The science behind the derecho
Dave Dildine, WTOP traffic reporter and weather researcher
WASHINGTON - The ferocious storm that blew through the Washington-area Friday night is a weather system known as a derecho.
It is not only an unusual weather event for the region, but the circumstances surrounding the storm changed its pattern characteristics, according to ABC 7 Meterologist Mike Stinneford.
Derecho is a Spanish word that means "straight." It is in reference to the storm's powerful straight-line winds.
Derechos typically form along the top of a hot air mass and can move an average of 70 miles per hour. That wind speed is normally associated with a Category 1 hurricane.
WTOP Traffic Reporter Dave Dildine is also a weather research analyst. He compiled this chronology of the derecho's movements Friday.
Timeline of Friday's derecho forecast
8:53 a.m. eastern time
The Storm Prediction Center places the Mid-Atlantic, including the Washington, D.C metro area, under a slight risk for severe weather. They note "considerable variability in storm evolution" but point to the possibility of "damaging winds spreading south-eastward" through the evening.
The National Weather Service in Sterling, Va., introduces increased chances for thunderstorms for the Washington area, stating that they "expect storms to continue to roll east … across much of Virginia."
The Storm Prediction Center begins tracking the evolution of a complex of severe weather in Illinois and Indiana. Operational forecaster mentions that the "extent of the severe threat should be limited to areas west of the Appalachian Mountains."
The Storm Prediction Center issues a severe thunderstorm watch for northeastern Illinois and the northern half of Indiana.
Fort Wayne International Airport records a wind gust to 91 m.p.h. Emergency officials in parts of Indiana begin reporting "massive damage" following passage of storms.
Local National Weather Service offices hold a conference call with Storm Prediction Center. SPC then upgrades parts of Indiana, Ohio, and West Virginia to a moderate risk of severe weather and warns of "significant winds." The National Weather Service office in Sterling, Va., warns, "late night/early morning hours will be the prime time for [severe weather in] our area."
The Storm Prediction Center classifies the "widespread/locally significant wind damage" in Ohio and West Virginia as a derecho.
Reagan National Airport soars to a record high temperature for the day and month of June of 104 degrees Fahrenheit.
ABC 7 Chief Meteorologist Doug Hill warns, "later tonight... we will have to be on the lookout for very heavy, potentially severe thunderstorms."
The National Weather Service in Sterling, Va., introduces increased chances for thunderstorms - some severe - for the region. Their forecast discussion mentions a growing concern over wind damage ongoing in the Ohio Valley.
The National Weather Service issues a severe thunderstorm watch for the region for the possibility of damaging winds.
SPC tracks the ongoing derecho across West Virginia and warns "storms to continue to roll beyond the east slopes of the Appalachians and across much of Virginia over the next several hours."
7:45 p.m. The National Weather Service issues a special statement for the area noting, "Storms have a history of producing major wind damage across West Virginia due to wind gusts over 75 m.p.h. along with a prolific cloud to ground lightning."
The National Weather Service in Sterling, Va., issues a large severe thunderstorm warning for far the western suburbs with a strongly wording warning of "destructive winds."
A severe thunderstorm warning is issued for the western suburbs. The Weather Service advises the public seek shelter immediately.
The National Weather Service sends the alert that "This is a particularly dangerous situation this evening. Residents and visitors to the region should start planning now to protect life and property through seeking shelter in a sturdy building when warnings are issued for your area."
The Weather Service issues a severe thunderstorm warning for the entire metro area and surrounding counties for destructive winds in excess of 80 m.p.h. Warning includes a stern message to the general public, "This is a dangerous line of storms... These storms are capable of producing destructive winds in excess of 80 miles per hour. This is a serious situation. You need to take cover now."
The weather observers at Dulles International Airport measured a wind gust to 71 m.p.h. Airport operations are on ground stop.
Automatic weather reporting station at the USGS in Reston, Va., measures a wind gust at 79 m.p.h.
Two storm-related fatalities are reported in Fairfax County, Va. One injury is reported on the George Washington Memorial Parkway near the Capital Beltway.
The Weather Service issues a severe thunderstorm warning for eastern suburbs including areas near the Chesapeake Bay.
A weather observer at Ronald Reagan National Airport records a wind gust of 70 m.p.h. as the storm moves into District of Columbia.
10:50 - 10:58 p.m.
Several automatic weather reporting stations in Prince George's County, Md. record wind gusts between 60 and 80 miles per hour. A wind gust of 76 m.p.h. was recorded in Seat Pleasant, Md.
Baltimore-Washington International Airport observers report a peak wind gust of 66 m.p.h. at the height of the storm.
Severe thunderstorm warning is allowed to expire for Washington D.C., metro area as storms push east over the Chesapeake Bay. The National Weather Service continues to receive delayed reports of widespread, severe wind damage from throughout the region.
"Outside a hurricane, a derecho is one of the worst storms that can run through the area," Stinneford says. "It's very unusual to have a derecho come this far south, also to come in the evening, also to come over the [Blue Ridge] mountains."
The storm that hit the D.C.area on Friday night started on the west side of Chicago and gained speed, following a jet stream east. This type of storm tends to die down by the end of the day, says Stinneford, and will lose steam.
Contrary to its historical pattern, Friday's derecho moved from west to east at a rapid speed, traveling up to 80 miles per hour at some points. It gained momentum heading into the Mid-Atlantic and hit after dark.
"This was a very large derecho," says Stinneford. "This is probably one of the biggest ones we've ever seen ... probably one of the worst storms ever in D.C. outside of a hurricane or the type of the damage you see from a major winter storm."
The major takeaway from this storm from a meteorlogical perspective was the size of its swath, he says.
"Outside of a hurricane, you will never see wind damage this widespread or this long-lived unless you had a land-falling hurricane."
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