Advertisement
YOU ARE HERE: WDBJ7 HomeCollections

The anatomy of Friday's "Derecho" wind storm

June 30, 2012|Meteorologist Brent Watts
  • Derecho begins as a cluster of thunderstorms with strong updraft or wind blowing into the storm.
WDBJ7 - First Alert Weather

The storms that moved through Friday night were part of a large, widespread wind storm called a "derecho" (pronounced "deh-RAY-cho"). This one may go down in the record books for causing the most damage.

WHAT IS A DERECHO?

Derechos start as a long line of storms that eventually take on bowing characteristics. (SEE GRAPHIC LEFT). On radar, the storm looks like an arch, with the strongest wind out ahead of the arch's point.

In Friday's case, the Derecho moved along the outer edge of high pressure, also responsible for the extended period of extreme heat.

WHEN DID IT FORM?

The derecho event started out as a cluster of storms, developing in Illinois around Noon on Friday. Travelling at 65mph, it made it to southwest Virginia in less than 9 hours.

Unless you watched the Noon/5/6 or followed the weather closely online and on mobile, you had no idea it was "on the radar." It developed that fast.

Advertisement

HOW STRONG WERE THE WINDS?

Winds ranged from 60 to 80mph as the storm raced through. Sensors at the Roanoke Regional Airport recorded a gust of 81 mph as the Derecho moved through. Other locations saw similar gusts, if not stronger. Below are the preliminary wind reports from the Storm Prediction Center. These are just the ones reported.

Derecho-Reports

 

THE HEAT MADE IT WORSE

The excessive heat wave was one of the reasons the storms had such reach, developed so fast, and caused so much damage.

The Derecho creates "Downbursts" when air is cooled by the evaporation or melting of precipitation in thunderstorms. Because the chilled air is heavier than its surroundings, it accelerates downward toward the ground.

In the case of this weekend's wind storms, the air in the clouds was WAY cooler than the 100° temperatures it was moving into. Therefor, the wind accelerated rapidly toward the ground and spread out, causing the massive damage.

HOW OFTEN DO WE SEE DERECHOS

 Derechos in the United States are most common in the late spring and summer (May through August), with more than 75% occurring between April and August.

Based on climatology, our region sees around one Derecho every four years. While each one is different, Friday night's storm certainly ranks as one of the worst on record to come through the region, causing the most widespread damage.

Derecho-Reports

wdbj7 Articles
|
|
|