ETA MODEL FORECAST SENSITIVITY TO INITIAL CONDITIONS FOR THE 22/23 FEB 98 FLORIDA TORNADOES CASE Michael E. Baldwin NOAA/NCEP/EMC/General Sciences Corp., Camp Springs, MD Operational forecasts from the Early (32km) and Meso (29km) Eta Models provided significantly different guidance for the tornadic thunderstorms that hit central Florida around 0300-0600 UTC 23 Feb 98. The Meso Eta, beginning at 0300 UTC 22 Feb 98, predicted strong upward vertical velocities and heavy precipitation in the vicinity of these thunderstorms at the time they were observed. However, the Early Eta, which began at 0000 UTC 22 Feb 98, forecasted weaker vertical motions and significantly lighter precipitation. Both models started out with a strong disturbance off of the Texas coast, with a coherent region of strong upward vertical motion associated with elevated convection. Evidence of this elevated convection can be found in the 00h Early Eta sounding near Houston, TX, which showed a nearly-saturated layer between 850-700mb. The disturbance appeared as a line of precipitation that moved across the Gulf of Mexico during the forecast. Although the forecast profiles were convectively unstable, the bulk of the heaviest precipitation was explicitly predicted by the model's cloud physics routines, rather than the model's convective parameterization. The convective scheme is designed to allow grid-scale (explicit) precipitation to take over once the vertical motion is strong enough to force grid-scale saturation. The Meso Eta maintained this region of intense upward motion as it moved across the Gulf of Mexico, while the Early Eta weakened it considerably after ~15h into the forecast. The model systems contained differences in horizontal resolution, domain size, boundary conditions, data assimilation techniques, and forecast model code. Experiments were performed to examine the impact of these differences on the forecasts for this case. The two models had slightly different formulations at the time of these forecasts. An experiment was performed with the Early Eta model code running at 29km with Meso Eta initial conditions. This produced nearly identical results as the operational Meso Eta forecast, therefore, these model differences do not appear to be the cause of the forecast differences. It is also possible that differences in domain size and/or boundary conditions were important in this case. The southern boundary for the Meso Eta was approximately 500km from the Gulf of Mexico, while in the Early Eta that boundary was approximately 1000km away from the Gulf. Boundary conditions for the Meso Eta were obtained by interpolating the 3-h old Aviation run of the Global Spectral Model (AVN), while in the Early Eta, a 12-h old AVN forecast provided the boundary conditions. The impact of these differences has not been explicitly tested. The initial conditions for the Early Eta were provided by an updated Eta Data Assimilation System (EDAS) which included 3D-variational (3DVAR) analyses, rather than the optimal interpolation (OI) analysis scheme which was used in the Meso Eta EDAS. One of the important advances in the variational analysis system is the ability to assimilate "non-traditional" data sources, such as GOES precipitable water estimates. These data were included in the Early Eta EDAS but not in the Meso OI analyses, other such data excluded from the Meso Eta include aircraft temperatures, VAD wind profiles from WSR-88D, SSMI sea-surface wind speeds, and surface winds over land. The initial precipitable water fields indicate the Early Eta was considerably drier over the southern Gulf of Mexico than the Meso Eta, most likely due to the inclusion of GOES precipitable water data in the Early Eta. This dry air advected northward during the forecast and interacted with the line of precipitation at the time it weakened in the Early Eta. An experimental rerun of the Early Eta EDAS without GOES-PW data confirmed this result, the ensuing forecast maintained the line of heavy grid-scale precipitation as it moved eastward across the Gulf, reaching Florida around 0300 UTC 23 Feb 98. Due to the complexity of NWP systems, it is often very difficult to link forecast discrepancies to specific model components or particular characteristics of initial conditions. Therefore, these results should not be considered as a suggestion that GOES-PW data be removed from the operational EDAS. The drier air found in the GOES-PW observations may have helped to maintain the severe convection over Florida. Typically, mid-level dry air is a prerequisite for strong downdrafts and the creation of low-level baroclinicity, which may be an important factor in the development of tornadoes. Forecast fields besides QPF and omega were not examined in detail and may prove to have verified better in the Early Eta than in the Meso. However, these results do show the sensitivity of Eta Model forecasts to different initial moisutre analyses and demonstrate that great care must be taken when assimilating new sources of data.