June 2012 North American derecho





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Published on Jul 1, 2012

The June 2012 Mid-Atlantic and Midwest derecho was an intense progressive derecho that tracked across a large section of the Midwestern United States and across the central Appalachians into the Mid-Atlantic States on the afternoon and evening of June 29, 2012 and into the early morning of June 30. It resulted in at least 12 deaths, widespread damage and millions of power outages across the entire affected region.

Storm overview

The thunderstorm complex which developed into a mesoscale convective system started as a small thunderstorm cell in central Iowa and continued into Illinois while mostly below severe limits at the time. Taking advantage of an extremely hot and highly unstable atmosphere with CAPE values in excess of 5,000 J/kg and temperatures on the south side of a stationary front well in excess of 100°F (near 40°C), it quickly intensified and the storms became severe as it crossed over the Chicagoland area early in the afternoon of June 29. As it tracked eastward into Indiana, wind gusts increased substantially, peaking as high as 91 mph (147 km/h) in Fort Wayne, equivalent to a high-end Category 1 hurricane, at least in gusts. By that time, the system had taken on a well-defined bow echo shape, the classic indicator of a derecho.

Moving east to east-southeast at about 60 mph (95 km/h) - a speed and trajectory that the derecho maintained for its entire lifespan - it followed the stationary front and increased in size as it entered Ohio in the mid-afternoon hours, affecting a large portion of the state. Since the derecho was clearly underway and expected to last a long time, the Storm Prediction Center increased the threat level to a moderate risk of severe weather at 3:37 p.m. EDT (1937 UTC) across Ohio, West Virginia and small portions of neighboring states, warning of the extreme wind threat. It crossed over Columbus shortly afterward, with winds reported to 72 mph (115 km/h) there, and winds in excess of 80 mph (128 km/h) reported at many other locations across the state. It maintained its intensity as it reached the Ohio River early in the evening, and the only known tornado associated with the system was also reported at the same time, which did minimal damage (in comparison) near Newcomerstown, Ohio at 6:20 p.m. EDT.

The derecho crossed the much more sparsely populated mountains of West Virginia and parts of extreme southwestern Pennsylvania and eastern Kentucky through the early evening hours. Despite the mountainous terrain and loss of daytime heating, the hot, humid air mass and high instability allowed the storms to maintain their high intensity throughout, even though the amount of energy decreased slightly (to a still-extreme 3,500 J/kg). They emerged into Virginia and Maryland later in the evening, where an even more unstable air mass awaited with CAPE values as high as 5,500 J/kg.

The derecho continued to expand as it crossed the Mid-Atlantic states late in the evening, impacting nearly all of Maryland and Virginia as well as Washington, D.C.. The most severe winds - reported as high as 87 mph (140 km/h) - were focused in the Baltimore-Washington metropolitan area and southwards towards Richmond. Widespread wind gusts over 70 mph (112 km/h), with some significantly higher, were reported across the large and heavily populated region. The damaging winds continued eastward across Chesapeake Bay towards the Atlantic Ocean, losing little strength despite the cooler marine layer. The derecho emerged into the Atlantic Ocean shortly before 2:00 a.m. on June 30, while still producing winds as high as 81 mph (130 km/h) right at the coast in New Jersey and strong and damaging winds on the Delmarva Peninsula. It dissipated in the pre-dawn hours south of Long Island.


Damage was widespread and extensive along the entire path of the derecho, especially in northern Indiana, central and western Ohio, Northeastern Kentucky, West Virginia, northern, central, and southwestern Virginia, Maryland, Washington, D.C., Delaware and southern New Jersey. In all the mentioned areas, many trees were uprooted or snapped, roofs were damaged, tents deployed to sell fireworks leading up to the 4th of July Holiday were collapsed, and power outages were extensive, with over 3.7 million customers losing power as a result. An Appalachian Power representative described the power outage as the worst the company had ever seen.


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