Mesoscale Precipitation Discussion 902
NWS Weather Prediction Center College Park MD
945 PM EDT Wed Sep 26 2018

Areas affected...Southern Texas

Concerning...Heavy rainfall...Flash flooding possible

Valid 270144Z - 270744Z

Summary...Numerous slow moving thunderstorms are expected to
develop and persist through the late evening and early morning
hours across parts of southern Texas.  Flash flooding is possible
given high rainfall rates and slow storm movement. 

Discussion...Recent radar imagery is depicting an organizing band
of very slow moving showers and thunderstorms between the Rio
Grande to near Victoria, and GOES-16 infrared satellite imagery is
showing an increase in overshooting tops over 50,000 feet over
this area.  There is a very slight overall southern movement with
this convection.  These storms are taking advantage of a very
moist and unstable airmass south of a stalling cold front. 

The anomalous low to mid level moisture and instability is
expected to pool along and ahead of the front, with the latest RAP
model analysis indicating PWs on the order of 2 to 2.25 inches
over this area through the early morning hours.  Large scale
ascent is being aided by right entrance upper jet dynamics,
boundary layer wind convergence, and 2000 to 3000 J/kg of MUCAPE. 
Effective bulk shear is on the order of 25 to 30 knots.

The latest high res model suite is indicating the potential for
some rather heavy rainfall totals by 3 am local time.  The ARW,
HRRR, and WRF are all supporting localized QPF maxima of 4 to 7
inches, especially between Laredo and Corpus Christi.  Although
flash flood guidance values are relatively high at the onset,
flash flooding is still possible given the potential for rainfall
rates to approach 3 inches per hour at times.

D. Hamrick

ATTN...WFO...BRO...CRP...EWX...HGX...

ATTN...RFC...WGRFC...

LAT...LON   29419626 29399581 29179548 28879537 28599559
            28339611 28049664 27799694 27309725 27059755
            26909829 26859893 26939957 27449988 27940040
            28380036 28569997 28749892 28919792 29199700
            29329661