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Autumn cold snap to bring wintry feel, snow to some in Eastern U.S.

It’s the coldest air of the season so far, and many will even see their first snowflakes of the season

A pocket of cold air, low pressure and spin aloft over the Northeast and Great Lakes. (WeatherBell)

A blast of Arctic air is descending south over the Lower 48, bringing a wintry feel and temperatures 10 to 20 degrees below average to nearly 200 million Americans. In addition to the coldest air so far this autumn, many will even see their first snowflakes of the season. Accumulating snows are likely in parts of the Great Lakes, with up to 2 feet projected in the Upper Peninsula of Michigan.

Forty million people from Kansas to North Carolina are under freeze warnings, too, with another 40 million included in freeze watches or frost advisories. The early-season cold air outbreak will probably spell an end to the growing season and kill vulnerable or exposed plants.

A few claps of thundersnow are even possible along the shores of the Great Lakes. Lake Erie, meanwhile, can anticipate scattered waterspouts as frigid air at the upper levels moves overhead.

The air mass has origins in the high Arctic north of Russia and Alaska, having spent the past week chilling over Canada en route to the Lower 48. The cold shot will last for three or four days, with a welcome reprieve coming as warmth returns next weekend.

How harsh will winter be? Six organizations issue forecasts.

In some areas, a garnish of fresh snowfall will decorate fall foliage nearing peak color, like in the Alleghenies and Appalachians. In the Upper Peninsula of Michigan, where a full-fledged snowstorm is ongoing, more significant snowfall will meet the fully leafed trees and cause damage and power outages.

What’s causing the cold

The anomalously cool weather can be blamed on an upper-level low, or a pocket of high-altitude cold air, low pressure and spin nestled within a dip in the jet stream. It’s parked over the Great Lakes at present and will spend about 48 hours in place and strengthen. That counterclockwise flow will swirl down frosty Canadian air, spilling it all the way down to the Mid-South.

Using NOAA’s HYSPLIT model, which simulates the paths traced by parcels of air, meteorologists can conclude that the air mass wafting over the Great Lakes and Northeast actually was over Siberia on Oct. 10. Since then, it’s passed over the Chukchi Sea north of Alaska and through Canada, performing a small loop before sliding south near the Hudson Bay.

The stubbornly chilly air mass, with winds out of the west-northwest, will linger through Thursday.

How cold it will get

Temperatures will hover 15 to 20 degrees below average for days. In Minneapolis, breezy winds and highs around 37 are in the cards for Monday. The average is closer to 58 degrees. The next several days won’t be much better.

Chicago was heading for a high of 40 degrees for Monday, with the chance of a spot rain or snow shower in the afternoon. Tuesday and Wednesday will be in the mid- to upper 40s. Averages are closer to the lower 60s this time of year.

Indianapolis, Columbus and Detroit will spend several days in the 40s before a gradual warm-up late week. Nashville, Raleigh and Washington, which should be in the upper 60s to around 70, will be in the mid-50s midweek with lows in the 30s.

D.C.-area forecast: Blast of chilly air arrives Tuesday; warmer by weekend

A number of nightly low temperature records could be challenged or broken. Most areas across the Dakotas, Nebraska and Minnesota will fall into the teens or lower 20s on Tuesday morning, including Aberdeen, S.D., where a morning low of 10 degrees is expected. That would beat the record of 12 degrees set in 1930, and would also mark the coldest temperature to be measured this early in the season since 1937.

Who will see snow

Lake effect snows were piling up in the Upper Peninsula of Michigan, where forecast totals of up to 20 inches may be realized by early Tuesday. Cold air sweeping out of Canada was extracting heat and moisture from the relatively warmer waters of the Great Lakes — the perfect recipe for factory-like snow production. Winter storm warnings were in effect.

Snowfall totals in Marquette, Mich., could rank among the top 10 highest on record during October, with 4 to 10 inches predicted.

A winter weather advisory was also in effect for Porter and Jasper counties in Indiana, where a localized filament of lake-effect snow is possible. The National Weather Service in Chicago noted that “there is a higher than normal degree of uncertainty with the snow forecast.” Subtle changes in wind direction will have significant bearings on where the snow band materializes and drifts, assuming it does at all.

Significant accumulations of a half foot or more are possible along the eastern shores of Lake Erie, and particularly from Erie to Buffalo along Interstate 90. An isolated rumble of thunder can’t be ruled out.

Several waterspouts are also possible on Lake Erie itself thanks to converging, or gathering, winds and a sufficiently unstable atmosphere. Lukewarm water temperatures will create lift and encourage surface eddies to be vertically stretched by convective updrafts, or pockets of air that rise into individual storm cells.

A few inches of snow are possible atop the Tug Hill Plateau in western New York state as well. In addition, the western slopes of the Alleghenies in West Virginia and the Panhandle of Maryland might see a dusting to an inch or two of snow too due to “upslope snow,” or air carrying moisture forced up the mountains. A warm-up is expected around Friday.