Tom Horne

The Mega-Derecho

July 2, 2012 by Thomas A. Horne, Editor At Large

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, www.spc.noaa.gov , 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.

8 Responses to “The Mega-Derecho”

  1. JJ Greenway Says:

    Great recap, Tom! The claims are rolling in, trust me!

  2. Fred Remer Says:

    This was a long duration derecho similar to the Boundary Waters Blowdown in 1999. It is truly amazing that these systems stay organized for so long. It’s remarkable what a little shear and instability will do!

  3. David Reinhart Says:

    Tom, remind me not to be anywhere near you if there’s lightning in the area. You’ve survived more hits and near-misses than anybody else I know!

  4. dick sanders Says:

    terrific article. I have been flying for many many years, g/a, military and airline and this is the first time that i have heard the term derecho. glad it has fund its way into my aviation lexicon.

  5. Allen Morris Says:

    Tom,
    Many thanks for the excellent reporting. Most of us had never heard of the derecho. It was one of those concepts that the news media would not bother with, since they likely felt it was too complex for the average viewer or listener.
    Allen Morris/aka Ace Abbott

  6. J janaitis Says:

    I was on a flight from Luray VA to TRI in Tennessee when this was approaching. Luckily I had WX weather on a 396 and was able to divert far enough to the east to get around this. I did my usual diligence watching the weather days before and got two briefings in the morning and early afternoon from Flight Service and this approching condition was never mentioned. I obviously did not look at the right sources but would expect some mention of the from Lockheed Martin. It seems this took many weather professional by surprise.

  7. John McCreight Says:

    Thanks for an informative article, much appreciated!

    If I may offer some constructive criticism, however, would it be possible to turn “mesoscale convective complex (MCC)” into a link to your previous article? I’m sure it was good too, and I’d like to see it. You might also want to write things like BTW out, not everyone in your audience is familiar with internet-cronyms.

  8. Kim Law Says:

    I was a passenger on a commercial jet the night of 7/7/1991 (Denver to Cleveland). We flew around in what I know now is called a derecho for over an hour. It was sheer terror. Wind shear, lightning hitting the plane, overhead compartments opened up, bottoming out, turning sideways, aborted landing, the works. We all thought we were going to die. After reading about the severity of the event, I consider it to be a miracle that we survived it.

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