A nation divided: Polar vortex to unload historic Arctic blast in the East as blistering heat roasts the West


A projection from the European model showing unusually warm air over the West and unusually cold conditions in the East on May 8. (Pivotal Weather)

A contorted jet stream — with a massive bulge of high pressure in the West and a downstream dip, or trough, in the East that resembles tall ocean waves — is cleaving the United States into two seasons. This weather pattern is leading to record heat in the West and Southwest, including Arizona, California, Nevada and New Mexico, while record cold descends upon the Midwest, Ohio Valley, Mid-Atlantic and Northeast as a result of a lobe of the polar vortex.

Both temperature extremes are unusually severe for this time of year. In the East, after a winter lacking in polar vortex-induced cold air outbreaks, a lobe of the tropospheric polar vortex is breaking off from the main circulation over the Arctic and is swinging down from Canada toward New England.

The low temperatures that will result are almost certain to break records.

Meanwhile, in the West, numerous weather observation sites are expected to eclipse the century mark Thursday and Friday, for example. High temperatures are running at least 15 degrees above average in a zone from Southern California to Texas, and these same areas have seen extreme heat since late last month, compounding the effects.

Highly amplified jet stream patterns are typically associated with weather extremes, and this one is no exception. It’s leading to a peculiar setup, one in which, on Saturday, Anchorage will be 15 degrees higher than Washington, and a cross-country flight from Los Angeles to New York would be a journey from midsummer to midwinter. (That is, if people were flying amid the coronavirus pandemic.)

The polar vortex finally shows up


Upper-air temperatures as simulated by the European model. Notice the lobe of cold associated with the polar vortex meandering south and east into the weekend, including May 9. (WeatherBell)

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