Extended Forecast Discussion (Part 2)

This is the second part of my Extended Forecast discussion, and will focus on the 2-3 week forecast range. A third part will be made for the final 4th week forecast this weekend.

For Part 1, click here.

AAM Transport
ESRL

The image here shows atmospheric angular momentum transport anomalies over the past few months. We want to focus on the anomalies in the top-right corner, the last few days of recording. On this graphic, we see a swath of yellows and oranges seeming to push diagonally upward along the image. Those yellows and oranges define positive AAM transport anomalies, and they are beginning their long-awaited movement to the north, which will throw a big wrench in our weather pattern. 

According to the oranges and yellows on the screen, the positive AAM anomaly transport values have made it as far north as about the 40N parallel, and they are strengthening. Because positive AAM anomalies generally can mean an enhanced jet stream, we might expect high pressure ridging to build northward with time as well, as the jet stream pushes north with the +AAM transport anomalies. This is already being observed with warm spells beginning to intrude upon the Central US.

This spells a predominantly warm weather period in the next few weeks, as this strong jet stream will eventually make it to the Arctic and keep the Arctic Oscillation (AO), and by extension the North Atlantic Oscillation (NAO) positive. Such a positive AO, which helps induce warm weather, is already on the forecast from the Climate Prediction Center, as shown below. 

CPC AO

The spaghetti-like alignment of the red ensemble forecast members for the Arctic Oscillation shows uncertainty associated with this pattern, as the very strong AAM transport anomalies push poleward, but eventually the ensembles should even out to a positive AO forecast over the next two or so weeks.

We can expand on the AAM for a more global look to this piece of the forecast.

ESRL
Relative AAM

Take a look at the chart above. Here, we see anomalies of the relative Atmospheric Angular Momentum (AAM) over the last few months, with a globally-averaged line on the bottom panel. If we look closely at conditions over the past few weeks, we can just barely see those yellow splotches of positive AAM anomalies that seem to be drifting in an upward-diagonal direction, meaning they're moving poleward. You wouldn't suppose this has to do with the AAM transport anomalies moving poleward too, would you?...

After the AAM transport anomalies make their move into the Arctic, guidance has been consistent on the idea of the AAM dropping well into La Nina-like negative territory. If this should happen, we would likely move into Stage 1 of the GWO stages. The stages, and their descriptions from the ESRL, are listed below.

The four primary phases of the GWO are described below, along with generally cold season (November-March) probable weather impacts for the USA. The GWO recurrence interval, or "time it takes to make a circuit", ranges from a broad 15-80 days. Two of the stages project strongly on El Nino and La Nina circulation states, which are also characterized by positive (Stage 3) and negative (Stage 1) global AAM anomalies, respectively.  Stages 2 and 4 are transitional.

Stage 1 (La-Nina like) – the global relative AAM anomaly is negative. The negative anomaly is primarily due to easterly upper level wind anomalies that extend from the Eastern Hemisphere tropics to the Western Hemisphere mid-latitudes. A retracted Pacific Ocean jet stream is a key feature in the total field.  Troughs are probable across the western USA with a ridge over the southeast.  High impact weather is favored across the Plains.

Stage 2 – the global relative AAM tendency is positive. This means that negative AAM is being removed from the atmosphere by surface friction and mountains. At the same time, westerly wind anomalies are intensifying in equatorial regions of the Western Hemisphere. Fast Rossby wave dispersion events in both hemispheres are a coherent feature of this stage and Stage 4.  A cold regime is probable across the central USA.

Stage 3 (El-Nino like) – the global relative AAM anomaly is positive. Westerly wind anomalies move into the Eastern Hemisphere, broaden in latitudinal extent and link up with deep westerly flow anomalies over the mid-latitude Western Hemisphere. An extended Pacific Ocean jet stream and southward shifted storm track is observed  favoring high impact weather events along the USA west coast.

Stage 4 – the global relative AAM tendency is negative. Positive (westerly) AAM anomalies are being removed by surface friction in the Western Hemisphere mid-latitudes and through mountain torques across the Northern Hemisphere topography. The next phase of the oscillation (if there is one) is represented by easterly wind anomalies intensifying over equatorial regions of the Western Hemisphere. This stage has enhanced subtropical jets and closed lows in the subtropics favoring rainfall events over the southwestern USA.

