Sunday, January 22, 2012

Severe Sunday Analysis

8:30 AM Monday Update...  I will have a new post soon dealing with the upcoming mid week system.

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It looks like the first significant severe weather episode will unfold late today and tonight across the mid south.  The fast moving system will have its biggest impact on areas along and east of the Mississippi River, but could still bring some severe weather as far west as central Arkansas.  Much of this depends on the amount of instability which develops day.  It's a cold start.  Warm and moist air will have to increase from the Gulf of Mexico and the modeling indicates this will happen.  Also, if there are any breaks in the clouds, this will increase the instability and the severe potential.

This situation is particularly dangerous on many levels which were outlined by Dr. Walker Ashley a few years ago.  This event will occur outside of what some consider "severe weather season".  This is a La Nina year and early and active severe weather seasons are typical.  Also, much of this will occur at night which increases the danger.  The storm system will affect an area of the country known as "fatality alley".  More people die from tornadoes in northeastern Arkansas to Tennessee to Mississippi into Alabama than any other place in the country.  Awareness is key to change this statistic.  I'll post a map below of what I'm talking about.

Right now, I expect storms to initiate late this afternoon across central Arkansas, then form into a line.  All of the storms should be out of the state before midnight.  Check out the maps below.

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This is simulated radar from HIRES model data indicating the storms will fire around 6 PM in central Arkansas
By 7 PM, the line has developed further from north central into central then into southwest Arkansas
By 9PM, the line is much stronger and has passed through central Arkansas heading into eastern areas of the state where conditions will be favorable for further strengthening.  A wind, hail, and tornado threat will be present.

By 11 PM, the line is almost to the Mississippi River and looks strongest, according to this model data, for east central and northeast Arkansas.
By midnight, it's exiting Arkansas
Instability values (green) are elevated across much of central, southern and eastern Arkansas according to the NAM by 6 PM.
The fast moving front pushes that instability to the Mississippi River by midnight
The EHI (Energy Helicity Index) which combines spin and instability shows elevated values across central and eastern Arkansas by 6PM.  This is the beginning of the tornado threat.
By midnight, the higher EHI values shift well to the east and the threat ends for most of Arkansas
The Storm Prediction Center shows a moderate risk where the highest levels of spin and instability will be present (red).  A slight risk is in yellow.
There is a 30% of severe hail (1'' or greater) in the red area within 25 miles of any given point.
There is a 15% chance for a tornado within 25 miles of any given point in the red shaded area
There is a 40% chance for severe winds within any given point in the purple shaded area.  This includes NE Arkansas.  There is a 30% chance for wind damage within 25 miles of any given point in the red shaded area including central and eastern Arkansas.
Dr. Walker Ashley identified an area of the country (in red) known as "fatality alley".  This area has more tornado deaths for several reasons including nighttime tornadoes and the perception that severe weather doesn't happen except in the springtime.

2 comments:

Paul Barys said...

very good analysis! stay safe!

Paul

jimmylee42 said...

Fanatic-

Been looking at the winter of 98-99 and the winter of 88-89. Both of these winters had mild months of January similar to this winter. The 99 January that had the LR tornado had a very cold few days in early January where there were highs in 20's. After the tornado in late January winter was very mild for February and March. There were no days in the two months with highs under 40 or lows under 25. No accumulating snow those months either.

Looking at the winter of 88-89 there was a closer match with this winter up to end of January. However, February turned out to have a frigid period early in the month a few days after an official high of 80 degrees. We had several days with afternoon highs in the teens at NLR, early in Feb with frozen precip on three of those days. School were closed for 2 days and would've been closed more except for the weekend. Feb ended up colder than normal and so did March.

I guess the snow lovers like myself will be pulling for the rest of the winter to be more like the 88-89 winter. As SBB says we shall see.

NOAA Winter Guidance