The Iowa bow echo of August 25, 2004

What happened?

On August 25, 2004, a bow echo moved across southern Iowa. This set of images is interesting because of the "bookend vortex" apparent on the northern edge of the bow echo on the RADAR scans.
A bookend vortex is a local eddy caused by large variations in the wind field over a small area--for example, a SW wind at 150 km/h next to a SW wind of 30 km/h. This shear induces a vortex to form, and often the most severe wind damage (as well as occasional tornadoes) is seen here.
The bookend vortex got its name because often there is a pair of them, one on the south side and one on the north side of the bow echo, both of them essentially bookending the bow echo itself.
The first bookend vortex can be seen from the beginning of the loops, with the vortex breaking off and moving just south of Des Moines. The second becomes apparent at about 18:31Z, with the curl becoming evident on the north edge of the bow echo. The vortices are visible on all three base reflectivity loops presented here.
The storm-relative velocity fields show this shear zone very strongly, and in fact, the signature is very similar to that of a tornado. For this reason, bow echoes often have tornado warnings associated with them.

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RADAR images

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