Monday, 27 October 2008
Madison Ballroom (Hilton DeSoto)
On May 24, 2004, a storm that formed near Hastings, Nebraska just southeast of the triple-point of two synoptic scale boundaries (cold and quasi-stationary warm fronts) and a surging dry line, moved into a region of rich low level and deep layer shear. This storm rapidly developed a low-level mesocyclone as it moved from southeastern Adams County into Thayer County, Nebraska and then underwent at least four reorganization cycles, producing multiple tornadoes during each. The Storm Prediction Center (SPC) has documented a total of 20 tornadoes associated with this storm in Adams and Thayer Counties, Nebraska, and Republic County, Kansas. Although there were no obvious boundary interactions as the Thayer County storm underwent cyclic reorganization and tornado production, NCEP reanalyses and subjective examination of the sounding, hodograph and observational data showed that (a) 0-6 km deep layer shear in southern Nebraska was great and of a magnitude that consistent with supercell development; (b) background 0-1 km shear values in southern Nebraska were exceedingly great, and consistent with those documented across strong boundaries; (c) storm motion carried the initial storm into an axis of instability within which values of surface based CAPE values exceeded 3500 J/kg southeast of Hastings, south of the warm front along the Interstate 80, and extending into north-central Kansas, where values of up to 4500 J/kg existed; and, (d) convective inhibition was strong south of the Kansas border, but was absent over southeastern Nebraska, just north of the strong capping inversion associated with the Elevated Mixed Layer. The proximity hodograph shows a sickle shape with a straight segment of long length in the lowest 1 km, consistent with the hodograph characteristics observed in some documented major cyclic supercell outbreaks, including that of 3 May 1999 in Oklahoma. The authors believe that this case represents a relatively unusual example in which the synoptic scale features not only provided the general thermodynamic and shear environment in which the Thayer County storm developed, but also produced the essential low level shear normally found in association with mesoscale or smaller features in cyclic tornadic supercell outbreaks. Proximity hodographs, soundings and high resolution radar imagery are presented to support these contentions and findings.
Supplementary URL: http://tornado.sfsu.edu/Activities/SLS_Conference_2008/index.html
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