The Relationship of Precursory Precipitation and Synoptic-Scale Mid-Tropospheric Flow Patterns to Spring Tornado Activity in Oklahoma

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Tuesday, 4 February 2014
Hall C3 (The Georgia World Congress Center )
Levi P. Cowan, University of Alaska Fairbanks, Fairbanks, AK; and M. D. Austin, J. Kurtz, M. B. Scotten, and M. Day

Winter precipitation and 500 hPa geopotential height are analyzed as potential precursory predictors of spring tornado activity in Oklahoma (OK). The Storm Prediction Center (SPC) tornado database is used to calculate tornado days for each of the nine climate divisions in OK. Using daily precipitation totals from the Climate Prediction Center U.S. Unified Precipitation dataset, Dec-Feb accumulated precipitation is correlated with Mar-Jun tornado days for each climate division. Insignificant correlations are found for all climate divisions, and statistical tests affirm that there is no significant difference in OK tornadic activity following wet versus dry winters. The synoptic-scale variability in the Rossby wave pattern over the United States (US) associated with OK tornado activity may explain the ineffectiveness of precursory precipitation as a predictor, but also suggests qualitatively that it stems from a driving synoptic pattern in relation to subsequent spring tornado activity. To investigate this, geopotential height at 500 hPa (Z500) from NCEP/NCAR reanalysis is also examined. A statistically significant and temporally consistent relationship is found between Z500 in the Pacific Northwest region and Mar-Jun statewide tornado days during 1981-2010 when Z500 is averaged over the preceding 4-month period (Nov-Feb). Persistent troughing (ridging) over the northwestern US and southwestern Canada during the winter is found to shift southeastward into the Rocky Mountains and enhance (suppress) OK tornado activity during the subsequent spring. This relationship strengthens as lead time is decreased, and may provide a method for predicting overall tornado activity in OK on a seasonal time scale. on 7-30-2013-->