Analysis – 1155 am

| April 26, 2011 @ 11:59 am | 167 Replies


The morning model runs are in now, and it still looks like a significant severe weather outbreak will occur over the SE US today and tomorrow.  I will primarily talk about Alabama here, but this is a large-scale event.

A line of showers is moving through central Alabama now, with no severe weather.  It will move on into east Alabama, and most of us, especially west of I-65, will get sunshine this afternoon.  With dewpoints in the 60s, and temperatures rising into the lower 80s from I-65 west, the air will become unstable.  There will be a small cap, but some storms will likely break the cap and become severe.  The main threat will be damaging winds and large hail, since the dynamics and wind shear are north and west of us this afternoon.

Things will start to change overnight and tomorrow, as the main upper-level system starts to move, bringing the cold front and upper dynamics closer.  By tomorrow morning, a cold front will be from near Indianapolis to Little Rock.  As a large upper-level disturbance and upper-level jet move over the front, a new low pressure area will develop to our west and move along the front.  The models disagree on the timing of this low, one moving it through Memphis early morning then into KY and OH, the other not until afternoon.  That will be one key factor tomorrow, because the highest storm-relative helicity values will be closer to the surface low and low-level jet, making storm rotation more vigorous.

Taking the average of the two, the largest storm-relative helicity, a very high 400 or more m2/s2, will be centered from northern MS into NW Alabama and middle TN by noon tomorrow.  The thing that makes this event more worrisome is that we’re pretty deep into Spring now, with strong sunshine and a warmer Gulf.  Dewpoints tomorrow will rise to near 70 degrees, and assuming we get sunshine, temperatures will rise into the 80s.  With cold upper-level temperatures, this will cause very unstable air.  CAPE will range from 2,000 to 4,000 J/kg over most of Alabama by early afternoon. 

With that combination of wind shear and instability, severe storms will develop, and in the areas of strongest shear, the storms will rotate vigorously and produce tornadoes.  Pinpointing the exact areas that will be the most vulnerable is still tough at this time, since it will depend some on where clouds linger longer in the morning, where today’s storms leave boundaries, etc.  But, the stage will be set most favorably for tornadoes over the northwest half of Alabama this time (it was the south half on April 15).  We are talking about areas northwest of a line from Demopolis to Clanton to Anniston to Gadsden. 

A gauge of the strongest combination of instability (warm, humid air) and wind shear is the STP (significant tornado parameter).  Anything above 1 is supportive of tornadoes, and above 3 indicates a serious threat for large tornadoes.  Here is the model-predicted STP plot for tomorrow at 1 pm:

This is a serious situation.  Just because there have been a lot of severe weather outbreaks this year (similar to 1998), and you have heard a lot of tornado watches and warnings, don’t get complacent.  Tornadoes were all over the place just south of BHM Metro two Fridays ago, and this time it could include TCL, BHM, ANB, Cullman, GAD, HSV, and MSL.  Have a plan for what you will do in a tornado warning at work, at home, wherever you will be.  The schools mainly have tornado plans in place.  Keep an eye on the weather carefully through Wednesday night.

At work, in large buildings, interior hallways on the lowest floors and stairways are often best.  Stay away from windows and outside doors.  In smaller offices, go to the lowest floor, in an interior room, away from windows and doors. 

In the home, the underground part of a basement is best, but get under a work bench, heavy table, etc.  Stay away from garage doors!  They are a weak point on your house in winds.  If you don’t have a basement, go to the lowest floor, interior room or hallway, away from windows, doors, and outside walls.  Hall closets, if on interior walls, are good.  Protect your head.  Put batting helmets, football helmets, bicycle helmets on the kids.  Stay low.  Falling trees can come through the roof, but often stop before coming all the way through.

In mobile homes, leave.  All (or almost all) of the fatalities in April 15 tornadoes in Alabama were in mobile homes.  If an outbreak starts, try to go ahead and go to a sturdy building ahead of time and wait it out.  You are better off in a ditch than in a mobile home.

There is no reason to panic.  Just have a plan ready, follow the weather, and if a warning is issued, execute your plan. 

Another update around 11 pm CDT.


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