the Equator. When they move eastward along the Equator, they can be either 'downwelling' or 'upwelling' waves.
Consider the explanation of a 'downwelling' Equatorial Kelvin wave as described by the NOAA (read the full article here):

"Normally, winds blow from east to west across the tropical Pacific, which piles up warm water in the western Pacific. A weakening of these winds starts the surface layer of water cascading eastward..."

In other words, if this wind pattern that blows winds from east to west breaks down, that warmer than normal water begins pushing eastward along the Equatorial Pacific. This anomalously warm water works its way eastward gradually and tends to sustain itself in the process. As a consequence, downwelling Equatorial Kelvin waves tend to be associated with El Nino events. You can see my annotations of downwelling Kelvin waves as solid lines on the above image.
On the flip side, an 'upwelling' Equatorial Kelvin wave can be thought of as the ocean waters trying to get itself a little more in balance in the wake of this very warm downwelling wave. Thus, an upwelling wave again features a Kelvin wave slowly progressing eastward, but this time it cools down the upper-ocean waters to a degree that upwelling Kelvin waves are generally associated more with La Nina events. In the above image, I've made an attempt to outline upwelling Kelvin waves by the dashed lines.

Given that we've had three clear downwelling Equatorial Kelvin waves traversing the Equatorial Pacific over the last year, as outlined on the above image, it's not necessarily a shock that we are in an El Nino at this time. In addition, we are able to use the above image to see that the emergence of cooler than normal water temperatures in the eastern equatorial Pacific appear to be the result of an upwelling equatorial Kelvin wave, as shown by the dashed line at around 110º West longitude.

I want to point out something that could endanger the El Nino by the time we reach late summer/early fall, however. Referring back to the above image, note how an area of cooler than normal water temperatures have developed between longitude lines 130º E and 160º E. I've circled this swath for two reasons: because it is a rather expansive area of cooler waters relative to previous below-normal anomalies in the other two more-apparent upwelling Kelvin wave episodes, and because it has brought about the strongest negative temperature anomalies in that portion of the equatorial Pacific in at least a year. The risk here is that this is the beginning of another upwelling equatorial Kelvin wave, which will move eastward with time and bring those below-normal temperature anomalies to the Nino regions. Should this occur (and it is not a certainty yet), it could endanger the El Nino, which is already in a more fragile position as a consequence of the existing sub-surface below-normal water temperature anomalies. We will have to monitor this in the coming weeks to see if it is indeed an upwelling Kelvin wave or merely an isolated area of cooler waters in the western equatorial Pacific.

As discussed, however, even if this is not an upwelling Kelvin wave beginning to form, there has been a material change in the ENSO phenomenon as of late that necessitates discussion.

Upper-ocean heat anomalies in degrees Celsius between 180º and 100º West longitude.
Source: CPC
The Climate Prediction Center has allowed us to compile those same anomalies shown in the previous image into a single graphic. This chart shows upper-ocean heat anomalies (in degrees Celsius) between the longitude lines of 180º and 100º West, from the surface to 300 meters down, if I recall correctly. In the presence of an El Nino, these anomalies should be positive, while a La Nina should bring these anomalies into negative territory.

During the month of May, we have seen a drastic shift in upper ocean heat anomalies, although in reality this shift began in mid-March. Indeed, after peaking at 1.5º C above-normal in the middle of March, those positive anomalies rapidly declined, to the point that they're now just barely in below-normal territory. The negative anomalies aren't strong enough to point to a La Nina, but the change from strongly positive values to marginally negative values is not a trivial one.

What does it mean? It's a good view of the evaporating above-normal temperature anomalies below the surface of the equatorial Pacific that we discussed earlier. The negative anomalies are likely a bit overdone and not reflective of the true nature of the ENSO phenomenon, given that there is a small yet significant area of below-normal anomalies around the 120º West line of longitude that is most likely distorting this graphic to the downside. As such, while the degradation in positive anomalies is accurate and noteworthy, the recent move into negative territory doesn't seem to be precise in my eyes.

Temperature anomalies between 2º North and 2º South latitude at the subsurface depths of 55 meters (left), 105 meters (center) and 155 meters (right).
Source: CPC
We are able to again see this material deterioration in the El Nino by looking at temperature anomalies along the Equatorial Pacific at three different depths: 55 meters, 105 meters and 155 meters below the surface. The 55-meter chart on the left shows the recent resurgence in below-normal temperature anomalies, erasing the above-normal anomalies that were in place as recently as late April. The 105-meter chart in the middle gives a better look at the underlying trend - indeed, steadfast positive temperature anomalies between 150º East and the dateline have deteriorated since late April, not to mention the elimination of well-above-normal anomalies centered around the 130º West line of longitude.
Perhaps most alarmingly for the viability of the El Nino, the positive temperature anomalies at a depth of 155 meters (right) have completely disappeared and have instead been replaced by marginally below-normal anomalies ever since late March. The lack of a solid underwater base for the El Nino does not bode well for its survival into fall and winter, especially if that aforementioned swath of colder than normal waters around the space between longitude lines 130º E and 160º E does turn out to be another upwelling Kelvin wave. Only time will tell, but this will certainly be something to watch as we move into the fall months.

We've analyzed a lot of observed data for the ENSO phenomenon, but scientists have put in a lot of hard work to create climate models that can anticipate the state of the phenomenon down the road. Let's take a look at these forecasts.

IRI/CPC suite of forecasts for SST anomalies in the Nino 3.4 region.
Source: IRI of Columbia University
There are two graphics of model guidance I want to go over. The first comes courtesy of Columbia University, and depicts a variety of weather models' forecast for sea surface temperature anomalies in the Nino 3.4 region from now until the January-February-March period of 2020. As stated at the start of this post, however we will only discuss forecasts going into November and December, as model guidance begins to diverge too much for my liking beyond that period.

Model guidance is in pretty good agreement on keeping the El Nino around through at least the August-September-October (ASO) period, with one or two outliers both to the upside and downside of the consensus. Beyond that period, however, divergence increases, although a general theme through the November-December-January period is that positive SST anomalies appear probable, especially relative to the potential for negative anomalies. It is worth making mention, however, of a cluster of models that prefer taking the Nino 3.4 SST anomalies into a level below +0.5º Celsius but above zero, a neutral-ENSO scenario. For now, though, we will side with guidance that prefers a weak El Nino through the fall months.

