Big weather pattern shift could blast East Coast with frigid air, snow in mid-December

Projection from the GFS Ensemble shows colder-than-average weather pattern.
Projection from the GFS Ensemble shows colder-than-average weather pattern.
Image: weatherbell analytics.

Everyone should be wary when weather geeks start getting excited about an upchartph.coming shift in the jet stream because it usually means inclement weather is ahead.?

After all, weather enthusiasts — whether they be armchair forecasters or professionals — tend to abhor boring stretches of "nice" weather.

Forecasters have reason to be psyched right now, given strong hints for what could be a major weather pattern realignment during the next two weeks.?

Computer models are increasingly showing the potential for a cold and possibly snowy weather pattern to develop along the East Coast of the U.S. during the second week of December. While there are many uncertainties associated with the forecast so far in advance, the general contours of what is likely to happen are bechartph.coming clearer.

Some of the building blocks for the cold weather are already in place, including a predominantly negative Arctic Oscillation, which favors — but does not guarantee — colder-than-average conditions in the eastern U.S.?

The Arctic Oscillation, or AO, is climate pattern that describes the atmospheric circulation over the Arctic and North Atlantic Ocean. During a negative phase of the AO, the polar vortex over the Arctic is weaker, resulting in a slackening of the upper level winds ringing the Arctic from west-to-east.?

This can allow frigid, Arctic air to spill into the midlatitudes, including Europe and the U.S.

For much of late November, we've had a strongly negative Arctic Oscillation with milder-than-average conditions in much of the U.S., which goes to show that other factors, including weather patterns across the North Pacific Ocean, also have an influence on winter weather in the lower 48 states.?

Computer model projections for 11 to 15 days from now show a strikingly favorable weather pattern for cold air to invade the eastern U.S., as well as parts of Europe and East Asia. Strong areas of high pressure at high latitudes, including one in the Gulf of Alaska and another monster high over Greenland, plus another across the Ural Mountains, will each act to help steer air masses around the world.?

A blocking high over Greenland is typically associated with some of the East Coast's most memorable snowstorms, because it helps direct cold air into the Northeast and Mid-Atlantic states while also preventing storm systems from quickly escaping out to sea.?

Other experts who have been examining the upchartph.coming changes in winter weather patterns are a bit more skeptical that they'll result in a blockbuster event. ??

Jason Furtado, a meteorology professor at the University of Oklahoma, says the weather pattern in the troposphere, which is the layer of air where most weather occurs, will go through "major changes" in the next week to 10 days.?

"The changes have started in the Pacific and will eventually propagate over into North America and strengthen the current blocking pattern across the North Atlantic (strong North Atlantic ridge)," Furtado said in a Twitter DM. "This overall pattern (ridges in Western US + North Atlantic and troughs in the Central - West North Pacific and Eastern North America) is overall favorable for a colder and stormier regime."

Furtado cautioned that just because the weather pattern will be ideal for generating snowstorms does not guarantee anything. "... The atmosphere will be primed, but we will need to watch the 'sparks' — the shortwaves/disturbances that ride along the jet stream that are the seeds for individual storms," he said.?

According to Ryan Maue, chief operating officer with Weather.US, the dip, or trough, in the jet stream over the eastern half of the U.S. in a week to 10 days is a "pre-requisite" for a snowstorm. But so far, there are no indications of a major storm during that time frame.?

Forecasters with Washington Post's Capital Weather Gang blog also raised the red flag on the upchartph.coming weather pattern change on Monday, saying it "screams winter weather potential." Two time periods experts there cited as analogs to the upchartph.coming weather pattern include December 1989, which set cold temperature records, and December 2009, which featured a blizzard known as "Snowpocalypse."?