Mesoscale Precipitation Discussion 0780
NWS Weather Prediction Center College Park MD
1142 AM EDT Thu Sep 06 2018

Areas affected...Central Texas

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

Valid 061541Z - 062130Z

Summary...Flash flooding will be possible into the afternoon in
parts of Central Texas, from the I-20 corridor in the rolling
plains, south into the Edwards Plateau, portions of Texas Hill
Country, and along the Rio Grande River in South Central Texas.
Rain bands and storms should meander very slowly, and will pose a
risk of flash flooding. Rain rates could reach (or locally exceed)
2 in/hr.

Discussion...Regional radars (KDYX, KSJT, KDFX) were showing
numerous small-scale convective rain bands and a few embedded
thunderstorms across central Texas. The convection was generally
moving very slowly, and was situated within a region of very weak
mid-upper level winds. RAP analyzed Corfidi vector and deep layer
mean wind magnitudes were near zero around Uvalde and Del Rio, and
below 10 knots over the remainder of the outlined area. A weak MCV
was noted on KSJT radar near Crockett County as of 1530Z, with a
circulation noted in the velocity data in the mid-levels of the
troposphere. The MCV was slowly lifting northwest, and may be
locally affecting the low-mid level flow patterns in the I-10
corridor. Increasingly channeled flow on the northeast flank of
the MCV may help focus convergence, and promote slow training of
convection near and north of I-10 in central Texas. There was some
evidence this was beginning to take place on KSJT radar. In the
stronger convection (Z > 40dBZ), ZDR was generally around 1 dB,
with CC values close to 0.99, implying large concentrations of
smaller more spherical droplets -- implying efficient warm rain
processes. This may lead to some radar underestimation of rain
rates in the tropical environment, characterized by tall and
relatively narrow CAPE profiles and PWATs above 2 inches (CIRA
blended PW and GPS-PW obs).

More broadly, a larger area of stratiform precipitation and cloud
debris were noted on both radar and GOES-16 satellite, and the
outer periphery of that was stretching from the MCV in Crockett
County, to near SOA, to near ECU, to near 5T9. This may evolve
into a differential heating boundary of sorts, particularly south
of I-10 where greater clearing was noted in the low-level inflow
region to the east. It also should not move much given the
stagnant pattern, so it may serve as a focus for continued
convective development into the afternoon. The setup for training
is less favorable south toward the Rio Grande, but given near-zero
backbuilding storm vector magnitudes and a deep layer mean wind
toward the northwest at only 5 knots, the mesoscale setup may
still be favorable for convection to persist in localized areas
for several hours. Therefore, flash flooding appears possible in
central Texas, but particularly in the vicinity of the
aforementioned MCV and on the edge of the existing stratiform
precipitation.

Lamers

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

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

LAT...LON   32509982 31609942 30859946 30149954 29529904
            28559958 28400045 29140102 29890088 30300115
            30660190 31290176 32260103