Tropical cyclone tornadoes: Synoptic scale influences and forecasting applications

Tropical cyclone (TC) tornado locations are normally referenced relative to their geographic locations or their storm-motion relative azimuth and distance (e.g., northeast region or forward-right regions being favored). Although favored "quadrants" are often referred to, climatologies show tornado locations to be clustered in larger regions (~140 degrees of azimuth). We have found that their locations relative to the background, deep-layer (200-850 hPa) environmental shear vector may be a better constraint. A very large fraction occurs within ~ +/-50 degrees of the downshear direction. This is especially true for environmental shears > 20 kt, shown in the figure. The attached figure compares tornado locations for the different reference frames, with the upward direction in the azimuthal plots pointing toward north, down-shear, and forward-motion, respectively. The color-coding distinguishes tornadoes by F-scale damage rating. This 200-850 hPa environmental shear vector (from GFS, current and forecast values out to 120 h) is routinely available in operations from the Statistical Hurricane Intensity Prediction Scheme (SHIPS) text file of predictor values, and therefore should be quick and easy for forecasters to consider. Some other parameters in that text file also appear promising for TC tornado applications. The D200 (divergence at 200 hPa, averaged around the TC after removing the vortex) tends to be substantially higher during time periods when TC tornadoes are reported than in other time periods when a TC is over (or very near) the U.S. For time periods having several TC tornadoes (at least 5 in a 6-hour period), the mean D200 value is almost one standard deviation above the U.S. sample mean.