An Analysis of Hurricane Opal’s Forecast Track Errors Using Quasigeostrophic Potential Vorticity Inversion

Abstract

Hurricane Opal?s landfall in October 1995 forms the basis of a serious hurricane forecast problem?the potential for hurricane conditions over land with insufficient warning time. Official National Hurricane Center (NHC, a division of the Tropical Prediction Center) forecasts predicted landfall and passage inland over the eastern United States at a later time than observed because of underestimation of the northward component of the steering flow by the National Centers for Environmental Prediction?s (NCEP) operational models and other hurricane track models. The goal of this paper is to isolate the cause of the poor forecast of meridional storm motion in NCEP?s early Eta Model by using quasigeostrophic potential vorticity (QGPV) inversion. QGPV inversion permits decomposition of the steering flow into contributions from different synoptic-scale features. The inversion procedure is applied to the Eta analysis and 48-h Eta forecast valid at 1200 UTC 5 October 1995. Analyses from the European Centre for Medium-Range Weather Forecasts form an independent comparison for the Eta Model forecasts and analyses. An extratropical cyclone to the northwest of Opal and a synoptic-scale ridge to the east are identified as being major contributors to the steering flow. The Eta Model underpredicted the intensity of the ridge positioned immediately downstream of the storm, resulting in a corresponding underprediction of the meridional steering flow by 5 m s?1. It is hypothesized that the Eta Model underforecasted the magnitude and extent of Opal?s outflow, and subsequent interaction with the downstream ridge, largely due to the model?s inability to correctly represent the convection associated with the hurricane in both the analyses and forecasts. Underforecasted upper-tropospheric temperatures downstream of Opal are consistent with this hypothesis. Accurate initialization of the model in the region containing Opal may have been hampered by the dearth of upper-air data over the Gulf of Mexico. Failure to properly resolve the hurricane is hypothesized to have resulted in the underforecasting of the downstream ridge and its associated steering flow.