NMME suite of forecasts for SST anomalies in Nino region 3.4.
Source: CPC
The NMME suite incorporates many of the models used in the IRI/CPC suite, but is worth going over anyway because of its variance with the first contingent of models analyzed. While this group of models foresees the El Nino as likely to persist into August, it seems as though there is a higher chance of SST anomalies dipping into the 0.0º through +0.5º region, not high enough to merit an El Nino classification but again more likely to exhibit El Nino conditions as opposed to La Nina conditions. Beyond September, though guidance diverges too much to gather an accurate forecast.

To Summarize:
- An El Nino is currently in place, with an El Nino Advisory declared by the Climate Prediction Center.
- Recent sea temperature anomalies below the surface suggest the El Nino may be undergoing a material degradation, potentially posing a threat to the survival of the phenomenon through the fall and early winter.
- Despite the apparent threats to the El Nino, model guidance sees the El Nino continuing into the fall months and perhaps into early winter. Beyond then, however, guidance diverges too much to ascertain what will transpire into the heart of winter 2019-2020.


Sunday, May 26, 2019

June 1-6 Potential Warming Trend

Model guidance is beginning to fixate on a broader trend away from cooler than normal temperatures in the central and east U.S. into a more seasonal, if not above-normal temperature pattern. Click on any image to enlarge it.

Forecasted 500 millibar geopotential height anomalies per the GEFS, valid 1pm June 1st.
Source: Tropical Tidbits
By June 1st, the GFS ensembles (GEFS) anticipate somewhat of a pattern change, albeit only to a modest degree, from the troughs continually dropping into the West, bowling east through the Plains and then shifting northeast through the Great Lakes. The ensembles here anticipate a trough to again drop into the Southwest, but this time mild ridging is projected to build into the Pacific Northwest as an upper level low migrates eastward from the Bering Sea into the Gulf of Alaska. Note that the ridge in the Pacific Northwest and southwestern portion of Canada will be tempered as the Pacific jet stream carves its way through west-central Canada, as opposed to carrying the trough down into the Southwest U.S.

Typical atmospheric pattern in a Rex Block pattern.
Source: National Weather Service
The result of this marginal shift in pattern is a Rex Block-like formation over the Western U.S. The image above shows a textbook Rex Block, where a ridge builds to force the jet stream northward, but an upper level low is positioned almost directly south of the ridge. The result downstream of the Rex Block is zonal flow, as shown by the almost-directly west-to-east isohypse alignment.

We see almost that same pattern in the GEFS forecast at the top of this post, with a ridge building into parts of the Pacific Northwest and southwest Canada, a trough to the south of that ridge, and zonal flow across the rest of the country as a result. Further, also seen in the textbook Rex Block image, we still see that stubborn upper level low positioned in southern Canada, the same feature responsible for repeated shots of cooler than normal air for swaths of the northern U.S. as of late. For the record, continued ridging over the North Pole affirms these bouts of cooler than normal weather will remain possible for the northern U.S.

Forecasted 850 millibar temperature anomalies as of 7am, June 4th.
Source: Tropical Tidbits
While the warming trend begins on June 1st as that zonal flow takes hold, the strongest above-normal temperature anomalies just a few thousand feet off the ground emerge on June 4th, with anomalies on a magnitude of 10 degrees Celsius above normal or higher forecasted by the GFS ensembles over parts of Oklahoma, Kansas, Nebraska and South Dakota. The entire Central U.S., parts of the Southeast and much of the Rockies look to be encompassed in these above-normal temperature anomalies before more seasonal weather begins to filter into the country's midsection.

After June 6th, model guidance begins to become more uncertain as to the direction of the pattern, but early indications are warmer-than-normal temperatures may sustain themselves in the Western U.S., leading to cooler than normal conditions for the eastern third of the country. I discuss the broad pattern outlook in a special long range outlook post here from last week, but already it appears that my early-June forecast may miss the mark for failing to catch this nascent warming trend.

To summarize:
- A warming trend is forecasted for portions of the Central and East U.S. beginning June 1st and continuing through June 6th.
- The warmest conditions look to be centered on the Plains, but temperatures gaining to at least seasonal levels appears likely from the Front Range in Colorado out into the Ohio Valley.
- Forecasts diverge beyond June 6th, but warmer than normal conditions may persist in the West while cooler conditions take over in the East.


Friday, May 24, 2019

Special Long Range Outlook: Early to Mid June

This is a follow-up on my previous long range outlook posted last weekend (click here to read). In this post, I'll be using convective activity across the Equator as a basis for the forecast, and then consolidate that with the long range outlook previously published. Click on any image to enlarge it.

**If you do not wish to read this (admittedly technical) discussion, please scroll down until it says to stop.**

Let's first review the concept of the Madden-Julian Oscillation (MJO), as this will prove critical to the rest of this article. The MJO is an oscillation that is based on the location of convective activity along the Equator. Depending on where this convection is, the MJO is said to be in one of eight phases.

Portrayal of enhanced (suppressed) convection around the Equator, shown as green/blue (brown) colors, for each of the eight phases of the Madden Julian Oscillation.
Source: NOAA
In a generalized sense, Phase 1 of the MJO typically sees enhanced convection along the Equator from South America into portions of Africa. In Phase 2, this enhanced convection shifts east into the Arabian Sea and marginally into the Bay of Bengal, with storms most focused directly south of the subcontinent of India. Drier than normal conditions are then normally seen north and east of Australian into Oceania. Phase 3 again sees an eastward shift of enhanced convection, now firmly into the Bay of Bengal and the broader majority of the Indian Ocean, more focused on the eastern half of that body of water. Additionally, we see further strengthening of that convection, maximized just southeast of India. Phase 4 sees the enhanced convection move into the Timor Sea and Arafura Sea. By Phase 5, the convective activity begins entering the far western Pacific Ocean, with calmer than normal conditions developing in portions of the Indian Ocean. Australia also experiences the enhanced convective activity in Phase 5, mainly over the northern section of the country. Phase 6 shifts the enhanced convection further east, and so on through Phase 7 and Phase 8.

However, a number of scientists have put in long hours of hard work to focus in on how these areas of enhanced and depressed convection vary, if at all, in location and intensity depending on the time of year. The Bureau of Meteorology in Australia has created these composites over three-month windows - for our purposes, we will use the May-June-July window so as to be centered over June.

