Analysis of estimated environments for 2004 and 2005 severe convective storm reports

Analysis of the mesoscale environments associated with observed severe convective storms can provide key insights into the character and predictability of severe storms, and the challenges to achieving successful forecasts. The NOAA Storm Prediction Center has begun efforts to create a database of severe storm environments for each severe weather report and to analyze these data with the goal of improving national severe weather forecasts. This portion of the broader effort focuses on analysis of estimated environments for all severe convective storms reported during the two year period 2004 and 2005.

The estimated environment for each severe report is based on hourly 40km horizontal resolution RUC analysis data above the surface combined with objectively analyzed surface observations using RUC surface conditions as a first guess. These grid point data are analyzed for a variety of kinematic, thermodynamic, and severe weather focused diagnostic fields using the NSHARP sounding analysis program. Environmental conditions are assigned from the nearest analysis grid point at the closest hourly time prior to the observed severe weather. For the two year period, this database contains environmental estimates for over 50,000 reports. The data are then stratified and analyzed based on report intensity, location, and environmental characteristics.

Preliminary results for the 25,000 reports observed during 2005 illuminate the diverse nature of the severe storm environmental parameter space. Bi-monthly and annual average continental United States plots of mean environmental conditions associated with subclasses of severe weather reports are used to more completely document the seasonally varying regional severe weather environments such as the southeast U.S. cool season low-CAPE / high shear tornado events, low-CAPE / high-shear events associated with warm season tropical systems, and low shear / high-CAPE “pulse-severe” warm season events. While statistical plots and national maps of reports associated with specific environments reaffirm some previous relationships, they also reveal new complexities and limitations of this knowledge when applied to forecasting severe storms nationwide. For example, the results indicate a strong association between significant tornadoes and environments with 100 mb mixed-layer CAPE less than 1200 Jkg^-1, 0-6 km shear greater than 18ms^-1, 0-1 km shear greater than 10ms^-1, and LCL's less than 1000m. However, this multi-variant relationship becomes more complex for significant tornadoes occurring in environments with CAPE greater than 1500 Jkg^-1, which are characterized by a much wider range of shear and LCL values.