May 22 - July 30
The Relationship of Precursory Precipitation and Synoptic-Scale Mid-Tropospheric Flow Patterns to Spring Tornado Activity in Oklahoma
Levi Cowan, Marcus Austin, Jonathan Kurtz, Matthew Day, and Michael Scotten
What is already known:
- Seasonal tornado forecasts are not yet offered in either an operational or an experimental capacity.
- Skillful seasonal outlooks could be very beneficial to emergency managers, operational forecasters, and the public.
- Little research has been focused on developing seasonal-scale predictive methods.
What this study adds:
- Local precipitation and jet stream configuration in the winter are investigated as potential predictors of spring tornado activity in Oklahoma.
- Local wintertime precipitation is found to be a poor predictor of springtime tornado activity in Oklahoma.
- The Nov-Feb 500 hPa geopotential height anomaly in the Pacific Northwest region is found to be useful for predicting subsequent Mar-Jun tornado days in Oklahoma. This relationship could contribute to the development of seasonal tornado forecasts.
Abstract:
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 precursory precipitation could be a statistically significant predictor of tornado activity in other regions of the US (Shepherd et al. 2009). 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.