Graphic for MPD #0785

Mesoscale Precipitation Discussion 0785
NWS Weather Prediction Center College Park MD
527 PM EDT Thu Sep 06 2018

Areas affected...South-Central Texas

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

Valid 062126Z - 070200Z

Summary...Slow-moving thunderstorms are likely to linger at least
for a few more hours in south-central Texas, and may be
particularly concentrated near the greater San Antonio area. Rain
rates of 1-2 in/hr will be common with the thunderstorms and heavy
rain bands, with localized rain rates of 2-3 in/hr possible. This
could lead to flash flooding, particularly in the area near and
northwest of San Antonio where soil moisture is higher due to
recent rainfall.

Discussion...A large area of relatively disorganized convection
was affecting much of Texas this afternoon with at least moderate
instability and elevated precipitable water values over much of
the state. A more concentrated area of convection was developing
in south-central Texas near San Antonio, where some slow-moving
and colliding outflow boundaries were acting to increase low-level
convergence. These thunderstorms were situated on the cusp of some
deeper moisture (GPS-PW observations 2.2 to 2.3 inches), and that
should yield some efficient rainfall production. MRMS rain rate
estimates were as high as 2-3 in/hr in some areas. The convection
was also becoming more concentrated in an area of reduced flash
flood guidance near and just W/NW of San Antonio. There has been
heavy rainfall in the past week in those areas (as much as 5-10
inches of rain), and the National Water Model indicates higher
soil moisture in the top 40cm in that same area. Therefore, the
antecedent conditions are also somewhat favorable for flash
flooding in the same area. Given that outflow boundaries are
driving some of the increase in convective coverage, the
concentrated area of convection may not linger for a long period
of time, but even a couple hours of heavy rain could be sufficient
to cause flash flooding. Elsewhere, further to the north in Texas
Hill Country and to the south toward Laredo, convective coverage
was not as high, but sufficient moisture and instability existed
for redevelopment of convection. Therefore, these areas have also
been highlighted for the possibility of flash flooding, even
though the near term (21-00Z) risk is highest in the greater San
Antonio area.

Lamers

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

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

LAT...LON   31519863 30629817 29789764 29009748 28199812
            27749920 27729930 27729998 28250044 29119989
            30270000 31329974


Last Updated: 527 PM EDT Thu Sep 06 2018