Thursday, 9 November 2006
Pre-Convene Space (Adam's Mark Hotel)
At 4:37 PM July 7, 2004, a backpacker in the Rockwell Pass area of Sequoia National Park in the southern Sierra Nevada, California, photographed the development of a tornado. Since the elevation of Rockwell Pass is approximately 4000 m (12000 ft), we believe that this is the highest elevation tornado ever photographed and documented in the United States. Evidence is presented that the tornado was mesocyclone-induced. Photographs indicate that the tornado was associated with cloud base rotational features consistent with those observed with supercell thunderstorms and also show severe hail, both falling and on the ground. Other photographs of the distant storm from the Highway 395 corridor show a crisp anvil backsheared to the west-southwest and flaring northeastward with an overshooting top near the tornado genesis area. Since the nearest WSR-88D radar in Hanford, CA (KHNX) was topographically blocked from observing this storm, we have assembled documentation on the possible supercell nature of this storm by using radar imagery from Edwards Air Force Base (KEYX), and a proximity sounding and hodograph reconstructed using NSHARP and upper air information provided from NCEP reanalyses and Vandenberg Air Force Base (VBG). The thermodynamic profile was favorable for deep convection in the monsoonal flow around a mid and upper tropospheric ridge located over southern Arizona. Deep layer shear ahead of a progressing disturbance moving across Southern California appeared not to be adequate for supercells in the southern Sierra Nevada. However, both effective deep layer and low level shear were greatly augmented by upvalley (southeasterly flow) in the valleys of the eastern Sierra Nevada on the afternoon of 7 July. The estimated curvature of low level hodograph (based at 3500 m) and observed storm motion (360o), 2 m s-1 produced storm relative helicity values of around 180 m2 s-2 consistent with values observed for tornadic supercells.
Supplementary URL: http://tornado.sfsu.edu/Geosciences/Geosciences_Docs/Activities/SLS_Conference_2006/index.html
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