Archive for July, 2012

The Mega-Derecho

Monday, July 2nd, 2012

Last Friday a monster wind took out houses, hangars, and electricity from Indiana to Delaware. The highest wind speeds at the surface reached 90 mph, or more! What caused this mayhem? A derecho (pronounced de-RAY-sho), something that’s fairly common in the central U.S., but rare in the east. A derecho isn’t a thunderstorm per se, although convection is at its heart. Nope, it’s a wind event, and a long-lived one at that.

The trouble starts when strong outflow winds from a thunderstorm complex–and yes, this could be a mesoscale convective complex (MCC) which we touched on before–form huge bow echos. Easily visible on infrared satellite imagery, I might add (I’m becoming more of a fan of IR images, BTW). Strong winds aloft, meanwhile, impart energy to the bow echos and start them moving. So now you have the forward motion caused by the outflows themselves, plus the windspeeds added by winds up near jet stream altitudes deep in the flight levels. It’s a two-fer! And it means big, destructive winds at and near the surface. Straight-line winds, by the way, not tornados. Derechos are not tornadic events. Although the wind damage matches what you’d expect to see with tornado activity.

How big were the winds? Reagan National Airport in D.C. recorded gusts as high as 71 mph, although surface winds as high as 80 to 100 mph have been recorded elsewhere in areas under the derecho’s influence. In yet another testimony to aviation safety, there were no accidents related to the derecho, at least not yet. Presumably because all received adequate warning, and any pilot with a particle of brain tissue made sure he/she stayed on the ground while the derecho did its thing. Hey, it’s not like it snuck up on us easterners–you could see it coming nine hours and 600 miles before it hit the Baltimore-Washington area.

Now you know why the Storm Prediction Center’s website, , is right up there on my favorite websites list. Not that I was logged on when the bottom fell out Friday night; I’m understandably afraid of a lightning hit. (I’ve lived through hits in two houses I’ve lived in, watched lighting go down a tree about five feet from where I was standing, lost numerous TVs and weather instruments to lightning hits, and watched lightning strike a barn on a farm I lived on, which subsequently burned the barn down, burning all the animals inside alive, and sending those living in the barn’s adjoining apartment screaming across open fields toward my house. In the dead of night. With the animals, trapped, bellowing in pain.) This was on December 1, 1974, the same night that  TWA 514 hit Mt. Weather on approach into Dulles International–but that’s another story.

So last Friday I was looking out the window, not surfing the net for cool Nexrad images.  I live very near AOPA HQ and the lightning was coming fast and furious–most of it cloud-to-cloud. Sure, there were wind gusts and plenty of rain, but thankfully no damage of the sort that happened in the Washington and Baltimore regions some 35 miles to my southeast and east, respectively.

As of Saturday some 750,000 were without electrical power. 911 services in Falls Church, Virginia and other nearby Virginia communities were out of service. Today I hear that the outages are down to 500,000 or so–but 911 still doesn’t work in those parts of Virginia. Traffic signals are out. Governments and schools

June 29, 2012 derecho progression

 are closed. Liberal leave is in effect. Clearly, the Boswash corridor is unprepared for derechos.

Meanwhile, friend and AOPA Pilot contributor Pete Bedell is meeting with an insurance adjuster this morning. Seems his Baron’s hangar roof departed the scene during the storm (no damage to the Baron, thankfully). I’m sure his situation is being duplicated all over the region. Not to mention the airplanes flipped over in their tiedowns, or, gasp, left unsecured. Those airplanes can be found in the nearest creek I suppose.

All because of a storm that began in Illinois. Though I didn’t capture any images during the derecho, here’s an image from the NWS that gives you the big picture, along with a chart showing the climatology of derechos.