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Farmers Almanac
The 2014 Farmers Almanac
Farmers' Almanac

What in the world is a Derecho?

What in the world is a Derecho?

As I’m writing this on Monday afternoon, thousands of people in the Midwest are still without power 10-days after a cluster of potent thunderstorms downed numerous trees and power lines.  The associated weather phenomenon that brought this destruction was a “Derecho,” a term seldom mentioned on radio and television weather casts, and all but unknown to many.

There are actually two types of thunderstorm complexes that can show up on radar.  One is an MCS or Mesoscale Convective System.  This is a cluster of strong-to-severe thunderstorms, much larger than one singular thunderstorm cell, though smaller than an organized area of storms affiliated with a significant low pressure system.  Such thunderstorm systems as an MCS can develop along and ahead of cold front.  The thundery/showery weather tends to move rather slowly; some MCS’s actually ramp-up in the late afternoon and evening, diminish as the air cools later at night, and then fire up again the following afternoon.

Although a derecho might at first glance look similar on radar to an MCS, it is completely different.  A derecho is caused by a sudden impulse of hot air that clashes with a moisture-laden airmass.  The result is an outbreak of thunderstorms, but unlike the slow-moving MCS, a derecho moves much more rapidly; so rapidly, in fact, that the area of thunderstorms gets pushed-out into the shape of a bow.  The combination of the intensity of the thunderstorms, plus their rapid motion can create widespread straight-line winds in excess of 100 m.p.h. Such winds can cause tremendous damage as is evidenced from what transpired on that Friday night 10-days ago, in the the Mid-Atlantic region.

Derechos can form at any time of year, but are most likely in the hot months of June and July.  And because you need a rather potent blast of heat to set one off, you don’t see them very often.  But as the nation swelters from extreme heat these past few weeks, it shouldn’t have been too surprising that one developed.

And with more intense heat expected over parts of the Great Lakes and Northeast over the next 6 to 10 days, will another Derecho rear its ugly head in the days to come?

Stay tuned!

4 comments

1 USAclimatereporter { 08.08.12 at 11:23 am }

i am still going to research more on a derecho and see how it happens thanks for the facts though

2 Melinda Mae Damon { 07.10.12 at 5:58 pm }

is there any more ba storms for northeast regan

3 christene h { 07.10.12 at 4:39 pm }

We also had no rain only strong winds. Why was there no time for any type of warning?

4 Julie T { 07.10.12 at 2:47 am }

Hi Caleb, why is it that the derecho that moved through here didnt produce any rain? we had absolutely no rain it was just very bad winds. So im taking it that this cool down is only temporary… I guess I better get prepared for the ugly heat predicted in the farmers almanac deeper into the month of July. Right again! this summer is indeed a scorcher

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