Mesoscale Precipitation Discussion 0640
NWS Weather Prediction Center College Park MD
1133 AM EDT Sun Aug 12 2018

Areas affected...South-Central and Central Texas including Hill
Country

Concerning...Heavy rainfall...Flash flooding likely

Valid 121532Z - 122115Z

Summary...Areas of moderate to heavy rain with embedded
thunderstorms are likely to continue through the day in parts of
south-central and central Texas. Many parts of the region have
received heavy rainfall in recent days, so additional rain could
cause flooding. Localized heavier rainfall associated with
thunderstorms could reach rates of 2-3 in/hr with the potential
for more focused areas of flash flooding.

Discussion...The ingredients are in place for rain and
thunderstorms to persist in a somewhat tropical environment into
the afternoon hours. RAP analysis placed a 50 knot southwesterly
jet streak over northwest Texas, and the right entrance region of
that jet streak is right over south-central and central Texas. The
RAP forecast maintains a decent upper level divergence signal
through the day in the region. At the same time, the RAP also
analyzed a weak low-level cyclone near the Texas Big Bend, with
broad low-level (925-850mb) southerly inflow from the Gulf of
Mexico. The streamlines generally spread out as they approached a
deformation zone stretching from just north of Del Rio up toward
the DFW metro area. Therefore, the nose of the warm, moist
low-level inflow was favorably aligned with upper level
divergence, and in concert these features should be sufficient to
sustain rain and embedded thunderstorms through the day. The
precipitable water values are near or just over 2 inches per
satellite analysis and observations. It is worth noting the 2.16
inch PW value on the 12Z DRT sounding, with a tall, narrow CAPE
profile and melting layer around 15kft -- a favorable profile for
efficient heavy rain production.

Cloud cover was more dense in an area of steadier rain in the
northern half of the outlined area, over Texas Hill Country. This
area would be most likely to see weaker instability and a
persistence of steadier rain with embedded heavier bands through
the afternoon as it should remain relatively near the nose of the
low-level inflow. Further south, from Del Rio and Eagle Pass up
toward San Antonio, breaks in the cloud cover are more likely, and
thus greater instability may build in the afternoon hours. This
area would be most likely to see stronger convection, with higher
rain rates and an increased potential for focused flash flooding
events. 850-300mb mean winds in the same area are very weak (5-10
knots), with Corfidi vector magnitudes less than 5 knots. This
favors meandering convective clusters with the potential for
back-building and stalling. KDFX radar has already shown that this
morning, with nearly stationary storms able to produce 2-3 in/hr
rain rates. This would be likely to produce flash flooding, and
especially around the five-county area of Val Verde, Edwards,
Real, Kinney and Uvalde, which have already seen numerous
locations receive 2-3 inches of rain in recent days. Therefore,
significant flash flooding is possible if a thunderstorm sits in
such an area, and significant flooding stranding people in trees
and on vehicles was recently reported from western Uvalde County
near the Nueces River.

A Moderate Risk of Excessive Rainfall remains in effect on the Day
1 ERO for much of this region, and people in the area should be
prepared to respond quickly to any flooding that develops.

Lamers

ATTN...WFO...EWX...FWD...MAF...SJT...

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

LAT...LON   31829915 31359788 30479783 29529814 28439983
            28780075 29350126 29840202 30870185 31400099