I italicized the Stage 1 description, as we would likely be heading for Stage 1 so long as projections remain consistent in coming days. Let's analyze that Stage 1 description as best we can. Reading through it, we notice that in Stage 1, the AAM anomaly is negative. With the forecast for the AAM dropping deep into negative territory, we can cross that off our checklist. Note how the description includes troughs in the Western USA, with a ridge over the Southeast. Model guidance has been latching onto that idea for a while now, as this shot of the GFS Ensembles 500mb height anomaly forecast valid for November 1st shows.

PSU

Lastly, check out the final sentence in that Stage 1 description. "High impact weather is favored across the Plains". 

Huh?

Well, let's decipher this. Stage 1 features the aforementioned troughs in the West US. If it also features a ridge in the Southeast, we can make a relatively safe assumption that they're talking about a predominantly-negative PNA pattern.

NCSU

The image above shows a typical negative PNA pattern across North America. Note the downsloping jet stream entering the West Coast; that tells us of troughing/storminess in the Western USA, just like we saw in the Stage 1 description. Conveniently, we also see a strong ridge positioned in the Southeast, oddly enough just like what we read in the Stage 1 description. Coincidence? No. 
Lastly, look where those two primary jet stream-like features lead- right into the Plains. That confirms our suspicions of a negative PNA pattern in coming weeks as we transition to that GWO Stage 1, placing the primary storm track over the Plains, and maybe a bit into the Rockies and Midwest at times.

SOHO

I want to briefly touch on what the Sun is doing right now. The chart above shows a sunspot identification image, where oranges and blacks indicate the presence of sunspots. Notice that large complex of sunspots in the center of the sun. This complex has been rotating to face Earth for the last week or so, and has finally gotten the Earth in its direct line of sight. The sunspot complex will be rotating away with time, but if it happens to expel a Coronal Mass Ejection (CME) or other expulsion of energy, our weather could turn warm rather quickly within the next month. For now, it's something to monitor.

JMA

The graphic above shows Outgoing Longwave Radiation (OLR) anomalies across the tropics regions, in a global view. Here, we can identify areas of tropical forcing, where enhanced or suppressed convection may be driving or enhancing our weather pattern here at home. Glancing over this image, we do find a swath of negative OLR anomalies, indicating enhanced thunderstorm activity, placed south of India and a bit southwest of the subcontinent as well. This swath of convection has an entirely different meaning for our weather pattern, which will be discussed later on in this discussion. But for now, we'll analyze it as the Madden-Julian Oscillation (MJO).

The MJO states that enhanced or suppressed tropical convection over certain parts of the Equatorial Pacific basin or Indian Ocean can have different effects on the global weather pattern. In this case, when comparing observed negative OLR anomalies in the image above with the Bureau of Meteorology's average negative OLR placement for each MJO phase, we can determine what MJO phase we appear to be in. See if you can figure it out on your own.

BOM

If you guessed Phase 2 into Phase 3, you're right! Phase 2 and 3 MJO events typically see enhanced tropical convection just south of India, very similar to the enhanced tropical convection we're seeing now in that JMA graphic above.

Now that we know what phase we're in, let's see if we can identify how this phase MJO is driving our weather pattern.

Americanwx

The image above shows typical 500mb height anomalies for Phase 3 MJO events in the month of October. Blues indicate stormy and cool weather, while greens, yellows and reds highlight warm and quiet weather. In "normal" October Phase 3 MJO events (the word 'normal' placed in quotes, since no MJO phase is really ever 'normal'), we tend to see high pressure extending from the West US into the Central US, with a swath of negative height anomalies from Greenland to the East Coast. Interestingly enough, we're currently seeing that same scenario play out, though not as defined to the naked eye as you might want it to be. Regardless, we are seeing that progressive ridge formation from the West extending into the Central US, with some stormy East US weather as a result. Though we might not make it to Phase 3 of the MJO, and the MJO isn't the only part of these negative OLR anomalies, we're certainly observing Phase 3-like conditions. Consequentially, if this continues, we might expect a continuation of such warm spells in the next few weeks or so in the US, as this may be enhanced with the strong +AAM transport anomalies driving north, and the resultant positive AO. 

That little section above was primarily from the first part of this Extended Forecast Discussion. Now that we're into Part 2, however, we can finally discuss the other use behind those negative OLR anomalies: The Hadley Cell.

