Asheville Meeting

Wednesday, 10 May 2000
Development of a low pressure index as a proxy for dry season severe weather in florida and its relationship with ENSO
Bartlett C. Hagemeyer, NOAA/NWSFO, Melbourne, FL
In a series of studies, Hagemeyer developed a conceptual model of the favorable synoptic environment for significant Florida tornadoes and demonstrated the importance of the jet stream. Subsequently, the author noted striking similarities between the location of the mean jet streams over Florida in the strong El Nino's of 1982-83 and 1997-98, and their subsequent impact on Florida's dry season severe weather (November - April). The most F2 and greater tornadoes in Florida history occurred during the dry season of 1982-83, and the second greatest number occurred in the 1997-98 dry season. During the strong La Nina's of 1988-89 and 1998-99 the mean jet stream was not present over Florida and only one F2 tornado occurred in 1988-89 and none occurred in 1998-99.

In the media frenzy following historic tornado outbreaks in early 1998 it was often suggested that El Nino was to blame. These media questions resulted in a backlash by many experts in the field who recoiled at the thought of linking El Nino to any particular tornado event. However, lost in the debate was the observation that it is quite likely there is a strong relationship between ENSO and atmospheric conditions favorable for significant tornadogenesis on a seasonal basis.

To test this theory, an extensive analysis of the relationship between ENSO data and dry season weather parameters known to favor tornado development in Florida was completed. Sea surface temperature , 250 Mb T, 200 Mb U and OLR anomaly values in the NINO 3.4 area were correlated with 250 Mb U, Mean Sea Level Pressure (MSLP), and 850 mb height anomaly values (1958-1999) in a grid encompassing the state of Florida. An index was also developed to measure the number and intensity of dry season low pressure systems affecting Florida and correlated with NINO 3.4. The seasonal number of tornado days, number of F2 and greater tornado days, and total tornado dollar losses were correlated with Nino 3.4.

Results showed significant correlation between dry season values of 250 mb U anomaly, Low Pressure Index, and tornado damage for Florida and NINO 3.4. The resulting multiple regression scheme using observed and predicted values of NINO 3.4 (NCEP CCA predictions) for May through April to forecast the dry season variables showed significant skill, particularly with regard to 250 mb U anomaly and the Low Pressure Index, which are effective proxies for tornado potential.

By the time of this conference the first full season of experimental forecasting will be complete and results will be presented. While there are certain risks in attempting seasonal severe weather forecasts, there are also definite benefits. An unpublished, qualitative seasonal severe weather outlook was done for Florida by the author for the 97-98 dry season and was successful in better preparing the NWS and State and County agencies for an unusually active season. The issues of the impacts of ENSO will only grow in the future and this study attempts to advance the ability to provide detailed regional seasonal forecasts in a more rigorous manner.

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