Mesoscale Precipitation Discussion 0803
NWS Weather Prediction Center College Park MD
1047 AM EDT Sat Sep 08 2018

Areas affected...South-Central and Central Texas

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

Valid 081447Z - 082030Z

Summary...Periods of heavy rain will continue today across
south-central and central Texas. The heavy rain may be more
focused in two areas: (1) from the Rio Grande near Eagle Pass
northeast toward the greater San Antonio area; (2) Texas Hill
Country / Concho Valley from near Sonora to near Brownwood. Rain
rates of 1-2 in/hr will be common and localized heavier rain rates
to around 3 in/hr will be possible. Many parts of south-central
and central Texas have recently received rainfall, and additional
heavy rain would likely cause flash flooding.

Discussion...Flash flooding is becoming increasingly likely again
today across portions of south-central and central Texas. Many
portions of the region, particularly in the Concho Valley and from
San Antonio metro west to Uvalde, have substantially reduced flash
flood guidance due to recent heavy rainfall. Most of the same
areas have received over 4-5 inches of rain in the past week, and
localized amounts in excess of 10 inches have also been observed.
Streamflow in the Uvalde to San Antonio region is running above
the 90th percentile right now, and on some streams above the 97th
percentile. Therefore, the region is hydrologically primed for
flash flooding given additional heavy rainfall, and some parts of
the area are historically vulnerable to flash flooding -- such as
the Balcones Escarpment that extends through San Antonio metro.

Even in the absence of favorable hydrologic conditions, the
atmospheric setup is favorable for flooding. GPS-PW observations
and the 12Z sounding from DRT showed precipitable water values in
the 2.2 to 2.3 inch range over south-central Texas. The DRT
sounding also showed a tall, narrow CAPE profile with most of the
positively buoyant region concentrated below -10C, a classic
signature for efficient rain production and flash flooding events.
Furthermore, the DRT sounding also showed mean cloud-layer wind of
just 1 knot, and effective storm motions across the entire region
should be very slow. All these ingredients are strong signals for
flash flood potential. The environment supports shallow
convection, with highest radar reflectivity focused below the
melting layer, and this could lead to radar underestimation of
rain rates.

The RAP forecasts a weak low-level cyclone (sfc-850mb) to drift
just north of Del Rio through the daytime hours, with a nearly
stationary deformation zone extending to the northeast. A band of
rain would be favored in this area of relatively strong low-level
deformation, and this rain band may also be able to lock into
certain areas for extended periods of time given the steady state
of the low and associated wind fields. Although the radar
presentation may, at times, seem more stratiform, the strong
low-level forcing and tropical moisture will favor highly
efficient warm rain processes in the convective bands and
relatively high rain rates. The soils underneath the deformation
band are already saturated or near saturated, so a nearly
stationary convective band seems likely to cause additional
flooding.

Further to the southeast and east, greater convective instability
is expected. A convective band with colder cloud tops and
lightning already stretched from Real County SSW into Maverick
County. This was showing a slight eastward drift with time, and
may be able to sustain itself in some form into the afternoon.
Additional convection may develop to the immediate east, or up the
I-35 corridor into central Texas. Hi-res models appear to be
underestimating the coverage and intensity of this convection, but
given the deep tropical moisture plume, there should not be a lot
of convective inhibition and the expectation is that thunderstorms
would continue to be favored along mesoscale boundaries. Flash
flooding appears likely with the ongoing deeper convective band in
south-central Texas, particularly where it can intersect the
Uvalde to San Antonio corridor -- an area that has received
significant rainfall in the past week.

Lamers

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

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

LAT...LON   32379890 32209664 30779710 29429817 28039913
            28040025 28990088 29730173 31160128