Michael Ventrice

The chart above shows 14 day-averaged omega anomalies on a pressure-by-longitude cross-section. Though it may seem daunting at first, it's not all that complicated. The line marked with the '0' is the Equator, while the 30S and 30N demarcations are lines of latitude. On this chart, we see a swath of blues and purples extending from the surface (~1000 hPa) to about the jet stream level (200hPa). Checking the legend at the bottom of the graph, we recognize this to be a swath of negative omega values. If you remember what the concept of omega usage is, negative omega values tend to indicate rising air, while positive values indicate sinking air.

Interestingly enough, we're looking right at a cross-section of the Hadley Cell.

This example image above, depicting the Hadley Cell, shows how this rising and sinking air works. We will see warm Equatorial air rising, as the negative omega values depict, usually rising into the upper atmosphere. From there, the air cannot rise further, so it is carried northward (or southward, depending on hemisphere) to around the upper regions of the lower latitudes, i.e. 30N or 30S. From there, the warm Equatorial air has cooled, and now sinks back to the lower atmosphere. This is how we identify the positive omega values on the chart above. From there, the cool air is carried southward (or northward, again varying with hemisphere) back to the Equator to start the process over again.

Applying that example graph to the omega chart, we can roughly make out where the Hadley Cell is currently placed. We see a body of sinking air around 40N, where the circulation has been weakening in recent days, with another swath of sinking air possibly forming such a cell in about the 20N region. The Hadley Cell circulation at the 30N area was much more defined in this graph a few days ago, shown below.

Old chart showing the defined Hadley Circulation Cell from the Equator to about 40N.

As the very-knowledgeable blizzardof96 pointed out with respect to this Hadley Cell, its placement around 40N is making it difficult for troughs to form in the area (you Typhoon Rule enthusiasts may recognize this as a warm signal for the US). However, with the cell apparently weakening, and model guidance already re-introducing troughs to East Asia in the near-future, it looks like our week 2-3 forecast is turning cooler.

Summarization

- Very strong positive AAM transport anomalies are moving poleward, meaning warm weather can be expected for the next week or two, before the anomalies hit the Arctic and sustain a positive Arctic Oscillation.
- The positive Arctic Oscillation will then provide the chance for warmer weather for the next few weeks, depending on how long that positive AO can be sustained.
- Dropping AAM anomalies after the passage of the positive AAM transport anomalies suggests a GWO Stage 1 alignment, supportive of a cold West US, stormy Plains, and warm East US in the 2-3 week forecast.
- The Hadley cell dissipation is likely to allow troughs/storms to return to East Asia, setting up the Typhoon Rule for a gradual return to stormy/cooler weather in that 2-3 week timeframe.

Look for the final installment, Part 3, to debut this weekend.

Andrew

Extended Forecast Discussion (Part 1)

This is an extended forecast discussion for the time period of today until approximately one month from today. Due to the length and technicality of this post, there will be multiple parts to this discussion. Today's discussion will focus on the projections for weeks 1 and 2.

We're going to begin with a look at the Atmospheric Angular Momentum readings, or AAM.

ESRL

The image here shows atmospheric angular momentum transport anomalies over the past few months. We want to focus on the anomalies in the top-right corner, the last few days of recording. On this graphic, we see a swath of yellows and oranges seeming to push diagonally upward along the image. Those yellows and oranges define positive AAM transport anomalies, and they are beginning their long-awaited movement to the north, which will throw a big wrench in our weather pattern.

According to the oranges and yellows on the screen, the positive AAM anomaly transport values have made it as far north as about the 40N parallel. Because positive AAM anomalies generally can mean an enhanced jet stream, we might expect high pressure ridging to build northward with time as well, as the jet stream pushes north with the +AAM transport anomalies. This spells a predominantly warm weather period in the next few weeks, as this strong jet stream will eventually make it to the Arctic and keep the Arctic Oscillation (AO), and by extension the North Atlantic Oscillation (NAO) positive. Such a positive AO, which helps induce warm weather, is already on the forecast from the Climate Prediction Center, as shown below.

CPC

The spaghetti-like alignment of red ensemble forecast members above that neutral line suggests the positive Arctic Oscillation phase for the entire forecast period, which extends for 14 days from today.

We're already seeing the effects of the positive AAM transport anomalies in the mid-latitudes, as 250mb zonal winds across the Northern Hemisphere and Southern hemisphere are markedly positive (stronger than normal jet stream) around the 30-60N and 30-60S parallels in each respective hemisphere, per the graphic below.

