Holiday showers?

The Alaska-Scandanavia ridge-bridge/AERAlaskans might want to keep the rain gear handy with polar-vortex guru Judah Cohen warning of a “Scandinavian-Alaska ridge-bridge” building over the top of the world.

Cohen, an MIT climatologist who forecasts for Atmospheric and Environmental Research, has been better than most at predicting winter weather over the long term in the North Hemisphere, which is no easy task. His forecasts focus heavily on the polar vortex and the Arctic oscillation.

The former, to greatly oversimplify, is the pool of cold, low-pressure air swirling counterclockwise above the pole as the planet goes spinning through space and the latter is the lable for the shifts in air pressure between cold Arctic air and warm ocean air, both in the Pacific and Atlantic, that steer the flow of the jet stream. 

Most Alaskans, or at least the majority who live in the urban core focused around Cook Inlet, are well familiar with the jet stream, and what it can do to 49th-state weather.

When the atmosphere bends the stream’s normal west-to-east route into a southwest-to-northeast route carrying warm, ocean air from down near Hawaii north across the Gulf of Alaska to slam into the state’s underbelly, the Anchorage metropolitan area tends to get all wet, although the matter of just a few degrees in temperature can determine if the precipitation comes as wet, heavy snow – the choice of winter sports fans – or rain, which can make the state’s largest city even darker, drearier and wetter than Seattle in December.

“Pineapple Express” is the common label now hung on these north-flowing atmospheric rivers, which have become rather common in Southcentral Alaska in the new millennium. 

The Scandinavian-Alaska ridge bridge could be setting the stage for more of them to flow north.

“…The models seem to be growing more confident in a Scandinavian-Alaska ridge-bridge the second half of November,” Cohen observed last week. “There are two important implications of this mid-tropospheric atmospheric circulation.  First, I believe that it increases the risk or odds of some extreme cold in North America, most likely in Canada, that could filter into the Lower 48 of the US.  Second that it is a favorable wave pattern to disrupt the polar vortex (PV) especially to cause it to stretch.  If the high latitude ridging in the Scandinavian-Ural region persists, then a larger PV disruption is possible starting sometime in December but more likely in January.”

Cohen’s forecasts are Outside-centric because that is where the vast majority of the readers of his forecasts live, but most Alaskans are familiar with the big climatic difference between here and there that dictates that when it is cold in America east of the Rocky Mountains it is almost always warm in Alaska.

This is because those atmospheric rivers of the pineapple express cool down as they make their way across Alaska and then ricochet off the cold air piled up in the Arctic to head southeast in the form of the dreaded “polar express.” 

This has sometimes led to milder winter weather in parts of Alaska than in the lower 48. In early January 2018, for instance, it was warmer in Anchorage – 49 degrees – than in Jacksonville, Fla. – 41 degrees.

“With the help of the jet stream, that wind flowing west to east along the northern rim of the planet, this storm has made its way southeast, coating Canada, the Northeast, and even Florida in snow,” Popular Science magazine reported at the time. 

“At the same time, says Mark Serreze of the National Snow and Ice Data Center, a ridge over the extreme western United States actually protected parts of southern Alaska from the cold. Ridges like these are elongated regions of relatively high atmospheric pressure.”

Think of those ridges as mountains in the sky that funnel winds in the same way that mountains on the ground do. With the ridging now in place, Cohen is predicting warmer than normal temperatures for much of Alaska through the rest of the month, but he admits this is all highly speculative.

“Interesting times weatherwise is the second half of November, the appetizer for the main course during the winter months or is it winter’s last gasp before meekly disappearing?” he writes. “I like to say Mother Nature likes to foreshadow what she has planned but often it turns out to be nothing but a head fake.”

As much as we might think we know about climate, our understanding of how the land, the ocean and the atmosphere interact to drive the day-to-day climate conditions we call  “weather” remains very limited. And it is only more so when it comes to anyone making reliable month-to-month or season-to-season predictions.

Cohen himself was by Monday Tweeting about the possibility of the Scandinavian-Alaska ridge-bridge growing to become a “Scandinavian-Dateline ridge-bridge” that “could bring extreme #cold to Asia in early December.”

The map he posted with that Tweet had Alaska painted pink and red for warm through the end of November, which is the classic pattern caused by the jet stream punching south from the Arctic into Asia only to rebound off the warm air in the Pacific and come oscillating back toward Alaska in the form of that pineapple express.





3 replies »

  1. The map looks informative, or would be if I had a clue what units are being measured and displayed. What the hell is CFS 500 hPA and the red/blue scale on the right represent? This info should appear in a caption right under the map so that the reader can interpret the map.

    • Can’t argue with you there, Martin. CFS denotes the Climate Forecast System, one of several models used to predict weather. hPA is basically a measure of “geopotential height” of global air pressure. You can kind of think of hPA lines as forming a topographic map of the atmosphere showing the mountains and valleys invisible to the human eye.

      The “troughs” and “block,” as they are normally called, that form in the atmosphere dictate how the air moves around the globe, and thus where the warm air and cool air go. There’s a short and decent explainer here:

      How these pieces interact is rather intriguing. During the Cretaceous period, when dinosaurs roamed Alaska, the average global temperature appears to have been only about 4 to 5 degrees C warmer than it is today, but the Denali area is thought to have been 10 to 12 degrees warmer than it is today.

      Shifting weather systems would explain how that might be.

      But I didn’t think it was worth getting this deep in the weeds to explain a pretty picture which by its colors sort of indicates warm and cold well enough for a layman.

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