Polar vortex rearing its chilly head in Pittsburgh region again

Evidently we have not seen the last of that ice-breathing creature known as the polar vortex.

After a relatively unremarkable weekend, the National Weather Service is forecasting a prolonged period of below-normal temperatures, with lows in the single digits and highs in the teens for much of next week.

"It's going to get cold. It doesn't look to be quite as cold as what we recently had, but we are forecasting a longer period of single-digit lows and highs in the teens," said weather service meteorologist Mike Kennedy.

A low-pressure trough parked over Canada will allow a series of weather systems to plunge south behind it, bringing the frigid air.

"We'll see snow, but the overall accumulations with each one of these fronts isn't expected to be great. It'll kind of nickel-and-dime us," Mr. Kennedy said.

One helpful factor is that Lake Erie has nearly frozen over, reducing the amount of moisture that will be picked up by the fronts.

State College-based AccuWeather is forecasting that the vortex, a spinning mass of bitter cold air that has always existed but has somehow gained national prominence this winter, will move farther south again, giving us a bout of extreme cold and blasts of nuisance snow.

"There is the chance the cold may rival that of early January in some areas," said a report by Alex Sosnowski, senior meteorologist for AccuWeather.com.

As the polar vortex dips to the south again, it "will continue to act like a giant pinwheel producing a series of weak storms with spotty snow and flurries, as well as brief waves of moderately cold air over southern Canada and the North Central and Eastern United States," Mr. Sosnowski said.

The polar vortex can be thought of as a cap above the Arctic Circle held in place by the jet stream, a band of strong wind that surrounds it. The jet stream gets its strength and energy from the convergence of cold air in the north and warm air in the south.

Under normal circumstances, the polar vortex spins at speeds of 100 mph or more. Meteorologists say it's like a child's spinning top: When it's spinning at full speed, it stays in place.

When a weak spot forms in the jet stream, the polar vortex can become unstable and wobble like a spinning top does when it's slowing down.

The National Weather Service forecast calls for a high barely above 20 today, and near 30 on Sunday and Monday before the bottom drops out on Monday night. Lows on Tuesday and Wednesday will be around 5 degrees.

Jon Schmitz: jschmitz@post-gazette.com or 412-263-1868. The Blade of Toledo, Ohio, contributed.

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