ESRL

Let's transition for a moment to the tropics.

JMA

The graphic above shows Outgoing Longwave Radiation (OLR) anomalies across the tropics regions, in a global view. Here, we can identify areas of tropical forcing, where enhanced or suppressed convection may be driving or enhancing our weather pattern here at home. Glancing over this image, we do find a swath of negative OLR anomalies, indicating enhanced thunderstorm activity, placed south of India and a bit southwest of the subcontinent as well. This swath of convection has an entirely different meaning for our weather pattern, which will be discussed later on in this discussion. But for now, we'll analyze it as the Madden-Julian Oscillation (MJO).

The MJO states that enhanced or suppressed tropical convection over certain parts of the Equatorial Pacific basin or Indian Ocean can have different effects on the global weather pattern. In this case, when comparing observed negative OLR anomalies in the image above with the Bureau of Meteorology's average negative OLR placement for each MJO phase, we can determine what MJO phase we appear to be in. See if you can figure it out on your own.

BOM

If you guessed Phase 2 into Phase 3, you're right! Phase 2 and 3 MJO events typically see enhanced tropical convection just south of India, very similar to the enhanced tropical convection we're seeing now in that JMA graphic above.

Now that we know what phase we're in, let's see if we can identify how this phase MJO is driving our weather pattern.

Americanwx

The image above shows typical 500mb height anomalies for Phase 3 MJO events in the month of October. Blues indicate stormy and cool weather, while greens, yellows and reds highlight warm and quiet weather. In "normal" October Phase 3 MJO events (the word 'normal' placed in quotes, since no MJO phase is really ever 'normal'), we tend to see high pressure extending from the West US into the Central US, with a swath of negative height anomalies from Greenland to the East Coast. Interestingly enough, we're currently seeing that same scenario play out, though not as defined to the naked eye as you might want it to be. Regardless, we are seeing that progressive ridge formation from the West extending into the Central US, with some stormy East US weather as a result. Though we might not make it to Phase 3 of the MJO, and the MJO isn't the only part of these negative OLR anomalies, we're certainly observing Phase 3-like conditions. Consequentially, if this continues, we might expect a continuation of such warm spells in the next week or so in the US.

ESRL

Going back to the atmospheric angular momentum one last time today, I want to go over the relative AAM values we've seen in recent days. The chart above shows relative AAM anomalies, with greens depicting negative AAM areas and oranges showing positive AAM values. If we look at the bottom panel, we see an average anomaly across the globe of these AAM values, and this is where we want to focus our attention now. In recent days, the AAM has been trending more towards neutral territory, as the AAM was expected to shift into El Nino-like states of high (positive) AAM. Now, however, the AAM has stopped pushing positive, and has leveled off in negative territory. Add to that the AAM tendency is in negative territory once again, and it looks like we'll be more confined to La Nina-like, low AAM stages.

ESRL
AAM Tendency

I'll go more in-depth into this discussion tomorrow, with our Part 2 segment, but here's the first part of the summary for the next two weeks' outlook.

Summary

- Warm weather is expected to continue as MJO Phase 3-like conditions provide a base for ridging in the West, pushing east into the Central US with time.
- This warm weather will also be sustained as positive AAM transport anomalies push north into the mid-latitudes in the next few weeks.

We only got to cover the first week or so in today's post, so weeks 2 & 3 will likely be covered in Part 2.

Andrew

Long Range Discussion, Concerns About Winter (Part 1)

Update: Click Here for the new Part 2 Post (ideally read after reading this post).

Today, we'll go over the latest outlook for the long range, and go over some new concerns I have about the coming winter. This is the first post of the two, dealing with the long range forecast portion. The winter concern post will come out tomorrow.

Let's first discuss the Global Wind Oscillation and Atmospheric Angular Momentum (GWO and AAM) concepts.

ESRL

The image above shows anomalies of the relative atmospheric angular momentum (AAM) since this past July to the present day. The image looks pretty complicated, and in reality it is, so we'll avoid any extremely difficult parts. In essence, positive AAM values can indicate enhanced areas of the jet stream, while negative values may indicate a weakened jet stream. As an example, take a look at the green swath around the 60N parallel in the first days after October. This green tells us of negative AAM anomalies in the area, so let's see if we can identify a cause behind it.

