Severe Weather Outlook for Tuesday, May 13, 2008
Valid 07z Wednesday - 06z Thursday

Synopsis....

Ongoing activity will make things tricky for the forecast period, but the overall confidence is fairly good for this forecast.  Main feature in this outlook will be the upper level low moving into TX.  Cold Front making its way through the Southern Plains will stall out and will retreat as a warm front across some portions such as the OK/TX border.  Dry line will also be a major player in thunderstorm development towards the afternoon hours.  Possibility of some weak elevated storms will exist across MT/ND/SD with a northern stream s/w.  That threat will be unorganized and not severe.

Southern Plains

TX/Southern OK/LA/AR/MO

The threat will be fairly complicated.  Ongoing activity will continue to threaten TX with large hail and damaging winds with the steep mid level lapse rates and dry air around 700mb contributing to DCAPE values approaching 1000j/kg.  MUCAPE values downstream of the activity remains high ranging from 2000-4000j/kg.  As stronger winds move in with the approach of the upper level low this activity will push eastward.  Storms in TX will push eastward continueing the severe threat downstream into AR/LA/MS.  This should allow some breaks in the clouds for areas in Central and Eastern TX before the next round of severe weather develops along the dry line and cold front. 

Along and ahead of the dry line in TX SBCAPE values are expected to be between 1500-3000j/kg.  These numbers could be lower if cloud cover becomes a problem, but either way it appears the CAP will erode with the moderate to strong instability, ample moisture, and large scale ascent.  One feature that will be a focus point for enhanced low level shear will be the sfc wave of low pressure in Central TX.  Winds ahead of this feature will back increasing low level directional shear.  Low level winds are weak but a narrow area of SE 25-30kts ahead of this wave of low pressure could be enough for a few super cells capable of producing tornadoes.  Just to the south of this feature is another focus point for a dry line bulge.  It appears the bulge is in responce to a speed maximum at 700mb.  This will enhance convergence and shear along the dry line.  Despite weak to moderate wind fields in the lower levels, deep layer shear and winds aloft will be strong enough to support rotating updrafts.  250mb flow increasing to 100kts will allow bl-6km shear values ranging from 50-70kts capable of supporting the development of super cells.  Low LCL heights and tremendous amount of CAPE in lower 3km along with 3km VGP values ranging from .2-.3m2s2 will support tornadic threat.

Further eastward strong LLJ structure in the LA/AR/MS area will support severe threat with ongoing activity. Jet diffluence aloft combined with low level confluence will also develop more activity.  Deep layer shear will be weaker than areas further upstream, but will be sufficient enough to sustain rotating updrafts.  Instability will also be weaker with mid level warming and ample moisture throughout the atmosphere.  Jackson, MS sounding shows PWAT values approaching 2", so the main threat will be for flash flooding with these storms capable of producing a tremendous amount of rainfall in a short time.  Damaging winds and an isolated tornado cannot be ruled out either. 

Further northeastward into TN/KY a strong frontogetic COL with an unstable PBL could lead to a few storms becoming severe.  However, weak vertical shear and weak wind profiles in general will allow these storms to be pulse.