A very active severe storm season could be shaping up

Most people have heard of El Niño, and it’s affect on our weather.  La Niña is the exact opposite and refers to the waters of the equatorial Pacific turning cooler.  The temperature of the Pacific ocean currents can have significant affects on weather patterns for both North and South America.  These phases of ocean temperature changes come around every two to seven years.  

NOAA - National Centers for Environmental Prediction

La Nina can bring increased stormy weather and warmth to Kentucky

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Probably the greatest reason why our weather in the Ohio Valley is affected by these El Niño and La Niña events is because they alter the average location of the jet stream.  The jet stream is the strong upper level current of air that directs most of our storm systems where they go.  When the jet stream changes its path, it takes storms with it. 

During a typical El Niño the jet stream will spend a lot of time flowing west to east across the southern U.S. and the Gulf of Mexico.  This can keep the severe weather focused across the deep south.  It can also have the benefit of preventing many hurricanes from growing stronger and striking the U.S. because the strong jet stream winds blowing into the Gulf and Caribbean cause wind shear which may tear the tropical systems apart.  However, with La Niña firmly in place the jet stream typically moves northward and flows into the Plains and then toward the Great Lakes and northeastern regions of the U.S.  This allows the general storm track to move into the Tennessee and Ohio River valleys.  It can increase the occurrence of tornadoes and severe thunderstorms here in our area as well as heavy rain and flooding.  The map attached illustrates how the jet can focus the stormy weather in certain parts of the country.  You can see how this La Niña pattern typically would cause warmer air and stormy weather to clash here in this part of the country.

This Winter, we’ve been in a strong La Niña phase.  The effects are starting to weaken a little bit and go back toward “neutral” over the coming weeks.  Many of the forecast models keep us in neutral to slightly La Niña conditions throughout 2011.  The question is, coming out of one of the strongest Winter La Niña cycles we’ve seen in a long time, could the weather pattern turn into one that produces a lot of severe weather here later this Spring?  I recently read an article by one forecaster who noted that some of the worst tornado outbreaks in U.S. history, including the “Palm Sunday Outbreak”, the 1974 “Super Outbreak”, and the May 3rd, 1999 outbreak all occurred in Spring seasons that followed a strong La Niña.  All of these outbreaks were deadly and had significant impacts in the middle part of the country. Indeed, our recent pattern is very similar to those years and it's enough to get your attention.

We certainly have a very active pattern going on already with frequent storm systems bringing rain and even some snow to Kentucky.  It’s easy to see that the general storm track for many weeks has been consistently focused right through the Ohio Valley.  As April draws closer and the temperatures start to warm across the southern U.S., there is some fear that we could be looking at a very active severe weather season around here.  Many factors are coming together that would support these thoughts.  Of course, the accuracy of long-range weather prediction is low and only time will tell if Kentucky will see above average damage from thunderstorms.  I wouldn’t be surprised at all to see some pretty serious outbreaks close to home in April and May.