Monday, February 09, 2009

Expert Opinions About Wednesday's Weather

I just took a look at the Area Forecast Discussions (AFD) and Hazardous Weather Outlooks (HWO) issued by various National Weather Service Offices across the South, including Huntsville, Birmingham, Jackson, MS, and Memphis. There are a lot of ideas and opinions out there on the internet, many of them valid, but it is best to go with the professionals.

The general consenus is that wind shear will be very high, even described as "intense" by Jackson and "almost...extreme" by Huntsville. The big question (as is usually the case this time of the year) is the level and areal extent of instability. The general consensus, based on models, is that there will be a narrow area of instability just ahead of the main squall line. For this reason, none of the NWS offices are predicting a major tornado outbreak at this time, despite the very high shear. However, they all do mention the possibility of a few isolated tornadoes spinning up.

Then there was what I consider a particularly important note of caution that was mentioned by the National Weather Service in Huntsville:

THE QUESTION FOR SVR WX POTENTIAL WILL BE INSTABILITY...AS DIRECT MODEL OUTPUT SUGGESTS THAT THE PREFRONTAL ENVIRONMENT WILL BE MODESTLY UNSTABLE AT BEST. HOWEVER...FEEL THE MODELS ARE UNDERDOING THE POTENTIAL HEATING...AND WITH SUCH STRONG WINDS IN THE LOW LEVELS...IT WILL TAKE LITTLE EFFORT TO CREATE WIND DAMAGE WHETHER IT IS CONVECTIVE OR NOT. THE FRONT WILL BE A FAST-MOVER...LIKELY MOVING INTO NW AL MID/LATE MORNING...AND PUSHING INTO GA MID/LATE AFTN.

To support this idea, there have been numerous tornado events in the past few years which occurred with less than impressive instability values combined with extreme shear. Last year at the Southeast Severe Weather Symposium at Mississippi State, this topic was mentioned during several presentations.

Steve Nelson from Peachtree City NWS office spoke about this topic in some detail.

Here are some of my notes from his presentation:

Cool Season Tornadoes in the Southeast United States - Steve Nelson - Peachtree City NWS

--Low instability during the cool season does not equate to no chance of tornadoes

--F2 or greater tornadoes must have high levels of shear to develop


--Tornadoes cause the majority of storm related damage and deaths during the cool season


--Tornadoes are more likely to persist overnight during the cool season than in the spring


--Mean level CAPE during cool season F2 events is 1300 j/kg


--Helicity values are at least 200-250 m/s during the cool season events


--CAPE values are often less than 100o j/k during cool season events


--Lightning output is low in cool season events


You can read more of my notes here.

Bottom line: Wednesday will be Alabama's first day of 2009 with the potential of some type of widespread severe weather threat. Please be weather aware and stay tuned.

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