Portrayal of enhanced (suppressed) convection around the Equator, shown as cooler (warmer) colors, for each of the eight phases of the Madden Julian Oscillation during the May-June-July period.
Source: Bureau of Meteorology
 The location of areas of enhanced and suppressed convection are about the same as in the general composite graphic shown first. However, particularly in meteorology, more data and more accuracy is almost always a positive when making a forecast. As a consequence, I will refer to this image later on in this post, as opposed to the first image.

It's reasonable to be skeptical as to why we should care about the location and intensity of convective activity along the Equator. I can imagine some of you are wondering how some showers and thunderstorms thousands of miles from you and your computer screen can possibly have any substantial impact on the weather pattern, much less be a driver of the pattern as a whole. This article will aim to show why we care about the MJO, and how we are able to use those showers and thunderstorms to make reasonable forecasts for the United States almost a month in advance.

Now that we've reviewed what the MJO actually is and how we can determine which of the eight phases it is in, let's see which phase the MJO is currently in so we can begin building this outlook.

Madden-Julian Oscillation (MJO) phase space diagram forecast, from the ECMWF model and ensembles.
Source: Climate Prediction Center
The above graphic might appear intimidating, but in reality it's rather simple. The diagram itself is called a phase space diagram, and it uses two different mathematical models that analyze the concentration of clouds and the behavior of winds at different levels of the atmosphere to diagnose what phase the MJO is in, and how strong it is within that given phase. Those two different models are labeled RMM1 and RMM2, as shown by the 'x' and 'y' axes of the graph. In other words, they help create the graph to tell us what the MJO is doing.
Reading the graph is relatively simple. The eight different phases are clearly marked, with locations typed out at different parts of the chart to indicate where enhanced convection is located when the MJO is in a given phase. You can confirm this by comparing the phase numbers and location names to the composite image(s) discussed earlier in this article. The graph also gives an idea of the strength of the MJO - the further it is from the circle in the center, the stronger the MJO is within any given phase. If the MJO is shown to be in that middle circle, it means the oscillation is too weak to definitively assign a phase to, and thus may not be a primary driver of the atmospheric pattern at that time. Luckily for us, model guidance has the MJO as being outside of that middle circle for the entire forecast period.

Viewing this phase space diagram, we can see that the MJO was actually inside of that middle circle during the middle of April, just over a month ago. From there, however, the oscillation seemed to come to life and strengthened into Phase 2 by late April. From there, through May up to this point, enhanced convective activity propagated eastward from the Indian Ocean, bringing the MJO through phases 3 through 8, where it now sits. Indeed, the MJO has been positioned in phase 8 for about ten days now.

This particular set of model guidance, from the European model (the ECMWF) and its ensembles, sees the MJO shifting into Phase 1 to wrap up May, and then transitioning back to Phase 2 in time for early June. Indeed, this guidance even sees convection moving eastward enough to set up in Phase 3 by about June 6th, though at that point discrepancies among ensemble members begin to materially degrade the quality of the forecast, not to mention those members have the MJO wobbling close to that circle in the center.
Of course, there is more than just one model that tracks the MJO this way:

Madden-Julian Oscillation (MJO) phase space diagram forecast, from the CFS model and ensembles.
Source: Climate Prediction Center
This graph uses the same chart set-up and parameters as the one before, although this time instead of the ECMWF model and its ensembles, the CFS model and its ensembles are displayed. Even so, the CFS takes the MJO through almost an identical path as the ECMWF during the same time period, dragging the oscillation through Phases 1 and 2 to kick off June before trying to move towards Phase 3 by June 6th.

This concept of the MJO moving into Phase 2 to begin June, after traversing Phase 1 to close out May, is more-or-less the consensus forecast across available models. As such, we will incorporate a Phase 2 MJO state into this outlook for the first five days of June or so. Now that we know what the MJO should do for the first several days of June, what does that translate to in terms of the weather pattern here in the United States?

More hard work by scientists has allowed us to have answers to that question.

500 millibar geopotential height anomalies as observed during a Phase 2 MJO event in June.
Source: Japan Meteorological Agency
The Japan Meteorological Agency (JMA) not only narrows down the typical 500 millibar geopotential height anomalies, 2m temperature, 200mb wind speed etc. patterns that occur by MJO phase, but also divides these into individual months. As a result, we are able to view the typical 500 millibar height anomalies seen when the MJO enters Phase 2 during the month of June. In the top image, cooler (warmer) colors represent below-normal (above-normal) height anomalies, which generally correspond to troughs and colder weather (ridges and warmer weather). We don't need to really worry about the bottom panel; it shows the outgoing long wave radiation (OLR) anomalies for a Phase 2 MJO event in June. It might sound complicated, but it's really the same thing as analyzed in the first two graphics of this post: in that bottom panel, cooler (warmer) colors represent areas of enhanced (suppressed) convection. We already know from earlier that a Phase 2 MJO event corresponds to enhanced convection in the Indian Ocean region, and this bottom panel is just reiterating that.

Back to the top panel in this graphic, it appears that the MJO entering Phase 2 in the month of June tends to bring about stormier than normal activity in Russia and northeastern China, while maintaining a tendency towards ridging over Japan and southeast China. On the other side of the Pacific, a June Phase 2 MJO event encourages a ridge well offshore of the West Coast and southwest of the Gulf of Alaska, while stormier/cooler than normal activity is predominant in most of the Western U.S. Eagle-eyed readers will recognize the positioning of that stormier activity as corresponding to the negative phase of the Pacific-North American (PNA) index (you can read more about the PNA by clicking here). As such, it's not surprising that the Central and Eastern U.S. then experiences a tendency for ridging and warmer weather during this state of the MJO for the month of June.
In other words, when the MJO moves into Phase 2 during the month of June, the western U.S. tends to be stormier and cooler than normal, while the central and eastern U.S. will have a tendency in favor of ridging and generally warmer weather.

But wait a second - that sounds pretty familiar to the forecast I published this past weekend, with cooler weather in the West, warmer weather in the East and storms riding a ridge through the central part of the country, doesn't it?  That's not by coincidence - if you've followed this blog over the years, you'll find that these articles have often made these kinds of confirming conclusions between teleconnections and oscillations, enabling these long-range outlooks to have at least a bit of justification behind them as opposed to throwing darts in the dark. Whether that justification ever ends up being accurate, though, is always a question mark!

