This is a follow up to a previous blog post “Derecho in Progress? Damaging Wind Threat in VA/NC later this Evening/Tonight?”
A violent line of organized severe thunderstorms called a derecho swept across the Midwest, Ohio Valley and Mid-Atlantic knocking down trees and power lines, damaging property, and leading to several fatalities. A derecho is a widespread, long-lived, straight-line windstorm that is associated with a fast-moving band of severe thunderstorms. The thunderstorms in a derecho create violent winds that generally blow in a straight line; the term derecho originates from the Spanish word for “straight” or “direct ahead.”
The derecho that moved across the eastern U.S. during the late afternoon and evening hours of Friday, 29 June 2012 appears to have originated from convection that developed in vicinity of a west-east oriented stationary front over Iowa and northern Illinois between 12-15Z Friday. Although unclear at this time, it is possible that the convection in Iowa and Illinois developed in association with the remnants of a mesoscale convective system which tracked from eastern Colorado into central Nebraska between 00-06Z 29 June. Convection initially over Iowa and Illinois Friday morning began to organize along outflow across northern Indiana by 18Z as diurnal heating south of the stationary front further enhanced a thermodynamic environment already favorable for pronounced cold pools. Organization along outflow persisted as the afternoon progressed, and the aforementioned convection evolved into a classic bow echo as it progressed east southeast into Ohio by 21Z. The bow echo expanded in size and intensity between 21-00Z as it tracked downstream into a highly favorable (and anomalous) thermodynamic environment over the Ohio Valley and Mid-Atlantic, producing an increasingly larger swath of wind damage. The resulting mesoscale convective system progressed east southeast through the Mid-Atlantic and eventually offshore by 06Z on 30 June.
The derecho can be seen rapidly moving east and southeast in the CONUS reflectivity radar loop below which runs from 0608Z 29 June, 2012 through 0648Z on 30 June.
The track of the derecho can be seen in the map below of all NWS watch, warning, and advisory products issued during the 24 hour period ending at 12Z 30 June.