What the stratosphere and tropical Pacific can tell us about December

For the better part of the last three weeks, our forecasters have been discussing and monitoring the potential for a pattern change throughout the hemisphere. Medium range model guidance has often been gung-ho with the pattern changing within 7 days. As is often the case, forecast models were too quick to change the pattern. Much of this has to do with poor forecasting of the stratosphere and tropical Pacific ocean. Not coincidentally, these two features can give us clues as to where the pattern is heading in the weeks ahead.

As it stands this afternoon, model guidance and ensembles are in good agreement that a -NAO (North Atlantic Oscillation) pattern will develop over the next two weeks. This pattern is often defined by large ridges or higher than normal heights in the atmosphere over Greenland and the North Atlantic Ocean. -NAO patterns can allow cold air to be displaced farther south into the Northern 1/3 of the United States, particularly the Central and Eastern United States.

Further analysis of the modeled -NAO pattern, however, shows that it is quite transient — and not necessarily a “classic” blocking ridge. Instead, the higher heights are a response to other storm systems, not their own entity — at least initially. The idea is that gradually, these ridges will become more established and less transient over time, but it remains to be seen if this has merit moving forward.

Forecast guidance suggests that cold air will become established in the Western US first during the early part of December in response to a -PNA.

Forecast guidance suggests that cold air will become established in the Western US first during the early part of December in response to a -PNA.

Instead, the atmospheric pattern across the United States is far more likely to be heavily influenced by a -PNA (Pacific North American Oscillation) pattern across the Western States over the next 1-2 weeks. This pattern forms when a large trough digs into the Western United States and far Eastern Pacific. A deep trough in these areas tends to pump up the Southeast Ridge over time.

All models agree that the processes will continue to domino forward as we move into early December, with an active storm track developing into the Central US including the Mississippi and Ohio Valleys. Exactly how the wintry weather potential evolves will depend heavily on the presence of blocking in the higher latitudes, north of the United States, to keep cold air in both the antecedent airmass ahead of storms and near storm systems as they develop.

How can we figure out if, when, and where that blocking will develop?

The Stratosphere

We can often find clues as to how the higher latitude pattern will behave in the stratosphere — way above the troposphere, where most of our sensible weather occurs. Here, the stratospheric polar vortex lies. This vortex has been weaker than average this Autumn, due to abnormally high heat flux and atmospheric wave-break events in the stratosphere.. The response in the troposphere has been notable; with some higher latitude blocking attempting to develop.

A major, sudden “stratospheric warming event” — which would significant disrupt the pattern and allow for some higher latitude blocking to become more established — doesn’t appear to be in the cards over the next two weeks. Zonal winds will be weaker, but will not be reversing from a westerly to easterly direction (one important signal for a stratospheric warming event). EP vectors are still pointing towards the equator at 10 hPA.

ECMWF model zonal winds at 10mb and 30mb and EPV at 10mb

ECMWF model zonal winds at 10mb and 30mb and EPV at 10mb


A recent attack has lead to a displacement of the polar vortex towards Eurasia with development of stratospheric Aleutian high. More polar vortex disruptions are forecast to continue occurring over couple weeks, due to the increase in WAF and development of a Siberian High.

This supports the development of a large ridge towards the Aleutian Islands within 14 days time. Not surprisingly, forecast model guidance has begun to suggest that very development, with higher than normal height anomalies signaled on both the GEFS and ECMWF ENS suites through Weeks 2-4. This is a major change from the current pattern in the Northern and Northeastern Pacific Ocean.


Weakening MJO/AAM

Tropical forcing, as it relates with the MJO (for more on the MJO click here), has been more active and propagating through the Western Hemisphere in Phase 8.  In November, this phase supports more troughiness over the Western US and ridging over the Eastern US with above normal temperatures. The MJO is forecast to propagate into Phase 2 next week. There is some uncertainty as to how strong the MJO and tropical forcing will continue to be beyond 10 days, but 200mb velocitiy forecasts show tropical forcing will be weakening at least slightly over the Indian Ocean. Nevertheless, more tropical forcing in the Indian Ocean is also more typical of a La Nina forcing pattern — nearly opposite to the El Nino-like state that has been recently observed.

The AAM, or Atmospheric Angular Momentum, is a measure of winds of around the earth’s atmosphere. The AAM has been in a super positive state, which often resembles a strong El Nino like forcing pattern. The GEFS ensembles forecast the +AAM to weaken over the next couple weeks, suggesting the global wind pattern will resemble a La Nina as we head into the early winter months. A weakening +AAM also supports less MJO progression, important because a large part of our winter forecasts is based of a very weak Central Pacific La Nina — supporting more ridging in the Western US and troughing East.

Global Wind Oscillation diagram for the Atmospheric Angular Momentum phase

Global Wind Oscillation diagram for the Atmospheric Angular Momentum phase

To summarize? The pattern is still in transition as we move towards the first few days of December. The first steps of a gradual step-down process towards a more wintry weather pattern across much of the Central and Eastern United States are finally underway throughout the hemisphere, but they will take some time to evolve.

Initially, without a very supportive source of arctic air and no stout high latitude blocking, winter weather threats will likely become more common across the interior — in the Great Lakes and Ohio Valley as well as parts of the Interior Northeast US — during the first week or two of December. Gradually, the pattern is expected to become more favorable for wintry weather in a larger majority of the Eastern US as well.

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Have a wonderful Thanksgiving!