THE MARCH 1993 SUPERSTORM CYCLOGENESIS - INCIPIENT PHASE SYNOPTIC-SCALE AND CONVECTIVE-SCALE FLOW INTERACTION AND MODEL PERFORMANCE

The incipient stages of the 12-14 March 1993 ''superstorm'' (SS93) cyc logenesis over the Gulf of Mexico are examined. Noteworthy aspects of SS93 include 1) it is the deepest extratropical cyclone ever observed over the Gulf of Mexico during the 1957-96 period, and 2) existing ope rational prediction models performed poorly in simulating the incipien t cyclogenesis over the northwestern Gulf of Mexico. A dynamic-tropopa use (DT) analysis shows that SS93 is triggered by a potent potential v orticity (PV) anomaly as it crosses extreme northern Mexico and approa ches the Gulf of Mexico. The low-level environment over the western Gu lf of Mexico is warmed, moistened, and destabilized by a persistent so utherly flow ahead of the approaching PV anomaly. Ascent and a lowerin g 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 Ra nge Forecast (MRF) model-initialized fields after convection begins sh ows that the MRF does not fully resolve important features of the pote ntial temperature, pressure, and wind fields on the DT in the incipien t SS93 environment. Similarly, the NCEP MRF 12-h/24-h forecasts verify ing 1200 UTC 12 March and 0000 UTC 13 March are unable to simulate suf ficient deep convection over the Gulf of Mexico, low-level PV growth i n the incipient storm environment, high-level PV destruction and the a ssociated warming and lifting of the DT over and downshear of the deve loping storm. Given that the MRF-initialized fields possess sufficient conditional instability, moisture, and ascent to trigger widespread d eep convection, the poorly forecast incipient SS93 development appears to be associated with the failure of the model cumulus parameterizati on scheme. A comparison of the MRF forecasts with selected forecast fi elds derived from the European Centre for Medium-Range Weather Forecas ts operational model supports this interpretation.