Roanoke Times
Copyright (c) 1995, Landmark Communications, Inc.
DATE: MONDAY, March 11, 1991 TAG: 9103110358
SECTION: VIRGINIA PAGE: A-3 EDITION: EVENING
SOURCE: MAG POFF STAFF WRITER
DATELINE: LENGTH: Medium
But the last two winters, which have been unusually warm, probably are an aberration in the long-term trend.
Chip Knappenberger, a state climatologist based in Charlottesville, said the trend over the past 45 years is to colder temperatures. The average has dropped two degrees since 1945.
The past few winters either could be a short-term cycle within the longer trend or the beginning of a different trend to a slightly warmer climate, he said. Climatologists have no way of knowing, any more than they can predict in advance, how the jet stream will set itself up for the season.
The jet stream separates arctic air from the North from warmer Southern air. Winter storms tend to follow its track as it moves south in the winter and retreats northward in the summer, Knappenberger said.
He said the jet stream usually follows one of two patterns during the winter.
Typically, it moves down over the Northwest, dips south as far as the Gulf of Mexico and moves up again over the East. There's a ridge in the West, which is warm, and a trough in the East, which is cold.
In the second scenario, which prevailed this winter, the jet stream creates a trough in the West, where it's colder than normal, and a ridge in the East, which is warm. Under this pattern, Knappenberger said, the jet stream stays to the north over the Ohio Valley and New England.
Once a jet stream pattern is in place, Knappenberger said, it tends to persist throughout the winter.
Although spring will not arrive officially until March 20, the months of December, January and February were unusually warm this season.
The National Weather Service reported that temperatures in each month were above normal: December by 5.5 degrees, January by nearly 4 degrees and February by 4.8 degrees.
Only five other winters since 1912, taken as a whole, have been warmer, according to the National Weather Service.
Highs of 74 on Feb. 5 and 65 on Feb. 7 set records for those dates. February brought a two-day cold snap as well, and a low of 9 degrees on Feb. 16 tied the record for that date. Wind gusts that morning dropped the chill index to minus 10-20 degrees.
It's also been a no-snow winter.
The National Weather Service said the season so far has brought only one "winter event."
That happened Dec. 27, when a combination of snow, sleet and freezing rain put an inch of mostly ice on the ground. The temperature rose immediately afterward and the glaze vanished the next day.
January brought 3.55 inches of rain with only a trace of snow, and February added 2.1 inches of rain and a touch of snow in the Roanoke Valley.
The National Weather Service said that in records going back to the turn of the century there have been just two winters with less than 2 inches of snowfall. Those were the back-to-back winters of 1918-1919 and 1919-1920, when just a trace of snow was reported.
Snow fell last month in outlying areas, however. The National Weather Service said higher elevations around the valley got a half-inch the night of Feb. 25, while areas to the north and east had up to 4 inches.
The National Weather Service said that for 43 of the past 78 years in Roanoke, March has brought an inch or more of snow. The most memorable March was in 1960 when 30.3 inches fell, including one storm that dumped 17.4 inches.
But the service said climatological records for Roanoke indicate that most of the snow that falls in March comes in the first half of the month.
The National Weather Service expects the mild weather to continue.
Its outlook for March calls for near-normal temperatures and precipitation. For March through May, the service predicts above-normal temperatures and below-normal precipitation.
by CNB