Indeed, long-range models see a pattern not dissimilar to that Phase 2 MJO composite evolving for the last bit of May and first bit of June:

Forecasted 500 millibar geopotential height anomalies from the ECMWF model (left), GFS model (center) and CMC model (right), all valid for the period from May 31st to June 2nd.
Source: Pennsylvania State University
Using these three models over the May 31 - June 2 forecast period, we can ascertain the general idea of troughing in the western U.S., signifying stormier weather, as well as a nascent ridge trying to form in the southern U.S. Note, however, that our forecast here seems to be scuttled by that large lobe of below-normal height anomalies and associated cooler weather in Canada. This seems to go right up against that MJO Phase 2 chart we were just looking at. What happened?

We have to go back to something I briefly touched on at the beginning of this post to understand why model guidance isn't showing what we expected. When analyzing the structure of the MJO phase space chart, I went over the circle in the middle, where an MJO phase can't be determined definitely because the oscillation is too weak at that point in time. Consequentially, the MJO may not be a primary driver of the pattern at that time. What we must now remember is that the MJO may not be a primary driver of the pattern at this time, even if it is in a defined phase. The atmosphere doesn't abide by any single oscillation or teleconnection, it is all one big puzzle.

In this case, models don't see North America getting a typical Phase 2 set-up because of Rossby waves sending much above-normal air into the Arctic Circle by way of those stout ridges over Eurasia and even potentially north of Japan. This forces the tropospheric polar vortex - already significantly weakened in the summer months, to be sure - to lower latitudes, as happens in the winter. Adding to that, we see a strong ridge positioned near Greenland (negative NAO pattern), which encourages cold weather in the upper latitudes to flow down to lower latitudes - in particular, the United States. These two factors (and likely more) combine to keep that swath of negative height anomalies locked out in Canada, bringing about that "unexpected" outcome. Additionally, during this forecast period, the MJO will be transitioning from Phase 1 to Phase 2, which makes it a little messy to use composite images. In any event, we now have three things to use in our forecast for early June:

- Phase 2 of the MJO
- Continuation of strong ridging over the Arctic Circle
- The negative phase of the North Atlantic Oscillation (NAO)

We'll make an actual forecast out of those factors at the end of this post. For now, let's move on to the mid-June outlook.

I'll begin by using another scary-looking chart which, in reality, isn't too tough to read.

Hovmoller of outgoing long wave radiation (OLR) between the latitudes of 5º North and 5º South, forecast period from May 22nd to June 19th, via the CFS model.
Source: North Carolina State University / Carl Schreck
First and foremost, let's break down what this chart shows.

For all areas above the solid black line positioned next to the May 22nd marker, all variables that will be described next are observed. For all areas below that solid black line, all variables are forecasted.
Next, we need to recognize that the solid colors here are merely showing the same thing we've looked at three times now: convective activity. As you might guess by now, green (brown) colors indicate the presence of enhanced (suppressed) convection.
The third thing to note about this chart is that we will not be paying attention to any of those red, pink and blue ovals and shapes - they indicate different types of waves and are juxtaposed with different bursts and drops in convective activity, depending on what kind of wave it is, but for our purposes we will disregard all that so it doesn't get too confusing.
The last thing to note about this chart so we can begin our forecast is that we do care about the black oval shapes. Remember back at the beginning of this post, when I explained that the MJO is an oscillation based on the location of enhanced convection along the Equator? Well, in this chart, those green areas show those same enhanced spots of convection at those same locations (see the longitude markers on the x axis) and also along the Equator (remember this chart shows convective activity between 5º North and 5º South latitude). The black ovals here are simply designed to highlight MJO waves (think of how the enhanced convection is a "wave" slowly moving east across the Indian and Pacific oceans).

That's about all that's critical to understanding that very-messy chart: knowing that part of the chart shows observed conditions and part of it shows forecasted conditions, knowing that the shaded colors show enhanced or suppressed areas of convection just like we've already gone over in other graphics, knowing that we can disregard the colored ovals and shapes, and knowing that the black ovals merely point out to you where the MJO wave is.


So, why do we care about it anyway? Whereas the forecasts of the MJO made using the phase space graphics only go out to June 6th, the CFS Hovmoller (name of this chart type) above goes out into late June, meaning we can use it for our mid-June outlook.

Let's start out at the June 5th marker on that chart. On June 5th, per this forecast, the MJO wave is forecasted to reach about the 85º East longitude region. If we refer back to the second graphic in this post, we can see that enhanced convection around 85ª - 90º East corresponds to the MJO being in Phase 2, nearing Phase 3. That lines up well with what the phase space forecasts anticipate by this time.

Beyond that, however, we run into some trouble. The forecast here has that convection, and the MJO wave as a whole, dissipating by June 12th, with the enhanced convection anomalies vanishing even before that, around June 10th. That poses a dilemma for us, as it means this model sees the MJO entering that center circle if we were to view this using a phase space diagram.

It seems like we're at the end of the road here. Of course, that's not the case.

Forecasted 200mb Velocity Potential for 1pm June 5th, using the GFS Ensembles.
Source: Tropical Tidbits
One way to identify the location of enhanced convection to track the MJO is by viewing anomalies in convective activity themselves, as we have done up to this point. Another way, however, is by viewing the upper air pattern to identify areas that are conducive for thunderstorm development. Recall that strong thunderstorms will build near the surface as air converges and rises, with the clouds of that storm then plateauing and spreading out high up in the sky when the air in those clouds becomes as cool as the environment, leading to the air high up in the sky to spread out and diverge. This is essentially what we're looking at here, only in this chart we're looking at those convergence/divergence motions only at the 200 millibar level (cruising altitude for most airplanes).
Green shading and associated spreading-outwards arrows indicates divergence aloft. This is a plus for thunderstorm formation, as it encourages air lower down to converge and create thunderstorms. Orange shading and associated spreading-inwards arrows indicates convergence aloft, which piles air into the column and suppresses thunderstorm formation. As such, you can think of green shaded regions as being areas where thunderstorm development is encouraged, and brown shaded areas where it is discouraged.