CPC

The graphic above shows geopotential height anomalies since the middle of this past June. The top panel gives an indication of these geopotential height anomalies on a time-by-height graph, where reds depict positive height anomalies (warm weather, high pressure) and blues indicate negative height anomalies (cold weather, low pressure). Check out that big swath of reds only a few days after the start of October. We saw significant ridging in the troposphere and even into the stratosphere, disrupting the jet stream and sending the Arctic Oscillation plummeting. As the jet stream significantly weakened with the ridging pushing through, the AAM reflected appropriately a negative AAM.

The overall AAM anomaly is shown by the bottom panel, and we can see how the AAM has been negative lately. However, it's been rising in recent days.

ESRL

The image above now shows the tendency of the AAM; among other things, it tells us whether the AAM wants to be in positive or negative territory. Lately, the AAM tendency has been in a positive state, and this is likely what the rising relative AAM is caused by. The tendency will take on a more significant role later on in this post.

ESRL

Take a look at the graphic above. Pretty complicated, right? Not quite. In the bottom left and right corners, as well as the top left and right corners, we see descriptions for regions that see convection in various stages of the Global Wind Oscillation (GWO). Phase 3 sees Indian Maritime convection, while the Dateline region observes tropical thunderstorms in Phase 5, and so on. More experienced weather junkies may realize these denotations are actually juxtaposing the Madden-Julian Oscillation (MJO) onto this GWO description.
As for the GWO itself, it does have eight phases, as the chart above entails. From another source in the ESRL, the GWO has been allotted into four primary phases. I've copied and pasted the four phase descriptions below:

The four primary phases of the GWO are described below, along with generally cold season (November-March) probable weather impacts for the USA. The GWO recurrence interval, or "time it takes to make a circuit", ranges from a broad 15-80 days. Two of the stages project strongly on El Nino and La Nina circulation states, which are also characterized by positive (Stage 3) and negative (Stage 1) global AAM anomalies, respectively.  Stages 2 and 4 are transitional.

Stage 1 (La-Nina like) – the global relative AAM anomaly is negative. The negative anomaly is primarily due to easterly upper level wind anomalies that extend from the Eastern Hemisphere tropics to the Western Hemisphere mid-latitudes. A retracted Pacific Ocean jet stream is a key feature in the total field.  Troughs are probable across the western USA with a ridge over the southeast.  High impact weather is favored across the Plains.

Stage 2 – the global relative AAM tendency is positive. This means that negative AAM is being removed from the atmosphere by surface friction and mountains. At the same time, westerly wind anomalies are intensifying in equatorial regions of the Western Hemisphere. Fast Rossby wave dispersion events in both hemispheres are a coherent feature of this stage and Stage 4.  A cold regime is probable across the central USA.

Stage 3 (El-Nino like) – the global relative AAM anomaly is positive. Westerly wind anomalies move into the Eastern Hemisphere, broaden in latitudinal extent and link up with deep westerly flow anomalies over the mid-latitude Western Hemisphere. An extended Pacific Ocean jet stream and southward shifted storm track is observed  favoring high impact weather events along the USA west coast.

Stage 4 – the global relative AAM tendency is negative. Positive (westerly) AAM anomalies are being removed by surface friction in the Western Hemisphere mid-latitudes and through mountain torques across the Northern Hemisphere topography. The next phase of the oscillation (if there is one) is represented by easterly wind anomalies intensifying over equatorial regions of the Western Hemisphere. This stage has enhanced subtropical jets and closed lows in the subtropics favoring rainfall events over the southwestern USA.

'Woah! Slow down! I don't know what this means!' is probably what some of you are thinking right now. Let's cut out the complicated parts and focus our attention on the underlined phrases above. In each stage, there's a description about the AAM and AAM tendency. We know what those mean, after discussing them above, so let's put it to use. Right now, the relative AAM is negative and the tendency of the AAM is positive. If we match that up with the underlined phrases, we find ourselves in a Stage 2 set-up, with the negative relative AAM described in Stage 1, but the positive AAM tendency described in Stage 2. Doing a quick evaluation, when we account for the fact that El Nino-like anomalies (Stage 3) are shown by high GWO values (Phases 5-8), and La Nina-like anomalies (Stage 1) display themselves in low GWO phases 1-4, we can estimate the GWO to currently be around Phase 3 or 4, given how Stage 2 (where we are now) is in between the high and low GWO phases.

