Whopper polar vortex can’t touch us
The polar vortex weather pattern dominated headlines at the end of January, producing the coldest air temperatures in a generation at many locations in the Upper Midwest.
But, thankfully, that kind of extreme weather doesn’t occur here.
The polar vortex is a large area of organized, counter-clockwise airflow north of the polar jet stream. For much of the year, the coldest air in this air mass is confined to the areas near the Arctic Circle.
Viewed from above, the polar jet stream looks like a snake circling the northern latitudes from west to east as a series of low-amplitude ridge and trough couplets. During winter, the polar jet and associated vortex drift southward toward the mid-latitudes.
The ridges and troughs typically move rapidly across the country, and the cold air in the polar vortex remains well north of the United States. The polar jet occasionally buckles, and a very high-amplitude ridge/trough couplet develops. Under the right conditions, this leads to the frigid weather that recently affected the Midwest.
The ridge must be located along the west coast of North America and extend well into western Canada. Northerly flow on the east side of the ridge then drives frigid dry air south from the Arctic toward central Canada.
On rare occasions, this high-amplitude polar jet will remain stationary for several days. This gives the Arctic air time to travel from central Canada into the Midwest states. In effect, a piece of the polar vortex is forced south into the United States.
A factor that allows polar vortex temperatures to drop so low is the dryness of the air mass that formed over the frozen Arctic. The lack of moisture means skies remain clear during the long winter nights. Any warmth delivered by the low daytime sun escapes the lower atmosphere quickly after sunset, allowing temperatures to plunge.
Nighttime temperatures during the most recent polar vortex dipped below -30 in many Upper Midwest cities. Daytime maximum temperatures also remained below zero.
The jet stream conditions responsible for the coldest winter temperatures in the Bay Area are similar to the Midwest pattern. The major difference is that the ridge over the western United States tilts, allowing the jet stream winds to flow from northeast to southwest.
This tweaked jet stream drives cold Canadian air across the Sierra Nevadas and into California. Fortunately, the long journey from Canada allows the Arctic air mass to warm somewhat. Local meteorologists often call our polar vortex event an Alberta clipper.
One polar vortex case that sticks out in my mind is the December 1990 cold wave, which sent nighttime temperatures in the Concord/Clayton area below 20 degrees for a couple of days.
Whenever a strong polar vortex circulation reaches the United States, record cold temperatures can occur. In the Midwest, that means several days of below zero temperatures. In the Bay Area, the threat is limited to a few nights of below freezing temperatures.
I much prefer our encounters with the polar vortex, by about 50 degrees.
Woody Whitlatch is a meteorologist retired from PG&E. Email your questions or comments to
clayton_909@yahoo.com
Woody Whitlatch
Woody Whitlatch is a meteorologist retired from PG&E. Email your questions or comments to
clayton_909@yahoo.com