In this forecast for June 5th, ensemble guidance sees strong divergence aloft over Latin America and between the 120º West and 60º West longitude lines. Additionally, we see some divergence aloft over Africa. Remember that this time period is when the MJO wave is dying out and we expect it to enter that center circle in the phase space diagram, where the MJO is technically too weak to be sorted into a phase.
Of course, the atmosphere could care less about what a bunch of humans say about phases - just because the MJO is seen as too weak to define doesn't mean the atmosphere will shut down convection, and certainly doesn't mean that convection in certain areas won't impact the U.S. until the MJO re-emerges into a given phase. So, since we see convection-favorable dynamics over Latin America and Africa at this timeframe, even though the MJO is technically now too weak to track, we are going to go into the mid-June forecast presuming that the pattern will reflect a Phase 1 MJO state, because the convection-favorable dynamics will encourage storms to form over Phase 1 locations. Again, the MJO will technically be too weak to track, but that definitely doesn't mean convection over the Phase 1 area won't produce Phase 1-like conditions.

So, what happens in a Phase 1 MJO event in June?

500 millibar geopotential height anomalies as observed during a Phase 1 MJO event in June.
Source: Japan Meteorological Agency
A Phase 1 MJO state during June typically sees a strong upper level low positioned over the Bering Sea and Aleutian Islands of Alaska, with weak ridging from Hawaii towards the Pacific Northwest and a corresponding tendency for stormier weather in the West and South United States. Some ridging is seen in New England.

200 millibar zonal wind anomalies as observed during a Phase 1 MJO event in June.
Source: Japan Meteorological Agency
What I find more interesting, however, is what typically happens to the jet stream during a Phase 1 MJO event in June. Looking at the north Pacific, we see quite a few contours with pink shading stretching from Japan all the way into the Gulf of Alaska. Those high positive contours and elongation from Japan to the Gulf of Alaska signal a strengthening and extension of the Pacific jet stream, a feature that acts to strengthen low pressure systems which drop into the Western U.S. on the back of this jet stream. I referenced the impact an extended Pacific jet can have when discussing the high severe weather threat earlier this week (click here).

Of course, we already saw earlier how the MJO is only one factor in a forecast, and that model guidance can (and often does) reflect this combination of teleconnections and oscillations in a way that is far from any one oscillation's "textbook" composite output (i.e. the composite for Phase 2 that we ran into trouble with earlier). Model guidance again is showing a different outcome by mid-June:

Forecasted 500 millibar geopotential height anomalies (left) and "spaghetti" plot of all members (right) from the GFS Ensembles, valid 1pm on June 8th.
Source: Pennsylvania State University
This is indeed a 384-hour model forecast, one of the big "no-no's" when it comes to making a forecast. While I agree that one should never base a forecast off of 16-day model guidance, I do believe that even these ultra-long-range model outputs can provide valuable hints about what the overall pattern will eventually be.

The GFS ensembles expect the pattern over the Arctic Circle to not vary too much by June 8th, with strong ridging still forecasted over Eurasia into the Arctic Circle, as well as a stout ridge now moving into Canada. Still, we do see some hints of that Phase 1-esque pattern, with tightened isohypses south of the Bering Sea seeming to signal a stronger Pacific jet stream and a valley in isohypses over the Western U.S. reflecting troughs still positioned over that area.

**If you scrolled past the technical discussion, STOP HERE.**

By now I've either put you through a lot of pain reading this, or you've gotten a satisfactory fill of technical forecasting for the day. You may also have decided to skip the technical discussion altogether. In any event, it's time to put these pieces together and make the forecast.

For the first several days of June, I expect a continuation of the current pattern. Strong ridging in the upper latitudes will keep the upper-level low over Canada in place and keep northern portions of the United States cooler than normal. Storm systems will continue to be deposited in the Western U.S., which will lead to a ridge to build in the East U.S. and allow those storm systems to ride northeast along the ridge, bringing repeated severe weather threats to the Central U.S. The presence of these troughs in the West will maintain predominantly cooler than normal and wetter than normal conditions. The pattern for the Southeast should remain relatively quiet, with this broad set-up not favorable for significant weather phenomena (i.e. big severe weather outbreaks).

From about June 8th through June 15th, I expect marginal shifts in the pattern, but for the broad North American set-up to be relatively unchanged. Ridging is preliminarily expected to continue in the Arctic Circle, which will keep the Northern U.S. under the gun for cooler than normal conditions. An extension and strengthening of the Pacific jet stream, especially relative to the weakened and highly-meridional flow seen to end May, seems plausible in this timeframe, which will then have the potential to re-introduce higher-end severe weather threats. At this point, the ridge in the Southeast should become at least a little less stout, which may allow more severe weather threats in the eastern-third of the country and perhaps give a reprieve to the Central U.S. Broadly-seasonal temperatures may be expected outside of the North and parts of the West (cooler than normal), but this could change if that ridge projected over Canada for June 8th becomes dominant and establishes a well-above normal temperature pattern for the central part of the country.

The longer-range forecast (i.e. beyond June 8th) will undoubtedly change, but hopefully this article gave you a look at how we can in fact use thunderstorms in the Equatorial Pacific to forecast the weather here many days in advance!


Wednesday, May 22, 2019

May 28 - June 1 Potential Storm System & Cooler Weather

*Note: This post is using a forecasting technique that, while similar to the 6-10 day storm forecasting method I was using previously on this blog, is somewhat different and subject to more uncertainty. As such, treat this forecast with a higher level of caution.

I am expecting a storm system and simultaneous influx of cooler than normal air into the contiguous United States to close out May and introduce June. Click on any image to enlarge it.

Observed 500 millibar geopotential height anomalies over Asia as of 7am central, Monday, May 20th.
Source: Tropical Tidbits.
As of this past Monday morning, an unusually-strong upper level low was observed over northeast China, with an individual disturbance rotating around the base of the low, seen here as entering western South Korea. This upper level low is situated between a pair of ridges, the stronger of which is placed east of Japan and near the Aleutian Islands of Alaska.

In this somewhat-different forecasting method, I am using a swath of eastern Asia (namely parts of China, Mongolia and Russia into Japan) to anticipate weather patterns over different parts of the United States in about a 10-day timeframe. To allow for the aforementioned uncertainty, I have widened this to an 8-12 day band centered around May 30th, ten days after the observed graphic above. This is quite similar to the idea that weather in and around Japan correlates to a similar weather pattern in the United States 6-10 days later, but this venture tries to identify where these weather events may occur, instead of broadly anticipating a storm system, cold wave or heat wave over the broad country (which, to be sure, has proved very effective). This is merely an attempt to build on that, and of course has the potential to end in failure. For now, though, there's only one way to hold it (and myself) accountable: by placing a forecast out using it.