As if that wasn't confusing enough already, we can actually forecast the AAM in the future!

ESRL

The graphic above now shows transport of the AAM. As if there aren't enough ways to examine the AAM, the transport of the AAM can allow us to delineate how AAM anomalies are making their way to the upper latitudes; I've drawn two arrows to illustrate this above. Notice how we currently see decently-strong positive AAM transports pushing northward. This tells us that we can expect a strengthening jet stream in the near future (if we recall positive AAM shows a strong jet stream), something confirmed by the forecast of the Arctic Oscillation, as we see below.

CPC

Ensemble members have the Arctic Oscillation pushing positive, something I wouldn't be surprised to see with the positive AAM transports in coming days.

-----------

Let's take a step away from the AAM now and move on to other long range factors.

PSU

The graphic above shows a 500mb mean height anomaly forecast from the combined GFS ensemble members, with individual ensemble member height contours shown in the smaller panels. On this graphic, valid for 60 hours out, we can see a strong trough dropping into the Gulf of Alaska, as the deep blue colors indicate. A small cut-off low occupies the Southwest, with a weak ridge persisting in the Plains. A trough is on its way to the Atlantic, as seen in the Northeast in the graphic above.
If you were to examine a loop of the forecast from these GFS ensembles, you would see several bursts of ridging in the West and Central US that initially appear strong, but quickly weaken and push east. If you're a close reader, you'll take a look at the description of the GWO Stage 2 and realize that these quick-dissipating ridges are actually the 'Fast Rossby Wave dispersion events'.

The atmosphere's just one big web of connections and correlations...

PSU

Fast forwarding to the GFS ensemble height anomaly forecast valid 264 hours out, we now see that Gulf of Alaskan troughing has pushed east into the West Coast, thanks to sustained ridging in the Bering Sea. As such, the ridging previously in the West/Central US is now finding a home further east, in the Plains. The New England region is seen basking in below-normal temperatures with the negative height anomalies.
Take a look back at our AAM section and give Stage 3 a look. As I had indicated, we're forecasted to head into positive AAM territory, which is the equivalent of Stage 3 in the GWO. Note how the Stage 3 description tells of an extended Pacific Jet Stream, as well as high-impact weather events in the West US. It's no mistake that the long range GFS ensembles are showing a stormy West Coast, fitting in with that Stage 3 description.

I'll use these forecast images again in tomorrow's Part 2 post, which will go over my concerns about next winter; my post here is already too long to extend it into the second topic.

Tropical Tidbits

Let's give our brains a break and use some of our simpler tools in the long range. The image above shows the ECMWF 500mb height anomaly forecast over the Western Pacific, valid this morning (Thursday). Note the trough digging into Japan. Using the Typhoon Rule and 6-10 day correlation, we can expect a stormy period in the US 6-10 days from today, in an October 22nd to 26th period.

Tropical Tidbits

But all's not well that ends well, winter weather fans. The graphic above shows the same ECMWF height anomaly forecast in the West Pacific, now valid for 10 days out. We see a massive - emphasis on massive - ridge forecasted to cover the eastern part of Asia, including over Japan. If this forecast comes to fruition, we might expect to see a prolonged period of significant warmth in the November 1st to November 5th timeframe. Again, that's if this comes to fruition.

JMA

One final piece concerning this forecast. I mentioned the Madden-Julian Oscillation (MJO) earlier in this post, and wanted to discuss it for a moment. In the image above, we see Outgoing Longwave Radiation (OLR) anomalies in the color shadings, with 200mb velocity potential contours and divergence with the arrows. Negative OLR anomalies indicate stormy weather, and are displayed as blues on this graphic. Oranges depict positive OLR anomalies, highlighting quieter than normal tropical convection. Notice how the strongest divergence is located just west of South America... and it doesn't even appear to be focused along the Equator! This tells us that although tropical forcing is strongest in the Western Hemisphere, the MJO is anomalously weak, as it has been for the last several weeks, per the graphic below.

CPC
40 day observed MJO

The MJO isn't expected to be a significant factor now or in the near future, per model forecasts.

