Mesoscale Precipitation Discussion 0118
NWS Weather Prediction Center College Park MD
240 AM EDT Sat Apr 13 2019

Areas affected...Portions of West Texas

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

Valid 130640Z - 131040Z

Summary...Slow cell motions and cell mergers could pose a flash
flood threat over portions of west Texas through the early morning
hours.

Discussion...Radars over west TX has shown an increase in
convective coverage in the past 90 minutes, and this is supported
by bursts of cold cloud tops (with temperatures near -65 C).
Currently, the storms are mainly hail/wind producers, as the low
and mid levels are quite dry (as evidenced by the 13/00z MAF/DRT
soundings). The MAF radar showed hourly rainfall rates as high as
1.50 inches, but based on the dry air in the mid levels, and the
output from the MAF HCA, these estimates are being skewed by the
presence of hail.

A low level east southeast flow continues to transports moisture
over west TX, with the axis of best moisture convergence pointed
north the of Big Bend region. Just south of this axis (and south
of a  cold front dropping south into west TX), there was an axis
of 1000-2000 J/KG of MUCAPE just south of the moisture axis. Local
VWP showed east to southeast low level winds, with mid/upper
levels winds from the west southwest (ahead of short wave energy
approaching from northern Mexico). This is resulting in slow and
erratic storm motions, and colliding outflow are resulting in new
convection as well.

As the short wave approaching west TX, increasing divergence could
support an increase in convective coverage here. Recent HRRR
soundings showed the column moistening in response to the moisture
advection, better supporting the potential for heavy rainfall.
There is a windows of 3-4 hours during which time slow cell
motions in the deepening moisture could allow for hourly rainfall
rates approaching 1.50 inches, especially where cell mergers or
short term training occurs. This idea is supported by the most
recent HRRR, showing the potential for isolated 2.00/3.00 inch
rainfall amounts through 13/11z.

One and three hour flash flood guidance values are fairly high
(generally above 2.50 inches), so cell mergers and/or short term
training could be needed to initiate flash flooding. As surface
low pressure gets better organized north of the Big Bend later
this morning, storm motions should increase, lessening the threat
for flash flooding.

Hayes

ATTN...WFO...MAF...SJT...

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

LAT...LON   32810240 32440130 31660076 31010110 30640166
            30100313 30600386 31580382 32340334 32430327
            32730300