In Johnsville, Frederick County, the roof of a dairy barn was destroyed and blown several hundred feet. Numerous trees were uprooted and large limbs were snapped off. Winds in this downburst were estimated to be around 80 MPH.
Here is the official damage report from the National Weather Service:
0538 PM DOWNBURST 3 SSE LADIESBURG 39.54N 77.23W 06/22/2012 E80 MPH FREDERICK MD EMERGENCY MNGR ROOF OF A LARGE DAIRY BARN DESTROYED BY THUNDERSTORM DOWNBURST WITH WIND OF AROUND 80 MPH. PIECES OF ROOF WERE BLOWN DOWNWIND SEVERAL HUNDRED FEET. MANY TREES IN THIS AREA WERE UPROOTED OR DAMAGED.
A downburst occurs when cold air high in a thunderstorm descends and accelerates. As the stream of cold air reaches the ground, it spreads out and can accelerate even further.
When environmental conditions are right, winds in a downburst can exceed 80 to 100 MPH, and such winds are comparable to or stronger than many tornadoes and can cause similar damage.
In some cases, hail and very heavy rain also accompany the downburst, leadind to flooding and significant wind driven hail.
The National Weather Service also received other reports of trees down and hail all across the viewing area from Friday Night's severe weather event.
Remember: A Severe Thunderstorm Watch means that conditions are favorable for the development of severe weather including hail greater than 1" (Quarter Size) in diameter, and thunderstorm wind gusts above 74 MPH, in and close to the watch area.
A Severe Thunderstorm Warning means that a thunderstorm has been detected by National Weather Service Doppler Radar and/or one of the following severe weather reports has been received:
- Thunderstorm wind gusts about 74 MPH
- Hail greater than 1" in diameter (Severe thunderstorms can produce hail as big as baseballs or eggs...)
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