Long Range Forecast Summary

- The AAM is expected to shift to positive in coming days, as the AAM tendency remains positive and positive AAM transports are pushing to the upper latitudes.
- Due to the positive AAM occupying the upper latitudes, strengthening of the jet stream is expected. This will result in a positive AO, and thus warmer weather.
- The GWO is expected to push into Stage 3 with the positive AAM, which will allow an El Nino-like set-up to evolve. Stormy weather in the West US, with warmth in the Central can be expected in coming weeks.
- East Asian signals tell us of a brief stormy period to end October, with potential significant warmth to start November.

Remember to look for Part 2 tomorrow afternoon!

Andrew

Spring, Summerlike Warmth Expected in Late April, May

I'm expecting the end of April and beginning of May to have seasonal to above-normal warmth across much of the country.

Tropical Tidbits

Shown above is the 500mb height anomaly pressure forecast over the West Pacific for April 26th. In this image, we see a strong ridge of high pressure stationed over Japan and East Asia, permitting the flow of above normal temperatures into the area. This event has significant implications on our weather here in the US. There is a rule, well explained by Joe Renken, that states a weather phenomenon in East Asia will be reciprocated in the United States 6-10 days later. This means that if there is a storm system in Japan on a certain day, we can expect a storm in the US 6-10 days after that. The same goes for high pressure and warm weather. So, if we use that rule, we can expect warmth over the United States around the May 1-5 period, if not for a longer time period than that.

The factors for warmth don't stop there- it goes far deeper, literally and metaphorically.

The image above may look complicated, but isn't that complex when explained thoroughly. Shown above is a graphical representation of a phenomenon called the Walker Circulation, also known as the Walker Cell. The Walker Circulation is most prominent in a La Nina situation, and forms when you have cool waters towards the Eastern Pacific, and warm waters near Oceania and Australia. The trade winds across the Pacific go from east to west in the Walker Circulation on the surface, due to the presence of high pressure in the East Pacific and low pressure in the West Pacific (think of it like a ball rolling downhill from the top of the hill (high pressure in the east Pacific) to the bottom of the hill (low pressure in the West Pacific), where the ball signifies the trade winds). When the trade winds reach the low pressure area in the western Pacific, the winds converge and begin pushing up into the atmosphere, creating thunderstorms. The air from the thunderstorms shoots up to roughly the 250mb to 200mb level before the winds spread out, as the air cannot rise further. From there, the air at 200mb flows from west-to-east, opposite the direction of the surface winds. Then, the high pressure and cool/dry atmosphere in the East Pacific pulls that air down back to the surface to start the whole process over again. As I said, this typically appears in La Nina situations, as cool waters in the East Pacific that are required for the Walker Circulation typically arise in conjunction with the La Nina.

The image above shows a view of surface pressure and surface wind vectors across the tropical regions of the world. We can use this image to identify the presence, or lack thereof, of the Walker Circulation. Taking a glance at surface pressures and winds from April 6th to April 10th, we do see surface winds flowing from east-to-west, a trait typically observed with the Walker Cell, and the low pressure center in Oceania also shows a characteristic of that circulation. This is rather odd, as we have been expecting an El Nino to arise, but the Walker Cell appearance here tells me that the El Nino has not hit the atmosphere yet, though it may start appearing in the surface temperatures. In recent days, the surface winds have been showing signs of weakening, indicating that the Walker Circulation may be weakening, which would herald the arrival of an El Nino if the winds reverse to a west-to-east stature.

For all intents and purposes, let's say that the Walker Cell's presence in the Pacific still indicates the presence of a La Nina (which is true in respect to the Walker Cell actually appearing, even though sea surface temperatures may argue otherwise). We can correlate sea surface temperatures in the Pacific to surface temperatures here in the United States. In the image above, the warm colors indicate a positive correlation of SSTs to surface temperatures (for example, warm Pacific SSTs would then correlate to warm temperatures in the US, or cool Pacific SSTs would lead to cool temperatures in the US). Blues in the above image then depict a negative correlation, where warm Pacific SSTs would lead to cool surface temperatures, and vice versa. In this case, with negative correlation signals across the US above, we can determine that below normal SSTs in the Pacific (La Nina) can then lead to above normal surface temperatures in the US, and vice versa. Thus, even though the SSTs in the Pacific indicate an El Nino, the atmospheric pattern is more supportive of a La Nina, which would then support warmer than normal conditions in the April-May-June timeframe. Despite this, since the Walker Circulation may be fading, this portion may just be a moot point in just a few weeks time.

