Blowin' in the Wind . . .
Posted March 10, 2006
For the past few days we have seen our share of windy weather with wind speeds averaging 20-25mph and gusts in the 30-35mph range. I guess it's the old March "coming in like a lion" weather lore we are familiar with. Back in 2004, on Sunday March 7th, a strong cold front, with the help of a strong upper level disturbance flew across the state in the evening hours from around 6:00pm until Midnight. Wind gusts over 50mph were reported at over two dozen official reporting stations with many unofficial reports over 60mph. Of course there were the normal countless reports of trees down, but significant little damage, and about 200,000 people with out power, at one time or another, over the course of the evening.
What I remember most about that evening was that the strongest winds were well in advance of the line of thunderstorms. Normally we see damaging winds, in this part of NC, associated directly with thunderstorms, tornadoes, or tropical systems, but this appeared to be something we don’t usually see in this neck-of-the-woods - a dry down burst. This expression refers to high winds with little or no precipitation. Downbursts occur in regions of a severe thunderstorm where the air is accelerated downward by very heavy rain which drags dry air down with it (a wet downburst) or as in this case, by exceptionally strong evaporative cooling occurring. When the rapidly descending air strikes the ground, it spreads outward in all directions in a circle, like fast-running water hitting the bottom of the sink.
Dr. Fujita defined downbursts as a surface wind in excess of 39 mph caused by a small-scale downdraft from the base of a convective cloud. He further subdivided downbursts into two categories macrobursts (greater than 2.5 miles in diameter from the initial point of downdraft impact) and microbursts(less than 2.5 miles in diameter). Many times, damage associated with a downburst is mistaken for a tornado, particularly directly under the downburst. However, damage patterns away from the impact area are characteristic of straight-line winds rather than the twisted pattern of tornado damage.
Temperatures dropped quickly from the lo 60s to the upper 40s in the wake of these cold, dry down bursts, and many of us were taken by surprise because they were occurring 10 – 15 miles in advance of the line of thunderstorms.