Forecasted 500 millibar geopotential height anomalies over the United States as of 1am central, Thursday, May 30th.
Source: Tropical Tidbits.
By early May 30th, current model guidance sees a similar situation developing, with an unusually-strong upper level low placed over Canada with an individual disturbance rotating around the base of the low: here, it is seen crossing from the western Great Lakes into Canada. In this forecast, colder than normal air is again seen over the northern Plains, Midwest and Great Lakes to end May and just begin June. Also matching with the May 20th observed conditions over Asia, a pair of ridges are seen surrounding this upper level low.

Indeed, model guidance sees below-normal temperatures a few thousand feet off the ground by the morning of May 30th over those same areas:

Forecasted 850mb temperature anomalies over the United States as of 7am central, Thursday, May 30th.
Source: Tropical Tidbits.

As such, this forecast is as follows:

- A storm system is expected to cross the central United States during the May 28th - May 30th period, with cooler than normal weather following over the northern portion of the country.
- Ridging over the Eastern Seaboard should keep that area relatively warmer to end the current month.


Tuesday, May 21, 2019

Active Pattern to Continue Severe Weather Threats

An active weather pattern is expected to continue threatening severe weather over the contiguous United States for the remainder of this workweek into the weekend and even next workweek, following up on my long-range forecast issued the other day (click here to read). Click on any image to enlarge it.

Latest analysis of the jet stream over the Northern Hemisphere.
Source: San Francisco State University
A look at the jet stream over the Northern Hemisphere spells out how the remainder of the week should bring continued threats for severe weather. The Pacific jet stream remains strong and extended west of the current trough that produced yesterday's severe weather, recording wind speeds in excess of 160 knots just off the West Coast. Maximizing that jet streak is the presence of another storm system making its way into the Pacific Northwest, which will be shunted south into the Southwest over the next day or two as a Rossby Wave blooms northward into the Gulf of Alaska. This particular storm system is expected to pose a threat for severe weather by the end of the workweek, something the Storm Prediction Center has already highlighted as shown below.

Severe weather outlook for Thursday, May 23rd.
Source: Storm Prediction Center
Long-range severe weather outlook for Friday, May 24th.
Source: Storm Prediction Center
As this storm system ejects into the Plains and Midwest to wrap up the workweek, a second trough is forecasted to make its way into the Southwest over the weekend.

500 millibar geopotential height anomalies for Sunday evening, May 26th.
Source: Pennsylvania State University
Indeed, on Sunday night model guidance projects strong negative geopotential height anomalies to once again be present over the Southwest, indicative of the presence of a storm system in that region. The positioning of a ridge over the Central and Eastern U.S. should locate this storm system's severe weather threat again over the Plains and Midwest to kick off the next workweek (about a week from today), essentially placing the same cities and states under the gun that have seen severe weather threats for the past few days now.

Just when it seems like it might quiet down, ensemble guidance then sees yet another storm system entering the Southwest by the end of May, which could pose yet another severe weather threat in these same areas for the beginning of June. However, model discrepancies become too large to have any material confidence in forecasts beyond the end of next workweek.

To summarize:

- An active weather pattern is expected to persist through this workweek and weekend into next workweek.
- Multiple severe weather threats are expected to evolve over the Southern and Central Plains into the Midwest and Great Lakes regions, as a train of upper level lows ejects from the Southwest and are pushed north into those areas due to a high pressure stationed over the Southeast.
- Severe weather threats are expected to continue through next workweek, but the potential for further threats may continue into the opening days of June.


Sunday, May 19, 2019

Long Range Outlook: May and June 2019

This post will discuss my thoughts on the outlook for the remainder of May and into June of this year.  Click on any image to enlarge it.

We begin with an analysis of the current atmospheric pattern across the Northern Hemisphere.
Five day (May 11 thru May 15) mean 500 millibar height anomalies.
Source: Japan Meteorological Agency (JMA)
The last several days have seen a rather volatile pattern positioned over the Arctic and similar upper latitudes. Most predominantly, there has been a trio of ridges pushing into the Arctic Circle, placed over Siberia, north-central Eurasia and far western Europe. The ridge placed over Eurasia has forced itself northward over the North Pole, disrupting the tropospheric polar vortex and splitting lobes of cold air to lower latitudes. Indeed, we have seen such an evolution take place, with strong negative geopotential height anomalies over the northern Atlantic Ocean, northeast Eurasia/Russia, and marginally in the Bering Sea and Canada.

This is a similar process as what takes place when the tropospheric polar vortex is disrupted in the winter, with colder than normal air being forced down to lower latitudes as ridging forces the polar vortex to be displaced from its usual resting place over the North Pole. Consequentially, colder than normal temperatures have indeed been observed in the lower latitudes, including here in the United States.

The movement of colder air into the United States has been driven more-so by a pattern resembling the positive phase of the Pacific-North American (PNA) oscillation, which is seen best by the presence of a ridge along the West Coast of the U.S., which encourages troughing and cooler air to shift southward from Canada into the central and eastern U.S. Additionally, a ridge near Greenland as of late has provided further impetus for the transport of colder air into the contiguous United States, a pattern which some of you may recognize as a set-up typical of the negative phase of the North Atlantic Oscillation (NAO). Again, both of these are patterns typically emphasized during the winter, but this provides a good example of how they can still function across seasons.

In summary, we're entering this forecast period knowing the recent atmospheric regime is tilted in favor of colder temperatures for the United States. Let's begin with the outlook itself.

Forecasted 500 millibar geopotential height anomalies from the ECMWF model (left) and GFS model (right) for the 8-10 day forecast period.
Source: Pennsylvania State University
 My goal for this outlook is to begin with computer model guidance and then expand outwards to look at individual oscillations to try and discern if these computer models might be missing something in their forecast, or may be right on target. In accordance with that framework, we'll begin with the projected 500 millibar geopotential height anomalies in the 8-10 day forecast period from the ECMWF model (the "euro" model; left panel) and the GFS model (the "american" model; right panel). In general, warmer colors portray ridging (calmer and warmer weather), while colder colors portray troughs (stormier and cooler weather).

