Sunday, January 14, 2024

Polar Vortex

Alaska's first -50°F measurement of the season occurred on Thursday in the well-known cold spot of Chicken, up at 1800 feet of elevation in the Fortymile country.  It's been chilly in the Yukon Flats too: well into the -40s in places.  In an interesting reversal of the December situation (when it was warmer to the east), this cold in eastern Alaska is just the western margin of an impressive cold pool across the border in Canada.

The cold across Canada is associated with a deep mid-atmospheric circulation that is sometimes (and controversially) known as the "polar vortex", and its migration across western Canada and then down toward the lower 48 has generated plenty of headlines.  Here's what it looked like in the 500mb analysis (courtesy of Environment Canada) at 3pm Friday:

Notice the sharp but strong ridge over Alaska, toward the top left of the image.  Here's a hemispheric view of 850mb (lower troposphere) temperatures at about the same time, with North America on the left:

The coldest air mass in the hemisphere was over western Canada - even colder than over Siberia - but parts of Alaska had 850mb temperatures above freezing at the same time.

Yesterday afternoon's surface analysis shows a vast area of high pressure from Greenland and the western Canadian Arctic down to the U.S. Plains; but low pressure over western Alaska.

It's ironic that the cold air of the so-called "polar vortex" is actually associated with high pressure at the surface, not the low pressure of a cyclonic circulation.  But the term is a misnomer in its popular usage: the "polar vortex" more properly refers to the wintertime circulation in the stratosphere, 10-30 miles above the surface.  Read more about this at NOAA's new polar vortex blog:

Getting back to near-Alaska conditions, the cold in northwestern Canada has been harsh in recent days: into the -50s Fahrenheit in many locations.  But despite several days in the -40s at Dawson, the Yukon River still isn't properly frozen over: here's the latest webcam image from noon today.

This is a repeat of the no-freeze-up difficulty of several recent years, with no ice road crossing between West Dawson and the main town.  There's a trail crossing upstream:

The webcam animation from Thursday shows lots of steam in the cold weather, but apparently no real action in terms of ice formation.

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