The 2009–2010 El Niño and Florida dry season severe weather—a reality check for the limits of predictability

Extratropical storms can produce various societal impacts during Florida's dry season (1 November – 30 April), including deadly tornado outbreaks, hailstorms, damaging thunderstorm winds, coastal flooding, hazardous marine conditions, strong gradient winds, and both beneficial and flooding rains. The lack of extratropical storms can cause drought and increased wildfire risk.

The author has documented a very strong relationship between the phase of the EL Niño Southern Oscillation (ENSO) and the number of extratropical storms affecting Florida during the dry season. He has made forecasts of the number of significant dry season extratropical storms based on ENSO phase since 2002 with considerable success.

While there is a strong correlation between dry season extratropical storms and the occurrence of severe weather such as damaging convective wind gusts, lightning and hail, it is the frequency of tornadoes that is of most concern. The two deadliest Florida tornado outbreaks, responsible for 63 deaths, occurred during the recent El Niño winters of 1997-98 and 2006-07.

Although explicit dry season forecasts of severe weather occurrence are not made, the Florida NWS offices use the forecast of an El Niño -- and the expected increase in storminess -- to raise awareness of, and preparedness for, tornadoes in their outreach activities. In the summer and fall of 2009 the Florida NWS offices, in partnership with emergency managers and the media, conducted an unprecedented campaign to prepare the state for the expected increase in severe weather during the upcoming dry season. The intent was to avoid the disasters of the previous El Niño winters.

As expected, the number of extratropical storms affecting Florida during the 2009-2010 El Niño was well above normal. Indeed, the 2009-10 El Niño winter tied the record of 20 extratropical storms set during the 1997-1998 El Niño. While the forecast of the number of extratropical storms was accurate, the tornadic activity was much below normal, with only 19 weak tornadoes reported. By contrast, 71 tornadoes, many of which were strong and violent, occurred during the 1997-98 El Niño dry season. The 2009-10 El Niño dry season was the first since 1972-73 without any severe weather-related fatalities in Florida.

Conventional thinking has been that sufficient warm, moist low-level air would be available during an El Niño winter to produce several significant severe weather outbreaks. The development of a persistent negative Arctic Oscillation and blocking pattern in late 2009 and early 2010 was responsible for significantly modifying the typical impact of El Niño on Florida through frequent incursions of cold continental air deep into the southern latitudes, resulting in cold and wet conditions. Indeed, there were two deaths attributed to exposure to cold in Florida. As a result of the deep penetration of the cold air masses, the warm sectors of the majority of the developing extratropical storms that affected Florida were unable to supply enough low- level moisture and instability to fuel the deep convection needed to produce significant severe weather.

The influence of the 2009-10 El Niño on Florida, as measured in jet stream position and extratropical storm frequency and magnitude, actually exceeded that of the 1997-98 El Niño. However, the persistent Arctic Oscillation pattern in 2009-10 likely spared Florida from the effects of violent tornadoes and instead brought record cold spells to the state. The dominance of the Arctic Oscillation, while rare, raises significant questions regarding the predictability of seasonal impacts based primarily on the ENSO phase and brings into sharp focus the need to improve the prediction of other intraseasonal oscillations such as the Arctic Oscillation, the North American Oscillation and the Pacific-North America pattern and related blocking phenomena.