Friday, October 24, 2014

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
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. 

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.

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.

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


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.

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.

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.


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.

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.

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.


- 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.


November 2-6 Potential Cold, Stormy Weather

I'm monitoring the threat for the season's first wintry blast of Arctic air in the next few weeks.

Tropical Tidbits
The image above shows the ECMWF model forecast for 500mb geopotential height contours and normalized anomalies, valid on the morning of October 27th. Blues depict areas of troughing, which lead to cold and stormy weather, while reds indicate the presence of ridging high pressure, which permits the formation of warmth and quiet weather. The graphic above shows a pretty strong upper level low dipping south into northern Japan on October 27th. We can observe this storm system pushing due east or a bit east-southeast as it moves in the aforementioned direction, and as it does so, the upper level low is expected to scrape northern Japan, if not actually centering itself over the northern part of the country.

When we apply the Typhoon Rule, which states weather phenomena in Japan is reciprocated in the United States about 6-10 days later, we might expect a pretty sharp cold blast of air in the November 2-6 timeframe.

Tropical Tidbits
For the sake of diversity, we'll analyze the GFS model's 500mb forecast as well, this one valid for the morning of October 28th. The GFS has this strong upper level low displaced a bit south of the ECMWF's location, and obviously slower than how the European model has the low progressing. But the kicker here is how the upper level low is stronger in the GFS forecast as opposed to the ECMWF. If the GFS ends up with the more accurate forecast, it's certainly a possibility that our cold weather in this November 2-6 timeframe may be more severe than currently thought.

To summarize:

- A sharp blast of cold air is expected to hit the United States around a November 2-6 timeframe, potentially with wintry weather.