Model guidance is in pretty good agreement over the evolution of the pattern over the United States into the end of the month, with both models suggesting a near-Rex Block formation along the western coast of North America. A Rex Block is traditionally exemplified by a ridge placed "on top" (to the north) of a trough. In this graphic, such a set-up would make it look like warmer colors would be positioned directly north of colder colors. While this sort of set-up isn't precisely laid out here, both models anticipate the development of zonal flow downstream of the Rockies, typically what happens downstream of a Rex Block.

Such a zonal flow and ridging over the eastern two-thirds of the country is rather intriguing given what is forecasted in the upper latitudes, with both models forecasting a continued disruption of the tropospheric polar vortex as well as a continuation of high pressure near Greenland. As discussed earlier, both of these factors encourage colder than normal air to propagate to lower latitudes.

So what's happening here?

Let's take a closer look at the projected pattern over North America. I noted how it seemed like a vaguely Rex Block-esque pattern was forecasted to set up over the western coastline of the continent. But we missed a key item when looking at the positioning of the ridge and trough along the coastline: the trough is forecasted to sit in the Southwestern U.S. In other words, this prompts a pattern downstream more typical of the negative phase of the PNA oscillation, a flip from the recent positive PNA phase laid out at the start of this post. In a negative PNA regime, the trough in the West U.S. encourages the development of a ridge in the East U.S., which is indeed what we're seeing in the model guidance projections.
This isn't too surprising when we consider that these factors that affect the PNA oscillation are upstream of the United States, where as the NAO and other factors more encouraging of cold air in this time period are either well north of the U.S. or are downstream of the country, limiting their impact.

As outlined, let's now expand on this a bit and look at forecasts for four key atmospheric oscillations below.

Forecasted states of the Pacific-North American (PNA) index, top-left; North Atlantic Oscillation (NAO), top-right; Western Pacific Oscillation (WPO), bottom-left; and the Eastern Pacific Oscillation (EPO), bottom-right.
Source: Earth System Research Laboratory (ESRL)
The projected state of the PNA oscillation is shown in line with what we extracted before viewing this forecast, where a recent positive-PNA regime would flip to a negative-PNA situation in the 8-10 day period. This ESRL forecast builds on that by anticipating the PNA oscillation to remain in negative territory until the turn of the month. Be sure to note the strength of this negative-PNA regime, as we'll return to that later on.

Moving to the NAO forecast, our earlier suspicions are again confirmed with the agency projecting this oscillation to remain in negative territory throughout the entire forecast period (indicating that ridging near Greenland will persist), albeit to a weakening degree by the end of the period.

The WPO and EPO phenomena can be noteworthy and impactful when they're strongly oriented in one way or another, but they are both projected to flip states multiple times and remain at relatively-modest strengths during the forecast period. As such, we will focus more on the PNA and NAO for this outlook.

We now have a pretty good idea of what is forecasted to evolve over the next ten days or so (feel free to scroll down to the end for a summary if it was a little tough to grasp), so let's now move into the two-week-plus forecast period.

Forecasted GFS ensemble 500 millibar geopotential height anomalies (left) and spaghetti plot (right) at the end of the forecast period (June 3rd).
Source: Pennsylvania State University
It is imperative to first recognize the degree to which this particular forecast graphic is imperiled as a consequence of its long time horizon. Rarely, if ever, do forecasts made sixteen days in advance actually come to fruition. Unfortunately, our computer models are simply not yet advanced enough and do not have enough real-time observational data to input to make such accurate forecasts. This is the caveat that sixteen days out is a very long time, and conclusions drawn from here are not to be fully invested in to.

The GFS ensembles project the pattern 16 days out to be pretty similar to the pattern seen developing by the 8-10 day period. Indeed, the GFS ensembles anticipate ridging persisting over the Arctic Circle but a trough along the West Coast will sustain a negative-PNA pattern for the United States. In other words, such output indicates warmer conditions may be expected for the southern portions of the country, relatively seasonal weather for the country's midsection, and perhaps seasonal to cooler-than-normal conditions in the northern slice of the country. Cooler than normal and stormy weather would be anticipated for the western half of the country, with storm systems shuffling into the Southwest only to eject into the Plains and form the basis for severe weather threats in the Central U.S.


I'm a believer in the idea that the atmosphere takes all kinds of oscillations and phenomena into account when creating the weather for a given area - the United States does not exist in a vacuum where a negative-PNA state automatically produces warmer than normal weather for the entirety of the eastern U.S., for example, or where a positive-NAO state automatically produces zonal flow over the country. I also believe, however, that some phenomena can impact certain areas to greater degrees than other phenomena based on the location and intensity of these phenomena. For example, while the negative-PNA state may not automatically produce a 'textbook' negative-PNA outcome, its position as being upstream of the United States means the country's weather is more likely to be dictated by what the PNA does than what the NAO (way off in Greenland) does.
These are not controversial beliefs by any means to any weather enthusiast, but it is worth stating because my outlook here will rely heavily on the latter belief. While my forecast for the first ~two weeks relies substantially on the evolution of a negative-PNA state, as discussed, I'll now shift the basis for my forecast to another indicator.


Consider the ESRL forecast for those four oscillations once again. Note how all of them seem to dwindle down to weak levels of whatever their respective states are forecasted to be. That makes for a tough forecast for the start of June, if four of the most impactful oscillations are expected to be too weak to make a significant impact. As such, I'll now briefly zoom out and review the broader atmospheric pattern currently in place across the globe: an El Nino.

Multivariate ENSO Index (MEI). Positive (negative) values indicate the presence of an El Nino (La Nina).
Source: Earth System Research Laboratory
Frequent visitors to this blog in past winters will readily recognize the importance of understanding the current state of the El Nino-Southern Oscillation (ENSO) phenomenon. The ENSO phenomenon is identified by observing sea surface temperature (SST) anomalies along the Equatorial Pacific. Cooler than normal water temperatures typically indicate the presence of a La Nina, while warmer than normal temperatures indicate the presence of an El Nino. Each of these states of the ENSO phenomenon broadly drive weather patterns around the globe, giving you an idea of just how material this is to any longer-term forecast.

Using the MEI metric above, we see that positive values have recently increased, indicating the presence of warmer than normal water temperatures in the Equatorial Pacific and, potentially, an El Nino event. For the purposes of this post, the June outlook will operate under the assumption that an El Nino is in place. What does that mean for the forecast?

