Sunday, March 11, 2012

Clouds Good, Sun Bad


Noon Sunday Update....  Well, the Storm Prediction Center has decided to continue a slight risk for severe weather for much of the state.    As I mentioned below with my earlier blog post, the instability continues to be lacking while there is plenty of shear.  Where these two combine is across southern Arkansas and even there, the instability should not be too high.  Also, as stated below, I like to play these things as "safe than sorry".  Everyone should stay weather aware today, but the threat is very much on the low end.  Check out this map below.

This is from the mesoanalysis page on the SPC website.  I know this looks messy, but ignore everything except the red lines across the Gulf coast.  This is the surface based instability we're watching.  This is far removed from Arkansas and will have to make a huge surge into northward to raise concerns for any significant severe weather.  Clouds and rain will limit the northward advancement.  It is possible some of this instability makes it into southern Arkansas late this afternoon so we'll continue to watch it for you.

That title was done in my Frankenstein voice!  That pretty much sums up today's severe weather potential.

There are a couple theories as to how this plays out today and I'll explain.  First of all, you need what is called "surface based instability" for the development of those nasty spring storms.  In order to get that, you need to build up warmth and humidity at the surface.  This is the fuel which helps drive thunderstorm development.  As of this morning, that's lacking due to showers and clouds!  Temperatures and dewpoints are in the 40s and 50s.  If any sunshine breaks thru those clouds, the chance for severe weather will be on the increase. If those thunderstorms can develop, there's plenty of spin in the atmosphere to raise a concern for tornadoes. That scenario seems unlikely, but it's at least possible and that's why SPC has placed much of Arkansas under a slight risk for severe weather.   So it's very conditional at this point and when you look at the radar and satellite, it's hard to believe we'll be able to get enough of that instability to develop.  The National Weather Service office in North Little Rock points out something very interesting in their morning discussion.  The NAM is the one model which shows things could get bad this afternoon as it develops enough instability for the formation of storms.  However, it's doing a poor job depicting this mornings rain and clouds and doesn't think it's there.  Well, look outside,  IT'S DREARY (good).  Other models, like the GFS, indicate the morning showers and show much less in the way of instability this afternoon. 

With all that said, it would be very wise for all Arkansans to monitor the weather today.  I always play these things from the perspective of "I would rather be safe than sorry".  If there's a place in Arkansas which concerns me more than others, it would be southern and southeastern Arkansas, but again, instability is lacking there as well.  However, there maybe enough in that section of the state to raise some concern.  You better believe I'll be watching it for all of you.

Below are a few maps explaining the whole situation.

This is the NAM late this afternoon which most are ignoring this morning.  However, it still needs to be watched.  This is showing enough instability surging into central Arkansas for the formation of some nasty thunderstorms.  This is called CAPE. (Convective Available Potential Energy).  This is measured in joules per kilogram.  This is approaching 1000 which is sufficient for storm production.
This is the NAM showing EHI levels (Energy Helicity Index).  This product combines shear and instability.  It indicates the possibility for supercells across central and southern Arkansas late this afternoon and if this happens, an isolated tornado or two can't be ruled out.
This is the GFS late this afternoon indicating the potential for instability.  This solution makes more sense due to the ongoing clouds and shower activity around the Natural State.   It indicates the fuel for storms staying south of the state.  However, I will be watching this carefully throughout the day, especially for southern and southeast Arkansas.
The Storm Prediction Center has placed much of Arkansas under a slight risk for severe weather.

1 comment:

Shack said...

As usual, nice job and thanks for putting in laymans's terms!


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