Florida's Unprecedented Dry Season "Significant Tornado Drought"

After the recent devastating tornado outbreaks, the discussion of the veracity and utility of developing regionalized seasonal tornado forecasts has resumed. The Florida dry season severe weather outlook is the only such forecast routinely made of which the author is aware. Since 2007 there has been an unprecedented minimum in significant tornadoes (EF2 and greater) during the Florida Dry Season (November through April). Since the Groundhog Day Tornado Outbreak of February 2, 2007 that killed 21 people in central Florida there has been only one dry season tornado death in Florida, and just two significant tornadoes. Those two EF2 tornadoes occurred on March 7, 2008 and killed one person (falling tree). Indeed, the 1,571 consecutive days between the tornado deaths of 7 March 2008 and 24 June 2012 (the latter associated with Tropical Storm Debby) is the longest period without a tornado death in Florida since before 1925! Population bias could not be a factor in this “significant tornado drought,” as the Florida population has increased by over 17 million people since 1925.

The author began producing experimental outlooks of dry season storminess (severe weather potential) for Florida in 1999, and has produced official outlooks since the 2007-2008 dry season. The outlooks are primarily based on the forecasts of the ENSO phase. Typically El Niño dry seasons are colder, wetter, and stormier than normal with an increased risk of severe weather, while La Niña dry seasons are drier and warmer than normal, with an increased risk of drought and wildfire, but a lower than normal risk of severe weather.

The five dry season during this extended period without significant tornadoes where characterized by two strong La Niñas, two weak La Niñas and one strong El Niño. The severe weather outlooks issued in the months prior to the start of these seasons were correct in predicting below normal activity four times and over-forecast severe potential in the one El Niño season. It is useful to review this case study of real-time forecasting of unusual seasonal tornado activity in Florida for insight into the technical challenges of their development and the societal challenges presented by their potential adoption and use by decision-makers. A discussion of the verification of these long-lead forecasts and the relationship with intra- and inter-seasonal climatic variability will be presented.