Global Ensemble Forecast Tracks for Tropical Storm Debby

Operational-model track forecasts for Tropical Storm Debby (June 2012) diverged as much as 180 degrees, making the National Hurricane Center forecast particularly challenging. Forecast tracks from the members of the National Centers for Environmental Prediction Global Ensemble Forecast System (GEFS) were similarly divergent; some forecast Debby to turn westward, some forecast Debby to go northward, and others forecast Debby to go eastward. Part of the divergence in the operational models can be from differences in model code, but the divergence in the GEFS is due only to differences in initial conditions. To understand the meteorological reasons for this model divergence, GEFS forecasts for Debby were examined in detail. Debby formed in the south-central Gulf of Mexico and then became a tropical storm by 1200 UTC 23 June 2012. Careful examination of the GEFS tracks showed that those storms turning east were stronger and deeper than those turning west. The storm strength was measured by the mean sea level pressure at the center of the storms and the maximum 10-m wind speed. The correlation coefficients between these two quantities and the longitude 12 and 24 h later were between 0.60 and 0.90, for all GEFS model runs initialized between 0000 UTC 24 June and 1200 UTC 26 June 2012. Further investigation showed that the initial midlevel moisture (specific humidity at 850 and 700 hPa) determined the subsequent strength and depth of the member storms. This variation in depth changed the effective layer for steering currents, which determined the direction to which the storms moved.