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Canadian cold takes aim at much of the U.S.

By Skip Rigney

After several chilly days, this weekend and Monday are forecast to be quite pleasant for late January in the Gulf South with afternoon highs in the 60s.  Without the aid of 21st century meteorological science and technology, we would be unaware that what is happening 4000 miles to our north will dramatically change the weather in the United States in less than 72 hours.

You may have heard or read about something called the polar vortex. The polar vortex is a large area of low pressure several miles above the polar regions, often a couple of thousand miles across.

When the winds in the counterclockwise circulation around the polar vortex are especially strong, the vortex usually stays more or less stationary. But, when the circulation of the polar vortex weakens, it begins to break into several smaller circulations, some of which begin to drift southward.

Some winters the vortex stays intact all season long. Other winters, there may be several breakups. Several things can cause the vortex to weaken and become unstable. One of these is a “sudden stratospheric warming” over the Arctic.

The part of the atmosphere where most of our weather takes place is called the troposphere. The troposphere is about 10 miles thick above us, but thins down to about 5 miles thick over the Arctic. Above it is the stratosphere, a generally stable and relatively warm layer. Every few years, for reasons scientists don’t fully understand, the polar stratosphere may warm 50-100 degrees in less than a week.

Such a sudden stratospheric warming took place back in December. The effects of these events tend to “drip” down into the troposphere, weakening and disrupting the tropospheric polar vortex for weeks.

On Sunday a very cold swirl of low pressure several miles above northern Canada, essentially a piece of the polar vortex, will begin rushing southward. By Monday it will cross the Canadian border into Minnesota. By Tuesday the cold front at the leading edge of the arctic blast will have raced all the way to the Gulf Coast. By Wednesday the eastern two-thirds of the U.S., with the exception of south Florida, will be immersed in cold air.

Several computer model forecasts are indicating that enough moisture could linger behind the cold front Tuesday to raise the possibility of some brief, light snow in parts of Mississippi. There’s a very slight chance that we could see some flakes here, but the better chances are in central Mississippi. Regardless of whether we get any precipitation after the cold front passes, Tuesday night will be one of the coldest of the winter with lows early Wednesday morning forecast to be in the 20s.

Folks in the upper Midwest will feel the brunt of this arctic outbreak. Even at their warmest on Wednesday, places such as Minneapolis, Milwaukee, and Chicago are expected to remain below zero. Mind-boggling wind chills of 30 to 50 degrees below zero are possible from the eastern Dakotas, Minnesota, Wisconsin, Iowa, and northern parts of Illinois, Indiana, and Ohio. Fortunately, the coldest air is not expected to hang around. In the Gulf South, afternoon temperatures are predicted to climb back to near 60 next weekend.