This image here actually is pretty complicated, but I'll do my best to explain it. This graphic shows the layout of the Global Wind Oscillation (GWO), and what factors indicate the presence of the GWO in one of a possible eight phases. For example, if the GWO is to be in Phase 1, we would look out for negative mountain torque (MT) values, as well as enhanced tropical convection in the Indian Ocean. An additional description of the GWO comes from the ESRL, as shown in the screenshot below.

I outlined the Stage 1 description, as not only does it fit in with the current conditions that we're experiencing, but it is indicative of a La Nina, like we talked about earlier in this post. How do we know for sure that we're in the Phases 1-3 of the GWO (which are traditionally La Nina-esque phases)? Let's take a look at the indicators that depict Phases 1 through 3 of the GWO.

Observed Mountain Torque

Observed Relative AAM

If we look at the first GWO image we discussed, as well as the screenshot description of the GWO, we can see what defines these key phases. We see that negative mountain torque and convection in the Indian Ocean is a key definition of these first three GWO phases. Looking at that observed MT chart shown above, we see that the net MT is now below average, as the black solid line shows. While the convection in the Pacific is more displaced in Oceania instead of the Indian Ocean, the description highlights that the relative AAM is negative, and that relative AAM image above shows a negative anomaly. Overall, this does indicate the La Nina phases, also known as GWO phases 1-3, strengthening that correlation image, as well as the Walker Circulation discussion. Additionally, in the same sense that positive East Asian mountain torque can lead to cooler than normal temperatures in the US, the current negative East Asian mountain torque anomaly in the graphic above (red line two images above) indicates warm weather is more likely.

Importantly, in that screenshot description image, we see that the La Nina-like stages indicate a trough in the Western US. Typically, when we see a trough/stormy weather in the West US, we get a consequential ridge of high pressure in the East US, which is a textbook negative PNA pattern.

The image above shows observed sunspots in accordance with the 30-day sunspot cycle. As I've mentioned on this blog before, there is a known inverse correlation between the 30-day sunspot cycle and the Pacific-North American index (PNA). When the 30 day sunspot number is anomalously high, we tend to see the PNA in its negative phase. In the same sense, when the 30 day sunspot number is anomalously low, the PNA tends to reside more in its positive phase. Composite images of both the positive PNA and negative PNA are shown above, courtesy of NCSU.

500mb height anomalies of a negative PNA on the left (warm colors= high pressure, cool colors = low pressure)
Surface temperature anomalies of a negative PNA on the right

500mb height anomalies of a positive PNA on the left
Surface temperature anomalies of a positive PNA on the right

If we look back at the sunspot image, we see that the 30 day sunspot cycle is just off the charts, indicating a very high number of sunspots, or at the very least, elevated solar activity. If we use our solar correlation with the PNA, we may be expecting the emergence of a negative PNA regime in the near future. If we look at the composite image above for a negative PNA, we see that warm weather tends to prevail in that sort of situation. This fits in well with the GWO description we went over earlier, the presence of the La Nina Walker Circulation, and the East Asian correlation tool.

The last piece to the puzzle is the presence of enhanced tropical convection over Oceania.

The image above gives a global view of rising air in conjunction with enhanced tropical convection (blue shading), also known as negative Outgoing Longwave Radiation (OLR), as well as sinking air, exemplified by the beige shadings, which show suppressed tropical convection. The green contours highlight divergence, or air rising and spreading out due to thunderstorms, while red contours depict sinking of air. We see that there is an anomalous swath of tropical convection over Oceania, shown by the blues/tropical convection, and the wind vectors at 200mb going away from that convection. The placement of this tropical convection fits into a certain phase of the Madden-Julian Oscillation (MJO).

If we consider that the current center of the tropical convection is around the 165 East longitude line or 170 East line, we can see which MJO phase is best represented here. It looks like the MJO is currently in Phase 6 or 7, as the line on the 165 East shows splitting through the two panels on the right that are circled.

What does a Phase 6 MJO mean in the month of April?

The Phase 6 MJO looks like a typical negative PNA regime, with low pressure anomalies along the West Coast, and strong high pressure/warm weather in the Central and East US. This supports the overall warm weather idea, only adding to the rest of the factors supporting a warm weather regime in the late April and early May timeframes.

Let's summarize all of this.

• Based on a number of factors, warmer than normal weather is expected for the latter part of April, potentially well into the beginning of May.

Andrew