Session 16A.2 Re-creation of historic Iowa EF-5 tornado environments using high-resolution workstation WRF output initialized with NCEP Reanalysis grids

Thursday, 30 October 2008: 1:45 PM
North & Center Ballroom (Hilton DeSoto)
Karl Jungbluth, NOAA/NWS, Des Moines, IA

Presentation PDF (2.5 MB)

Even though tornadoes occur regularly in Iowa and the upper Midwest, tornado outbreaks, especially those producing EF-4 or EF-5 damage, are extremely rare. This limits forecaster experience with the mesoscale environments that produced the high end tornado events. A number of disastrous F5 tornadoes occurred in Iowa and the upper Midwest in the days before real time mesoanalysis and high resolution numerical models made it possible to get a comprehensive understanding of their near-storm environment. In addition, much has been learned about the environmental conditions most relevant to high-end tornado development since these events occurred, and associated studies were undertaken. Example cases include the F5 Jordan, Iowa tornado on June 13, 1976 and the Charles City, Iowa tornado in 1968.

This paper explores the utility of using short-range high resolution workstation WRF output to paint a picture of the mesoscale conditions which preceded a number of F5 tornado events, or tornado outbreaks that included a tornado of at least F4 (now EF-4) intensity. NCEP reanalysis grids are used for initialization of the WRF model. Output is analyzed in much the same way that forecasters approach looking at a severe storm environments today, with emphasis upon instability and shear parameters in various layers. In addition, basic WRF output fields such as mean sea level pressure and 500-hPa heights are compared to historical analyses, serving as a double check to ensure that WRF output has produced a reasonable synoptic scale solution; one that can be downscaled to approximate the mesoscale severe storm environment.

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