November 25, 2009

El Niño Brings Increased Risk of Severe Weather Over Florida

Bart Hagemeyer, meteorologist in charge of the National Weather Service Forecast Office in Melbourne, FL, has demonstrated a clear connection between El Niño and winter tornado outbreaks in Central Florida. Once again tied to changes in the jet stream patterns, El Niño creates an environment of higher upper-level winds and increased vertical shear (winds changing direction with height), conditions which are necessary for the development of strong and long path tornadoes. The two of the deadliest tornado outbreaks in the history of Florida (February 1998 – 42 dead, February 2007 – 21 dead) both occurred during El Niño winters.

Cooler temperatures expected, but not necessarily severe freezes
In addition to the increased rain and storminess, El Niño is also associated with cooler winter temperatures over the entire Southeast. Winter temperatures generally run from 2 to 4 degrees F cooler than normal during El Niño winters. The greater number of cloudy and rainy days are primarily responsible for the cooler temperatures, as the change is seen more in afternoon high temperatures rather than morning lows. The cooler temperatures lead to greater accumulation of chill hours, which are necessary for proper reproduction and fruit setting for flowering fruits such as peaches, blueberries, and strawberries.

While El Niño is know to bring cooler temperatures, the risk of extreme cold weather or damaging freezes is actually lower than normal. We believe that the same jet stream patterns that lead to frequent storminess also tend to “block” the intrusions of frigid arctic air masses that usher in extreme low temperatures. Of the dozen or more catastrophic freezes to hit the Southeast in the last century or more, almost all happened when the Pacific Ocean temperatures were in the neutral range rather than El Niño or La Niña.

El Niño brings excess rain and storminess to parts of the Southeast
Normally beginning in November, El Niño affects the jet stream pattern in a manner that leads to frequent winter storms and frontal systems, cooler temperatures, cloudier skies, and much above average rainfall. Winter storms tend to develop along the Pacific jet stream and track into California. From there, the disturbances often slide along the Southern U.S., where the low pressure systems can tap moisture from the Gulf of Mexico and reintensify as they track along the northern Gulf Coast and up the Southeast Atlantic. The result is frequent winter storm passages, increased rainfall and cooler temperatures over much of the Southeast.

In December through March El Niño typically leads to rainfall 40% to 50% greater than normal over the peninsula of Florida and up to 30% greater than normal over coastal Alabama, South Georgia, and coastal North and South Carolina. El Niño’s influence is especially strong in the southern two-thirds of the state. The mountainous region of north Alabama, North Georgia and middle and east Tennessee is a transition zone. Depending on where the transition zone occurs this winter, the mountains will experience drier-than-normal, near-normal or wetter-than-normal conditions. These winter impacts of El Niño are generally stronger than any other time of year and more consistent among past El Niño events; therefore the winter forecast can be viewed as the most reliable compared to other times of the year.

For more detailed information on El Niño climate shifts in your particular county, please refer to the Climate Risk Tool at AgClimate: Climate Risk Tool

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