Topper's Blog: The Derecho


A year ago Saturday the June 29th,we were tracking a line of thunderstorms, 100 miles from tip to toe that barreled thru the Metro DC area Friday night from 9:30 - 11:30 PM packing straight line winds of 50 to 80 mph. This same 'clump' of storms hit Indiana, southeastern Ohio and West Virginia with hurricane force winds Friday evening. The technical term is Mesoscale Convective System or Derecho, usually seen in the Midwest and not in the Mid Atlantic. National hit a daily record of 104 and also set an all time June record high. This system of thunderstorms held its shape and intensity from the Ohio River to the Potomac River and across the Bay into the Ocean resorts in coastal Delaware and Maryland. The intense heat ahead of the storm complex fueled the storms as they raced eastward Friday night.

'Derecho' means straight and these types of systems travel in a straight line with a bit of a bow to them. The winds are straight line winds that blow from the direction of the movement of the line of thunderstorms, west. Sometimes small tornadoes are embedded in the line of storms but it is very difficult to distinguish the damage from that of the straight line winds. There were a few tornado warnings that Friday. In fact we first went on the air with the tornado warning in Washington County. These straight clusters of thunderstorms are possible from spring to late summer in the U.S. and are a bit more common in June and July. They form when it is hot and humid. Ironically, it was hot Friday but not humid but as the line approached the Metro Area dew points rose as higher humidity moved in ahead of the actual event.

Damage with Derechos is widespread and not just in a couple of neighborhoods. In a more typical line of storms one or two storms within the line might produce severe weather in two confined areas. Everyone in the Metro Area experienced winds from 50 to 80 mph that night, resulting in damage over tens of miles. It really was like a thirty minute, minimal strength hurricane. This was a one in sixty years event.

We were out at the AT & T National and during the 6 PM show I commented that I had never seen the atmosphere so unstable. At 6:58 PM I tweeted out 'prepare for winds 50 - 80 mph' and we went to a Red alert on the 7 PM show. We then stayed on the air until 2 AM.

Two million customers lost power. Streets were littered with downed trees and tree limbs and looked like war zones. The third round of the AT & T National somehow went on as scheduled but no fans were allowed on the premises for safety reasons. Some sixty volunteers worked feverishly overnight and through the morning removing massive trees from the fairways of several holes at Congressional. The 104 that day was a daily record and a new monthly record high for the month of June and greatly contributed to the instability of the atmosphere that night.


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