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Friday, May 8, 2009

Massive derecho and classic bookend vorticity (radar shots)



A massive derecho that started in north central KS very early this morning is currently hammering the TN valley area and points east. The radar grab above was taken this morning when the Springfield Missouri area was getting hit. You can clearly see the "bow echo" and near the top or north end on the echo there is a very interesting area that we call bookend vorticity.
In its most basic form, it is when the top end, bottom end or both ends of the "bow echo" fold in on its self.
Under the right conditions this folding can and generally does cause that part of the storm to rotate and can help lead to tornadogenesis.

Bookend Vorticity with tornado warning shows classic hook echo via StormLab Supercharged


Also of real interest is the "hole" that is clearly seen in the composite mode as well as the SRV mode!
Radar shot below is from GRLevel3 and also shows the couplet and inflow notch created by the wrap around vorticity. At one point this particular couplet had almost 110 kts gate gate!


Damage has been pretty much been reported with this system since it first began and tornado reports are also still ongoing.
For those not familure with a derecho, A derecho (from Spanish: "derecho" meaning "straight") is a widespread and long-lived, violent convectively induced straight-line windstorm that is associated with a fast-moving band of severe thunderstorms in the form of a squall line usually taking the form of a bow echo. Derechos blow in the direction of movement of their associated storms, similar to a gust front, except that the wind is sustained and generally increases in strength behind the "gust" front. A warm weather phenomenon, derechos occur mostly in summer, especially June and July in the Northern Hemisphere. They can occur at any time of the year and occur as frequently at night as in the daylight hours.
The traditional criteria that distinguish a derecho from a severe thunderstorm are sustained winds of 58 mph (93 km/h) during the storm (as opposed to gusts), high or rapidly increasing forward speed, and geographic extent (typically 250 nmi (460 km) in length).[1] In addition, they have a distinctive appearance on radar (bow echo); several unique features, such as the rear inflow notch and bookend vortex, and usually manifest two or more downbursts

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