Signals strengthening for Mid December pattern change

Good afternoon! After a long and relaxing Thanksgiving weekend, the meteorological community almost always looks ahead to the weeks to come as December quickly approaches What lies ahead are some of the more critical weeks for many sectors, including construction, energy, natural gas and agriculture just to name a few. The weather pattern that occurs during this time frame is often critical to the winter forecast – not just in the short term, but also in the weeks beyond.

As you may recall, when we released our Winter Forecast in early November, we spoke of the December pattern as “higher than normal” confidence. The anticipated shot of cooler than normal air and episodes of higher latitude blocking have occurred, and we now approach an anticipated period of moderation. What happens beyond this, however, is the key piece to a forecast that features a flip to colder than normal temperatures across the United States. The evolution of the pattern throughout the hemisphere is beginning to fall into place – so lets discuss!

The pattern throughout the hemisphere over the past several weeks has been a bit of a conundrum. As recently as last week, there weren’t very many takeaways from the pattern in October or November that could guide us in any sensible direction towards Winter. Long-standing periods of warmth were replaced by brief cold; sometimes intense, sometimes not, eventually replaced with warmth once again. This is a bit of a change from the past few years, when we began to analyze the evolution from a very early part of the ballgame so to speak. Over the past several days, however, things have changed quite significantly.

The first and most notable changes have occurred in the Pacific Ocean. An extremely stagnant pattern has undergone major changes, and the wave-train throughout the North Pacific Ocean is evolving in a completely different manner than it was 15-30 days ago. As discussed in our Winter Forecast, this type of change was heavily favored when using an MJO, ENSO SSTA and high latitude match blend. Nevertheless, it is exciting to see it finally playing out. Over the next several days, a ripple effect of small wave breaks will occur from the Bering Sea towards the Aleutian Islands. These will undoubtedly begin to have an effect upon the pattern downstream.

Multiple “waves” move through the Pacific Ocean over the next several days, indicative of an upcoming pattern change across North America.

These kinds of things can be complicated, but are best understood by using simple analogies. Think of the ridge and trough patterns in the Pacific Ocean literally as waves in the Ocean, or ripples moving away from a central cause in a pond. How these ridges and troughs (waves) behave in the North Pacific can offer us key clues as to how the pattern will be affected and how the pattern will behave in North America. With some analysis on the evolution, we can understand that the wave breaking pattern evolution, working in conjunction with a few other things (mountain torque and siberian low flux’s to name a few) will eventually affect the Northern Hemisphere pattern to support the return of Western USA ridging.

As these waves continue to progress eastward, the brief moderation in the weather pattern across the United States will begin to break down. Large troughs in the Bering Sea (way on the left side of the image) will aid to push heat flux northeastward and allow ridging to build on the Eastern Pacific and Western USA Coast. This is a critical piece of the forecast in the middle latitudes.

Changes, meanwhile, are already underway in the stratosphere as well. While high latitude blocking that developed over the past 10 days did not necessarily produce a snowstorm, mega-cold outbreak, or freeze the Great Lakes, it had some very significant impacts on the weather pattern as a whole. Upward-moving effects in the stratosphere were widespread, with the wave events making progress to affect the stratospheric polar vortex, which is now already perturbed and weakened compared to normal. Basically, the high latitude ridging that developed in the troposphere was effective in disrupting the polar vortex in the stratosphere. As the feedback of this process continues over the next few weeks, the suggestion is quite strong that the stratospheric polar vortex will be heavily disrupted.

The GEFS and, to a certain extent, the ECMWF EPS suggest that zonal winds will reverse (or near-reverse) at multiple levels. This will leave the polar vortex extremely vulnerable. As the stratospheric polar vortex becomes perturbed and the higher latitude heights become more susceptible to movement and breaks, medium and long range forecast models are beginning to suggest the potential for much higher than normal heights in the higher latitudes (a.k.a. blocking). This is most significant as we are seeing signals for higher latitude ridging both on the Pacific and Atlantic sides, from British Columbia into Western Canada and Greenland towards the Davis Strait.

It is important, at this juncture of the forecast, to take a bit of a step back and understand where we are. While the evolution of these things seems to be 10-15 days away (really, the potential for anomalous cold or storminess still is) the wheels are already in motion. Refer back, for a moment, to the North Pacific Ocean graphic. Carefully watch the westward retrograde of higher than normal heights in the arctic, and the eastward movement of the short-circuit wave train over the Pacific Ocean.

As the stratospheric effects begin to work in conjunction with tropical forcing, feedback both from the ENSO regions and the Indian Ocean with the MJO progressing through the latter stages beyond Phase 5, an alignment of extremely favorable hemispheric indicators begins to become apparent. The more important thing is that these favorable indices make sense given the evolution of the pattern across the hemisphere in multiple levels. It is one thing to see a -NAO or -AO on a GFS run. It is another thing to understand that based on how the hemisphere is behaving, based on the evolution of tropical forcing and stratospheric behavioral indicators, and based on a very solid conglomeration of analogs — this is all supported heavily.

Week 3 Temperature Departure on our new, higher resolution hand-made maps which will debut in the coming days.

While the specifics, breadth and duration of this particular “Event” remain uncertain, and while specific storm threats are still very far outside the forecasting envelope at this time, it is becoming increasingly apparent that a well-defined period of cold with the potential for multiple winter weather events will evolve from Mid December onward from the Great Lakes and Ohio Valley into the Northeast States. Over the next several days, it will be important to continue to monitor our global pattern indicators and keep a close eye on ensemble guidance trends.

The beginnings of this cold period is reflected in our Week 3 Forecast, and we will continue to monitor closely for storm threats as well as the longevity and timing of the colder than normal air as it approaches. Have a wonderful Sunday, friends!