Rare Class Of Thunderstorm Responsible For South Jersey Damage
By Pat Loeb
PHILADELPHIA (CBS) - Many of us were quite happy never to have heard the word "derecho" (pronounced duh-RAY-sho) until Friday night's incredibly damaging storm, which, we learned, belonged to that class of weather system. The storms are hard to predict or even define.
"Exact definitions can vary," Steve Corfidi, with the Storm Prediction Center, told KYW Newsradio. "It's a little bit like pornography; you know it when you see it."
The term "derecho" comes from the Spanish word for "go straight." So, just as a tornado is a wind that turns, a derecho barrels ahead in a straight line. It's roughly defined as a repeating series of strong thunderstorms traveling in a straight line at least 250 miles. Friday's went about 600 miles.
Corfidi is one of the nation's leading experts on derechos, but he admits he can't predict when they'll occur. "To keep that system going for that length of time and space depends on factors which are very small scale and we just don't sample the atmosphere with that degree of precision."
He says the conditions that can produce them -- cool air above, hot air below -- occurs in our area in June and July, but they don't usually lead to derechos.
"If you were to forecast a derecho for every time you got the ingredients that are involved in their development, you would definitely overforecast."
So, while derechos are unusual, he can't rule out another.
