What La Nina means for the severe weather season
La Niña is a coupled ocean-atmosphere phenomenon similar to El Niño. During a period of La Niña, the sea surface temperature across the equatorial Eastern Central Pacific Ocean will be lower than normal by 0.5 °C. In the United States, an episode of La Niña is defined as a period of at least 5 months of La Niña conditions.
So far during the end of 2007 and early 2008, La Nina has been extremely strong, likely the cuase of our extreme weather patterns so far this Winter. La Nina traditionally causes the Great Lakes, Plains and Midwest to experience much colder and snowier winters. With La Nina currently strenghening, I still expect and that is why I have been forecasting the next 2-4 weeks to be the worst we have seen so far this winter.
Now wth La Nina remaining strong through the Spring, I expect a volatile severe weather season across the Eastern half of the United States, at least through May.
We have already seen unprecidented severe weather outbreaks this year with the major tornado outbreak across portions of Tenn, Kentucky and Arkansas back in February and with Indiana seeing tornadoes in both the months of January and February for the first time in decades.
La Nina typically provides a split flow jet-stream, with the Northern branch and the Pacific/Southern Branch. This type of pattern set-up causes strong storms to develop along the Southern branch and clash with colder air and often times phase with systems on the Northern branch of the jet-stream. This in turn causes significant severe weather outbreaks out ahead of the weather systems. So get ready as severe weather season is almost here.