Thursday, March 27, 2014

Severe Weather Update


3:50 PM Thursday Update.... This video goes over hi res model data for this evening's severe weather threat and the threat later Friday.  There is high confidence a line of storms will develop over northwest Arkansas.  There's a lower degree in confidence of storm development in front of that main line across west central areas of the state.  This video explains.



At this point, I'm still not expecting an "outbreak" of severe weather, but it does looks like we will have some in Arkansas later today (Thursday) and again later Friday afternoon.  Wind and hail should be the primary severe weather threat, but an isolated tornado or two can never be ruled out.  

This episode of rain and storms revolves around the presence of a frontal boundary and 2 waves of energy.  The first arrives late today and tonight, then another later Friday afternoon.

I have put together 4, 4 panel charts explaining this situation thoroughly for you. These maps come from weatherbell.com and the Weather Prediction Center.  Before I get into these, let me explain.

The first panel in the upper left is the 4KM NAM simulated radar

The second panel in the upper right shows you surface based CAPE (Convective Available Potential Energy).  This is the instability needed for thunderstorms.

The 3rd panel in the lower left is the surface dewpoint.  For severe thunderstorms this time of year, you generally need to see those values in the mid 50s and higher.

The 4th panel in the lower right is the frontal position according the the Weather Prediction Center.  This panel will not match the exact time of the other 3 panels, but it's very close and gives you a good idea

Remember, I'm using one particular computer model and it's never perfect, but I think this has a good handle on the situation.  You may need to click to enlarge each one.

By 10 PM Thursday, there are a few showers and storms across central Arkansas, but we're really watching the NW corner of the state here.  Notice in the upper right, there's a nose of higher CAPE values shooting into western and northwestern Arkansas.  Dewpoint values (lower left) are up there in that corridor, but notice the lower dewpoints in the extreme northwest corner of the state.  This is the airmass behind the boundary and it's stable behind it.

By 4AM Friday, the line of thunderstorm along the boundary has fired up and is situated across central Arkansas.  Also notice the CAPE values have decreased and they are highest along and ahead of the boundary which is situated from SW to central into northeast Arkansas.  The drier air in the lower left panel is making a move into central Arkansas behind the storms.  Remember, this will not be perfect, but it's a good general idea.
Now for the next round.  The boundary has settled across southern Arkansas with stable air over the north.  The CAPE values at noon (instability) are really increasing along and ahead of the front due to daytime heating.  Dewpoints are also high along and south of the boundary.  Simulated radar in the upper left shows storms already coming back and in southwest Arkansas.
By 4PM, the greatest instability is confined (according to this model) to areas south and east of a line from near west Memphis down to Magnolia.  The front and wave of low pressure is in south central Arkansas with very high instability values ahead of it.  As the low sweeps to the east by early Friday evening, the severe threat will decrease for everyone.  Notice the rain and storms behind the boundary into central and northern Arkansas.  These storms could be quite strong with hail the main threat, but we're also watching storms in the SE with all that warmth and moisture available.

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