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!