Published: June 22,2015
Derechos are large clusters of thunderstorms that produce widespread straight-line wind damage, usually as a result of one or more curved lines of thunderstorms known as bow echoes.
According to severe weather expert Dr. Greg Forbes, (Facebook), an ordinary thunderstorm produces a swath of damaging winds usually only a mile or two wide and a few miles long, but derechos can produce damage swaths tens of miles wide and at least 250 miles long.
Derechos in the U.S. are most numerous during the late spring and summer, with May (over 4 per year), June (3-4 per year) and July (over 4 per year) the peak months, according to a 2005 study by Walker Ashley and Thomas Mote.
Average number of derechos in the U.S. by month from 1986-2003. (Ashley and Mote (2005))
Frequency of derechos in the U.S. from 1986-2003.(Ashley and Mote (2005))
Dr. Forbes says due to the widespread nature of high winds in a derecho, the impact is somewhat like that of a landfalling hurricane, and affects a much greater area than most tornadoes.
The extensive swath of downed trees and power lines can require a major cleanup and restoration effort that takes days to weeks and often requires relief workers to come in from other states to aid in these efforts in the worst derecho events.
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Ashley and Mote found derechos claimed 153 lives in that 18-year period. Interestingly, three northern states - New York (23), Michigan (17) and Ohio (16) - comprised over a third of all derecho deaths.
Almost 70 percent of derecho fatalities occurred in areas other than a permanent building, including in vehicles (30 percent), in boats (19 percent), under trees (11 percent) and camping (9 percent).
Derechos can sweep through parks, forests and other recreation areas packed with those on summer vacation, downing hundreds, if not thousands, of trees. They can also occur in the overnight and early-morning hours when folks are sleeping.
To give you an idea of what kind of impacts these damaging thunderstorms complexes can have, here are some examples of significant derechos in history.
Historic Damaging Derechos
June 29, 2012: A massive derecho swept from the southern Great Lakes and Ohio Valley to the Middle Atlantic.- Over a roughly 13-hour period, a line of severe thunderstorms with damaging winds raced east-southeast at a forward speed up to 65 mph starting west of Chicago around 10:30 a.m. CST on June 29, reaching the Mid-Atlantic coast around midnight.
- Along this roughly 800 mile-long swath, over 600 reports of high winds or wind damage came in from 11 states.
- Power outages were on a massive scale. At its peak, 4.25 million customers were without power.
- States of emergency were declared in Maryland, D.C., West Virginia, Ohio, and Virginia. Dominion Power compared the restoration effort to that from a hurricane.
- Compounding the misery, a heat wave lingered in the Mid-Atlantic States for the next week, with daytime highs in the upper 90s and 100s, as those without power tried to sweat it out.
- The first one on July 19 packed wind gusts up to 92 mph. Thirty people were injured at the baseball stadium where the Cardinals were preparing to play the Atlanta Braves.
- Power was knocked out to 560,000 customers.
- Service had been restored to about 270,000 power customers when the second derecho hit the St. Louis area on July 21, driving power outages back up to around 560,000!
- Over 338,000 lost power in the Memphis area.
- Some people were without power for up to 15 days.
- This derecho moved at speeds of up to 75 mph.
- Wind gusts up to 92 mph were measured along its path.
- The massive damaging windstorm resulted in 7 fatalities.
- The total damage estimate was placed at nearly half a billion dollars.
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