Forecast errors associated with Hurricane Rita (2005)

The 2005 hurricane season in the Atlantic Basin broke numerous records for frequency, intensity, and duration of storms. Seven storms reached major hurricane status, with four storms making landfall along the United States coastline at category 3 intensity or higher. In the case of Hurricane Rita, a dramatic shift in the post-landfall forecast track occurred about 24 h before landfall on September 22. Previous to this time, model consensus was for the storm to stall over the Texarkana region for several days, resulting in the expectation of heavy flooding in this region. In contrast to those forecasts, the bulk of the precipitation associated with Hurricane Rita began to accelerate poleward after landfall, impacting regions of southern Canada and the northeastern United States within 36 to 48 h after landfall.

Preliminary results suggest the primary mechanism responsible for this dramatic change in forecasts centers around a positive potential vorticity (PV) anomaly to the west-northwest of Rita. The strength of this feature was under forecast by the majority of numerical forecasts, perhaps due the presence of convection in the model forecasts. Concomitantly, the upper-level ridging associated with Rita was over forecast, perhaps due to the failure of the numerical guidance to capture the response to the weakening of Hurricane Rita prior to landfall. The combined effects of these two errors was to produce a forecast of weaker steering flow over the storm versus verification. Numerical simulations an PV inversions will be utilized to quantify the impacts of these features.