Re: Arctic weather? Its complicated
Posted by Jeff Va on January 16, 2021, 3:00 am, in reply to "Arctic weather?"
By Alex Sosnowski, AccuWeather senior meteorologist |
Published | Updated Jan. 15, 2021 10:29 PM CST
As January reaches the midway point, AccuWeather forecasters are continuing to track the behavior of the polar vortex and monitoring closely for signs that some of that frigid Arctic air could come charging down into the lower 48 states. When exactly will the polar vortex make a move and send below-normal cold into the United States? Well, it’s complicated, but here’s what forecasters are seeing.
One sign of a disturbance in the vortex, a weather system that hovers over the North Pole and contains some of the coldest air on Earth, will be a wave of cold air originating from over Canada that will spread from the Upper Midwest to the Northeast this weekend behind a powerful storm that was traveling from the Northwest to the Midwest this week. However, the air will turn milder as it passes over the relatively mild waters of the Great Lakes. This event will be more of a bellwether of what’s to come than an actual weather maker itself. "This is the first sign of a change across the northern latitudes following the polar vortex displacement," Pastelok said.
The air mass with this first displacement from the polar vortex will not be all that cold near the surface and will follow an air mass that brought above-average temperatures across much of the Midwest, which in turn warmed the ground.
"We will see a drop to near-normal or even still slightly above-normal temperatures in the Midwest and Northeast this weekend,” Pastelok said. “Then the cold air looks to deepen in the Northeast early next week with temperatures going a few degrees below normal Monday and Tuesday while the Midwest moderates.
Pastelok and his team see signs that another blast of Arctic cold from the polar vortex is on the way, but it won’t happen on the same timetable that was initially forecast a couple of weeks ago.
Major weather changes developing over the North Pacific Ocean over the next week are expected to allow very cold air to build over northern Canada. So far this month, temperatures have been well above average across much of Canada, but once Arctic air takes root over Canada, the stage will be set for some of that bitterly cold air to be sent southward later in January.
Pastelok said that “the most persistent frigid air” will grip parts of central and southwestern Canada, the northern Rockies and the interior Northwest of the U.S. from late January into early February. “Temperature departures of 10-20 degrees below normal" will occur across these regions with the rush of Arctic air, he said.
Once that cold air breaks into U.S. territory, it will take some time for it to work its way to the eastern third of the nation. Pastelok said he expects the “Arctic air to arrive during the weekend of Jan. 23-24, with temperatures dropping well below normal across a wide area.” And it won’t happen all at once.
Some Arctic air will likely come in waves, Pastelok said, due to the potential for storms rolling through the Midwest to the East. In between shots of cold air, temperatures will moderate some. AccuWeather's long-range forecasting team expects temperatures to drop about 8-16 degrees below normal across areas in the East during the period from Jan. 24-28, but that blast of cold will be relatively short-lived.
The average high in Chicago, for example, on Jan. 24 is 31 degrees. For the same date in Cleveland, the average high is 35 degrees, and in New York City, the mercury tops out at 38 on average.
"In the northern Plains and Midwest, high temperatures could plunge 10-20 degrees below normal on the coldest days. It's possible the high will reach only the teens or low 20s in Chicago on a few days," AccuWeather Senior Meteorologist Dave Samuhel said. "Highs are expected to drop as much as 8-16 degrees below normal across areas farther east during the last week of January."
In New York City, highs could reach the 20s or near 30 on a few days during the last week of the month amid the brief but bitter blasts of Arctic air.
The harshest cold will occur in Canada and the northern reaches of the U.S. As January comes to a close, temperatures are likely to fall 15 to 20 degrees lower than normal for several days in Alberta, Canada, and down into parts of the U.S., like Montana. High temperatures could plummet to near zero, and lows could plunge as low as minus 20 F as the month ends.
During the first week or two of February, the core of the cold air may remain rooted from the central and northern Rockies to west-central Canada, but temperatures are likely to trend upward farther to the east in the U.S.