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Tornado season: why so active?

By: Dr. Jeff Masters, 4:22 PM GMT on April 10, 2006

Friday's tornado outbreak in Tennessee featured at least 46 tornadoes across eight states. Three strong F3 tornadoes touched down (winds of 158 to 206 mph), including the Gallatin, Tennessee tornado that killed nine people and damaged or destroyed 700 to 900 homes in Sumner County. The April 2 outbreak had six F3 tornadoes among the 56 reported, and the March 9-13 outbreak had 11 F3 tornadoes and one F4 tornado among the 84 that touched down.

Figure 1. Departure of Sea Surface Temperature (SST) from normal in early April for 2005 and 2006. SSTs were near normal in 2005 in the Gulf of Mexico, but were about 1-2 degrees C warmer than normal this year. Other things of interest: the cool water signature of La Nina is obvious in the Eastern Pacific in 2006 but not 2005; water temperatures in the Caribbean and tropical Atlantic are cooler this year than last year, which may mean a delayed onset to this year's hurricane season compared to last year. Image credit: NOAA.

Why has this year's tornado season been so violent? According to NOAA,, "The difference this year is the abnormally warm temperatures and dry conditions during the winter throughout the southern and central United States that kept water temperatures warm in the Gulf of Mexico." If we look at a plot comparing this year and last year's change ino Sea Surface Temperature (SST) from normal (the anomalies, Figure 1), we see that the Gulf of Mexico is indeed significantly warmer this year. Plenty of warm moist air from the Gulf of Mexico certainly doesn't hurt, but the primary reason that this year's tornado season has been so bad is that we've had an unusually active and strong jet stream. The reasons for this are complicated and not well-understood, but have more to do with large scale global circulation patterns such the the presence of La Nina and the negative phase of the North Atlantic Oscillation (NAO) that has predominated this Spring.

The latest 2-week forecast from the GFS model shows that the jet stream is now expected to grow less active for the next 10 days, and no further major tornado outbreaks are expected during that period. A more active pattern may return late in the month, however.

Jeff Masters
Tornado Damage
Tornado Damage
Automobiles at car dealership in Gallatin, Tennessee, resulting from toranado which hit April 7, 2006.
Tonado North of Winona Mississippi
Tonado North of Winona Mississippi
Taken from McDonald's on Highway 82 at 5:35pm on April 7, 2006


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