Dr. Jeff Masters' WunderBlog

Tornado season: why so active?

Posted 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 (pluto)

Automobiles at car dealership in Gallatin, Tennessee, resulting from toranado which hit April 7, 2006.

Tonado North of Winona Mississippi (StonevilleHead)

Taken from McDonald's on Highway 82 at 5:35pm on April 7, 2006

Categories: Tornado

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101. HurricaneKing 9:37 PM GMT on April 11, 2006
CARIBBEAN/GREATER ANTILLES/SERN MEXICO... THE CAN/GFS ARE BACK TO FORMING A SUBTROPICAL/TROPICAL LOW NR HISPANIOLA DURING THE SHORT RANGE PD AND TRACKING IT WWD THROUGH THE WRN CARIBBEAN AND SRN GULF OF MEXICO. SINCE AN UPR CYC IS CLOSING OFF ATTM NR CUBA...A SFC LOW IS PSBL. FOR NOW...PER COORDINATION W/TAFB...HAVE INTRODUCED AN ELY WAVE MOVG THRU THE WRN CARIBBEAN AND SRN GULF OF MEXICO. THE CANADIAN FCST A SIMILAR SITUATION BACK IN FEBRUARY...AND NOTHING MORE THAN A SFC TROUGH DVLPD. SEE THE LATEST DISCUSSIONS FROM TAFB/TPC ON THIS SYS. ROTH/CLARK Member Since: July 6, 2005 Posts: 71 Comments: 2430

102. ForecasterColby 9:42 PM GMT on April 11, 2006
Well, I'll be darned! Shear's too high ATM, but it does look like it has more chance to develop into a TC than my knee hair.

103. ForecasterColby 9:44 PM GMT on April 11, 2006
Ack, what am I saying? I was reading the stupid shear map wrong pokes self in eye We might see something kick up out of this.

104. rwdobson 9:51 PM GMT on April 11, 2006
boy it sure doesn't look impressive on the water vapor loop. looks like there is plenty of dry air and shear around. Member Since: June 12, 2002 Posts: 0 Comments: 1574

106. rwdobson 10:01 PM GMT on April 11, 2006
it won't. even if it forms a TD, which is unlikely, it won't become a named storm. Member Since: June 12, 2002 Posts: 0 Comments: 1574

107. globalize 10:31 PM GMT on April 11, 2006
Don't need a knee hair mental image, thanks. Member Since: August 30, 2005 Posts: 0 Comments: 1150

109. ForecasterColby 10:55 PM GMT on April 11, 2006
I, also, get that error. Mail WunderYakuza about it, he's the site's technical guy.

110. DAVIDKRZW 11:13 PM GMT on April 11, 2006
my blog is update

112. RL3AO 11:35 PM GMT on April 11, 2006
I hope this doesnt add gas to the fire...

113. globalize 11:47 PM GMT on April 11, 2006
Michael STL..that's probably because I don't have a blog site..just like to visit this one now and then. I have a question about ocean water salinity and acidity and its relationship to heat absorption. I may do a few personal experiments in a week or two in the Keys. Somebody clue me in. Member Since: August 30, 2005 Posts: 0 Comments: 1150

114. DAVIDKRZW 11:54 PM GMT on April 11, 2006
where is dr m update to day any way

116. Snowfire 1:27 AM GMT on April 12, 2006
With shear like this, I can't understand all the excitement. Until it drops below about 20kt where it matters, I wouldn't take all this too seriously. Member Since: June 29, 2005 Posts: 24 Comments: 300

118. MZT 2:30 AM GMT on April 12, 2006
If anything shows up in these conditions, though - it'd be another shallow - 'Greek' type storm. Member Since: September 24, 2005 Posts: 0 Comments: 775

120. oriondarkwood 2:07 PM GMT on April 12, 2006
Just posted my first hurricane prediction on my blog: http://www.wunderground.com/blog/oriondarkwood/show.html Member Since: July 5, 2004 Posts: 51 Comments: 41

121. caribbeantracker01 9:13 PM GMT on April 01, 2012
Quoting Snowfire: With shear like this, I can't understand all the excitement. Until it drops below about 20kt where it matters, I wouldn't take all this too seriously. but this says 2008 Member Since: May 21, 2008 Posts: 0 Comments: 442

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