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 most cases, forecast tracks of these systems at the point of landfall were reasonably accurate. However, 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. As such, an investigation as to the mechanisms responsible for this shift in forecast behavior is undertaken.

Preliminary results suggest the primary mechanism responsible for this dramatic change in forecasts centers around an extratropical low pressure system centered several hundred kilometers to the to the north-northwest of Hurricane Rita. The intensity of this system was almost universally underforecast by the numerical guidance. 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. Particular attention will be given to the unusual behavior of overprediction of the diabatically induced ridging.