An application of potential vorticity inversion to the position forecast problem of Hurricane Opal

Accurate forecasting of hurricane motion is required to prepare for hurricane landfall. The underpredicted acceleration of Hurricane Opal by the National Centers for Environmental Prediction's (NCEP) operational eta model during landfall and over-land passage is investigated using quasi-geostrophic potential vorticity (QGPV) inversion. We identify and explain model errors in the evironmental steering flow using a systematic procedure including removal of Opal's cyclonic circulation that builds upon previous work.; The eastern ridge associated with the anticyclonic QGPV was persistently forecasted too weak and too distant from Opal, though the forecast of this feature improved at shorter ranges. The anticyclonic contribution to the retrieved flow increased from {dollar}-{dollar}11% of the observed vector (inhibiting the motion) to 15% (aiding the motion). This increase of 3.8 m s{dollar}sp{lcub}-1{rcub}{dollar} is alone sufficient to explain the slow forecast.; The eta forecasts likely did not sufficiently resolve the advection downstream of storm outflow and subsequent ridge building. This error propagated through the forecast cycle and prevented phase-locking with the ridge and increased flow. Representation of the ridge improved following advection of the outflow over the more dense U.S. upper air network.; This study emphasizes the need for accurate upper-air analyses and offers a real-time application of QGPV inversion that decomposes the steering flow. The need for further research into the intimate relationship between storm intensity and subsequent storm track is stated.