Severe weather update – 232 pm

…TORNADO WATCH MAY COME OUT SOON FOR NORTH ALABAMA…

We are watching the atmosphere very carefully this afternoon, as severe weather is possible tonight, mainly between 4 pm and 3 am.  I have become a little more concerned about the threat for isolated tornadoes looking at the latest surface maps and models.  This should not be a major tornado outbreak, but it is a weather situation that deserves your attention.

Here’s the deal.  An upper-level disturbance will move through tonight, centered north of Alabama.  This will cause rapid pressure falls over north Alabama, and winds will respond by flowing northward.  These sudden changes are harder for the atmosphere to adjust to, so wind shear this evening will become rather large.  The short-term models show helicity values over 400 m2/s2 this evening.  So, we’ll have plenty of wind shear for severe storms and even tornadoes.

As usual in February, the limiting factor will be instability.  However, the upper-levels are rather cold, so it doesn’t have to be as warm and moist at the surface to be unstable.  With sunshine, our temperatures have warmed to near 70, and will probably hold in the 60s this evening with increasing winds.  The big question is low-level moisture.  Dewpoints are still way too low for severe weather now (52 at BHM, 55 at TCL), but upstream where our air is coming from, dewpoints are in the lower 60s in southwest MS and central LA.  How much of that higher-dewpoint air gets in here will determine the severe weather threat.

Models indicate that dewpoints will get as high as 62 or 63 near I-20 this evening, with CAPE values over 1,000 J/kg.  That seems a little aggressive to me, but even a CAPE of 500-750 J/kg with this kind of wind shear produces an energy-helicity index of around 2…enough for isolated tornadoes.  If the models are right, EHI could reach 3.

I want to emphasize that this is not Apr 27, 2011.  EHI that day was 10.  We are mainly expecting scattered thunderstorms with localized damaging wind gusts and hail.  But, with the wind shear being this high, and cloud bases being very low at night, isolated tornadoes are possible.  This is an evening you should have a source of weather information handy at all times, and if you plan to be out somewhere, keep an eye on it.  I’ll have another update around 445 pm as the low level moisture field develops and we can tell more about what is happening.  Models start to become useless, as we just need to look out the window as this evening gets closer.

Category: Severe Weather