Monday Night's Weather Question: About Derechos
So many people are still asking about the Derechos, especially if storms are now going to be as bad as the June 29th storms.
Derechos are not a rare event. They do happen each year, they just don't typically hit the Mid-Atlantic, and the derecho that did hit our area was an extremely strong derecho. A derecho is defined as a large cluster of extremely strong storms. To be classified as a derecho the cluster of storms has to have lasted for at least 6 hours. This cluster of storms is rapidly moving, and tends to take on the "bow" shape where the storms push forward, a clear indication of extremely strong winds. The storms must also provide a path of damage at least 250 miles long, and most storms have to include winds more than 58 mph. (That's classified as being a severe thunderstorm).
The winds basically produce downbursts, you may have heard of a microburst. One happened in Woodstock last August. Downburst happen in thunderstorms when the downdraft is so strong in a thunderstorm, those strong winds get transported down to the surface.
Here is a graphics of storm reports of wind damage from the Storm Prediction Center.
Those blue dots represent storm reports!
Many times in a derecho you can see winds upwards of 100 mph!
In our area we had 87mph winds clocked at Wintergreen, and 69mph in Harrisonburg!
It's not uncommon to see some tornadoes within the outer edges of a derecho but mainly it's straight line damaging winds.
Again, just because there is a lot of wind damage never assume it's a tornado! Strong straight line winds can produce extreme damage.
How often do they happen?
They do tend to happen more in the summer months. Here's the setup for June 29th. We had a large upper level ridge of High pressure over the Ohio Valley. When that cluster of storms formed over Iowa, they followed the upper edge of that ridge which sent them into a highly unstable environment. Because they move so quickly they were in West Virginia in a matter of a few hours. This cluster was not weakening, and we had a major storm system on our hands. The question then was, how are the mountains going to interact. We believe the derecho was so strong and moving so rapidly into such an extremely unstable environment, there was no time for them to weaken, and they didn't.
The storms produced very little rain and extreme winds. As the rain evaporated as it attempted to fall to the surface, it cooled. A term we call "evaporative cooling" which also aided in strengthening winds that hit the ground.
If you are a camper or a hiker it is extremely important to pay attention to the weather especially in the summer months. Thunderstorms can form quickly and strengthen rapidly. Don't think it can't happen to you.
So how often have they hit the Virginia area?
The last derecho to hit our area was in June of 2008. That storm complex started in southern Indiana and traveled due east through Kentucky right into northern Virginia.
There were also two other weaker derechos in May of 2004.
Take a look at the span of this derecho in 1980.
The path looks slightly longer than the 2012 derecho, but eerily similar.
If there is a lesson to be learned, it's that just because we live in the mountains, doesn't mean they always protect us. Evidence is the tornado outbreak of April 2011 and this most recent Super derecho of June 2012.
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