The purpose of this article is to present simulation results of sub-tropical storm Allison using different combinations of microphysics, cumulus, and boundary layer parameterizations available in the MM5 numerical model and compare them with observations. Allison was a long-lived, early-season tropical storm that produced exceptionally heavy rain over a widespread area from Texas eastward to Florida then up the eastern seaboard of the United States in June of 2001. Allison went through multi-stages of structural evolution over its two-week life span. The focus of this article is on its “sub-tropical” stage as it moved east-northeast across the land mass of Louisiana and southern Mississippi. Surface stations indicated that the central pressure of Allison was falling while animation of Doppler radar images depicted the formation of a squall line on the southeast flank of the cyclone followed by the emergence of an eye-like feature near the cyclone center. Given these observations, numerical simulations using the mesoscale model MM5 were carried out. The model was set up with two nested domains at 81km and 27km horizontal grid spacings and 23 vertical levels. Various combinations of microphysics, cumulus, and boundary layer parameterizations were attempted. Preliminary results indicated that the Grell cumulus scheme combined with the MRF boundary layer scheme produced the best overall sea-level pressure pattern with a well-defined hook-shaped rainband around the cyclone’s east and southeast flank. Other physics combinations produced either disorganized or, at best, an ill-defined line of vertical motion to the south and east. Details on these simulation results will be presented at the conference.