Shifting Jet Stream Brings Cold Temperatures Southward

January 15, 2009

January 15, 2009 — Baby, it's going to get cold outside.

Temperatures at the end of the week are scheduled to drop into the single digits, according to University of Virginia Climatologist Philip J. Stenger.

The jet stream, Stenger said, maintains a barrier between colder northern air and warmer southern air, but the jet stream moves around the globe in a wave-like pattern. Currently the wave is at a higher latitude in the western part of the country and dips in a very southerly direction in the east. This allows could air to move farther south.

"A common configuration over North America is to have a northward bend in the jet stream over the western part of the continent — a ridge— and a southerly dip — a trough — over Eastern North America," Stenger said. "That makes it colder in the east than in the west, and now there is a considerable temperature difference in this case."

Stenger said the lowest temperatures should arrive late Friday night and early Saturday morning.

"It will dip well into the single digits, and in some places it will approach zero," he said.

The cold air will be assisted by cloudless skies at night, which allow warmer air to rise into the upper atmosphere, and a lack of wind, which would mix the warmer air in the higher atmosphere with the colder air closer to the ground.

Higher elevations may be warmer. "The cold air that builds up overnight is denser than the warm air above it and it tends to flow downhill, accumulating in the low-lying areas," Stenger said.

The cold snap will not last long, however.

"It looks as if the cold conditions will start to dissipate on Sunday," Stenger said. "By Sunday night, we should see the lows climb out of the teens."

While this weather seems bitterly cold, Stenger said it is not record-setting.

"The record temperatures went well into negative numbers as measured at the McCormick Observatory," he said.

The all-time record low for McCormick Observatory, where climatologists have been tracking temperatures since the 1890s, was set Jan. 19, 1994, at 10 below zero . The record low for Jan. 17 was 5 below zero in 1982, and the record for Jan. 18 was 3 degrees below zero in 1982.

Stenger said 1994 was the last year a below-zero temperature was measured at McCormick Observatory.

"We hit single-digit temperatures in about two-thirds of the years, so it is more common to have single-digit temperatures than not," he said.

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— By Matt Kelly