http://www.erh.noaa.gov/er/lwx/events/svrwx_20120629/
This is an excellent summary of not only what a Derecho is, but how it manifested itself on 29 June 2012!
For those that didn't experience this line of storms -- it was easy to notice the power of the gust of wind ahead of the line, with hurricane strength gusts. It was quite impressive, indeed. The official wind-gust at Dulles International Airport (IAD) was 71 mph.
Just a taste, note this splendid radar sequence:
Radar Mosaic of the 29 June 2012 Derecho. Keep an eye on Chicago, Illinois. It is at that point that the line of storms turns SE and charges to the Mid-Atlantic!
Live well!
It's common knowledge that unstable low pressure systems (i.e. storms) typically pop up around the edges of a high pressure system. But the NOAA site which you listed above says that, in the summer, derechos often form along the NORTHERN edge of a high pressure system. You can clearly see this here. There's a big area of emptiness (i.e. high pressure) centered around Arkansas and/or Tennesse, and the storms are skirting along its northern edge. I wonder if the clockwise turning of the high pressure system is mainly what helps the derecho to accelerate eastward. NOAA seems to say that a derecho's acceleration is just a product of widespread air instability out in front of the storm, suddenly ignited by a storm's oncoming cold-air outflow, i.e. its gust-front. But it looks to me like there's something else is at work in that massive acceleration eastward: A jet stream? High pressure wedging under the low pressure and the low pressure then 'falling off' of it? (I have no idea.)
ReplyDeleteDavid, great thoughts and questions! I have to confess that I don't know the answers myself -- though what you say makes some sense. Hopefully my expert sources on weather can give me some thoughts...
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