Surface temperature anomalies during summers where an El Nino was observed.
Source: Earth System Research Laboratory
The ESRL provides a neat tool whereby the user is able to analyze a variety of atmospheric variables that can be broadly expected given an El Nino or La Nina, given if it is occurring in the winter or summer. This provides a substantial advantage here, as we can glean what a "typical" El Nino in the summertime will give out in terms of temperature anomalies. The outcome is rather impressive, as it lines up well with what had already been established in the forecast through the end of May. Indeed, a "typical" summertime El Nino results in cooler than normal conditions in the northern U.S. as well as in the Rockies, with a small area of stronger cooler-than-normal anomalies in the Southwest. This fits well with the idea of a negative-PNA set-up, where troughs and cooler weather persist in the Southwest into the far northern U.S. as the jet stream then curves upward over the Plains due to ridging in the Southeast, a consequence of the aforementioned West U.S. troughing.

As such, it may be prudent to begin the June forecast with a basic outline akin to a negative-PNA state, with cooler and stormier weather in the Western U.S. extending to the northern Plains, severe weather opportunities in the Central U.S., and broadly seasonal to warmer conditions in the Southeast. Now that we've zoomed out to examine the overarching pattern, it's time to return to individual oscillations. While the PNA, NAO, EPO and WPO all may be too weak by the start of June to dominate the broad pattern for the U.S., another oscillation looks to be strong enough at this point in time to shape the June outlook for the contiguous 48. This is the Madden-Julian Oscillation (MJO).

Forecasted state of the Madden-Julian Oscillation (MJO) from mid-May through the end of May from the GFS ensembles (left panel) and the ECMWF ensembles (right panel).
Source: Climate Prediction Center
The Madden-Julian Oscillation (henceforth MJO) is categorized using a phase space diagram. In this case, that means that the MJO can take one of eight different phases. Why so many? The MJO phenomenon is identified by the placement of convective activity along the Equatorial Pacific, similar to the ENSO phenomenon. However, with the MJO, the zone of monitoring extends across the entire Equator, not just in the Equatorial Pacific. Each phase of this oscillation represents a different location for convection along the Equator, with Phase 8 and Phase 1 typically seeing convection placed over and around South America and/or Africa, and Phases 4 and 5 corresponding to convection north of Australia. Each of these eight phases has been found to produce different impacts on weather patterns around the globe, including here in the United States. We'll use that to our advantage a little later on.

Both the GFS ensembles and ECMWF ensembles anticipate the MJO to move near or into Phase 2 by the beginning of June. The models differ as to the strength of the oscillation by this point in time, however, with the GFS expecting a notable Phase 2 state but the ECMWF bringing the oscillation weak enough to enter that circle in the middle of the graph, indicating that the MJO is too weak to be definitively placed into a certain phase. Model divergence like this beyond two weeks out is to be expected, but because both models have the MJO either approaching or being within the Phase 2 state, we will continue the June forecast under the assumption that the month will begin with the MJO in a Phase 2 state.

As I noted earlier, each of the eight phases of the MJO affects global weather patterns in their own ways. The hard work of a number of scientists has enabled us to create composites for variables such as temperature, precipitation and upper-air flow for each phase of the MJO, so that we can have an idea of what kind of weather a certain phase of the MJO will bring for any given month. Let's view the surface temperature anomaly composite for a Phase 2 MJO state during the month of June.

Surface temperature anomaly composite for a Phase 2 MJO state during the month of June.
Source: American Weather (americanwx.com)
Allow me to restate again the importance of recognizing that no one oscillation controls the weather pattern - indeed, the atmosphere can be thought of as a web of different phenomena, all of them impacted or imposing impacts someway, somehow by/on to others. As such, it's critical to remember that we are still trying to glean a general framework for the June forecast, rather than definitively identify what *will* happen. After all, if we knew what would definitively happen, I would be on a private island with an expensive yacht somewhere!

As the composite image above shows, a Phase 2 MJO state occurring in June has historically resulted in warmer than normal temperatures over the southern and central Plains, extending northeast through the Great Lakes and Northeast regions. Slightly below-normal temperatures have been seen in the Pacific Northwest in such a situation.
This brings about a pattern similar to the negative-PNA pattern that we have discussed rather extensively in this post, and seems to encourage the idea that the pattern forecasted to end May could continue into the opening portion of June.

500 millibar height anomaly composite for a Phase 2 MJO state during the month of June.
Source: American Weather (americanwx.com)
Indeed, when looking at the 500-millibar geopotential height anomalies for a June Phase 2 state of the MJO, a pattern similar to a negative-PNA set-up emerges, with slight hints of a trough in the Western US aligning with ridging in parts of the Eastern U.S. Note the upper level low in place near Greenland, though, which represents the positive phase of the North Atlantic Oscillation (NAO) and tends to encourage zonal flow & warmer conditions over the contiguous United States. It remains to be seen if this will transpire into June, as ESRL forecasts have the NAO maintaining a slightly-negative state into next month, which would then suppress ridging over the Eastern U.S. somewhat.

Beyond this point, model guidance becomes too unreliable to rely on any combination of oscillations, and as such the remainder of June will expect a pattern broadly in line with a summertime El Nino, with recognition that individual oscillations could (and likely will) easily change such a pattern as next month draws closer and model guidance becomes more accurate as to the state of oscillations.

Forecast Summary

- Western U.S.: Expect warmer than normal weather to shift to stormier- and cooler-than-normal in coming days, with this regime change complete by seven days out. This stormy and cooler pattern should persist through the end of May. Tendency for stormy and cooler weather should continue into the opening days of June, with this same pattern encouraged by the El Nino for the entire month. Individual oscillations and other discrepancies may change this forecast for June, however.

- Central U.S.: Expect a continued active weather pattern through the end of May, with ample severe weather opportunities as storm systems eject eastward from the Southwest and ride an upward-curving jet stream over the Plains. Temperatures will vary, with a tendency for cool weather in the north Plains and seasonal to warmer weather in the south Plains. Precipitation should be expected to be above normal.

- Eastern U.S.: Expect warmer than normal conditions to evolve through the end of May and into June before a retrenchment to broadly seasonal conditions as the negative-PNA pattern breaks down. Precipitation anomalies are not seen dramatically going either way, though the Southeast U.S. may be monitored for somewhat below-normal precipitation. Broadly seasonal to somewhat warmer than normal conditions are possible for June in the aggregate.