The March 1993 superstorm cyclogenesis : Incipient phase synoptic- and convective-scale flow interaction and model performance

The incipient stages of the 12-14 March 1993 "superstorm" (SS93) cyclogenesis over the Gulf of Mexico are examined. Noteworthy aspects of SS93 include it is the deepest extratropical cyclone ever observed over the Gulf of Mexico during the 1957-96 period, and existing operational prediction models performed poorly in simulating the incipient cyclogenesis over the northwestern Gulf of Mexico. A dynamic-tropopause (DT) analysis shows that SS93 is triggered by a potent potential vorticity (PV) anomaly as it crosses extreme northern Mexico and approaches the Gulf of Mexico. The low-level environment over the western Gulf of Mexico is warmed, moistened, and destabilized by a persistent southerly flow ahead of the approaching PV anomaly. Ascent and a lowering of the DT (associated with a lowering of the potential temperature) ahead of the PV anomaly contributes to further destabilization that is realized in the form of a massive convective outbreak. An examination of the National Centers for Environmental Prediction (NCEP) Medium Range Forecast (MRF) model-initialized fields after convection begins shows that the MRF does not fully resolve important features of the potential temperature, pressure, and wind fields on the DT in the incipient SS93 environment. Similarly, the NCEP MRF 12-h/24-h forecasts verifying 1200 UTC 12 March and 0000 UTC 13 March are unable to simulate sufficient deep convection over the Gulf of Mexico, low-level PV growth in the incipient storm environment, high-level PV destruction and the associated warming and lifting of the DT over and downshear of the developing storm. Given that the MRF-initialized fields possess sufficient conditional instability, moisture, and ascent to trigger widespread deep convection, the poorly forecast incipient SS93 development appears to be associated with the failure of the model cumulus parameterization scheme. A comparison of the MRF forecasts with selected forecast fields derived from the European Centre for Medium-Range Weather Forecasts operational model supports this interpretation.