I guess the question on everyone's mind lately is.. What the Heck Is Going On??
The biggest factor I could find behind the increased storm activity is a weakening LA NINA cycle. La NINA is cooler than normal waters in the equatorial Pacific which is caused by strengthening of the trade winds over the pacific. This graphic below shows what happens in a LA NINA phase. Note the wetter than normal conditions over the SE USA with Dry conditions over Texas. Well this matches up nicely with what has unfolded thus far.
The LA NINA pattern produces a MUCH weaker Jet stream over the country that we saw last fall. This allowed for rapid fluctuations in the jet stream and introduced more in the way of Low Level Jets which allowed for moist air off the Gulf of Mexico to be transported north. It's noteworthy to mention the Gulf of Mexico ran above normal with surface temperatures rather high all of last summer and with a lack of hurricane activity to transport that heat northward to higher latitudes it became a ticking time bomb. Remember! The Earth constantly looks to achieve equal balance from the lower and higher latitudes. With available heat and moisture to be tapped into with a fluctuating LA NINA cycle toward neutral this was enough to tie into the reasoning behind this active tornado season!
To confirm this thought.. Lets go back in the past! April-September at years where a La Nina Winter followed an El Nino Winter (1965, 1971, 1974, 1989, 1996, 1999 and 2008) we found that several significant severe weather outbreaks occurred in or near the same areas that were hit hard this year during those years. In 1965, the Palm Sunday Outbreak occurred, which was one of the more significant tornado outbreaks in the 1960s. In 1974, the SuperOutbreak of tornadoes took place, with roughly 150 tornadoes in one day. In 1996, the violent Oakfield F5 tornado struck in Fond du Lac County, which was the last F5 tornado to impact the area. And just a few years ago in 2008, 26 tornadoes developed in south central and southeastern Wisconsin.
Why was April So Active?
This is soley on a hunch of mine that this quick change from a rather vigerous La NINA cycle to more neutral conditions could have aided in such an Active April for the Lower Plains.
Here's the Data from NOAA:
2010 | 1.7 | 1.5 | 1.2 | 0.8 | 0.3 | -0.2 | -0.6 | -1.0 | -1.3 | -1.4 | -1.4 | -1.4 |
2011 | -1.3 | -1.2 | -0.9 |
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** NOTE The fact we go from a strong -1.4 during the winter months to a -.9 by the spring. That is NOT a gradual switch to Neutral conditions over the Pacific.
Look at the two images below. First one shows sea surface temperatures from March which indicated some warming. However, there is an abrupt reversal in the LA Nina pattern by early April and a moderate trend towards neutral conditions. Could this abrupt reversal be to blame on extreme weather over the lower parts of the country?
ABRUPT SWITCH TO NEUTRAL CONDITIONS...
How Does This Affect Hurricane Season?
La Niña produces easterly wind departures at upper levels of the atmosphere and westerly wind departures at lower levels, across the eastern tropical Pacific Ocean and tropical Atlantic.
La Niña contributes to fewer eastern Pacific hurricanes and more Atlantic hurricanes.
El Niño produces westerly wind departures at upper levels of the atmosphere and easterly wind departures at lower levels, across the eastern tropical Pacific Ocean and tropical Atlantic. Over the eastern Pacific these wind patterns are opposite those normally seen in the region, and results in lower vertical wind shear.
During cold events in the eastern Pacific, the odds are significantly higher that the U.S. will experience greater impacts because of a larger number of tropical cyclones and higher intensities for each storm. Over the 73 years covered by this st udy, the total numbers of tropical storms and hurricane landfalls were 58 during El Niño years versus 82 during La Niña years. The average Saffir/Simpson category of landfalling tropical cyclones (counting tropical storms as zero) is 0.93 du ring El Niño years and 1.33 during La Niña years.
I will go into more detail about Hurricanes in a future blog.
