Mesoscale Precipitation Discussion 0811
NWS Weather Prediction Center College Park MD
557 AM EDT Sun Sep 09 2018

Areas affected...SRN TX

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

Valid 090957Z - 091457Z

Summary...Heavy rainfall was escalating in coverage this morning
over the Texas Hill Country and south Texas. Flash flooding is
likely, with rapid accumulation of 3-plus inches of rainfall.

Discussion...At 0945Z the number of flash flood warnings from
local offices was increasing ahead of two mesoscale convective
vortices (MCVs) located one near San Antonio and the other along
the Rio Grande to the south. These vortices were the result of
latent heat release from prior overnight convection feeding back
on the weak synoptic mid level flow. An upstream 250-mb trough
axis placed this region under a difluent flow regime aloft,
further supporting organization and gentle lifting of the near
2.25 inch precipitable water values per the SPC mesoanalysis.

All cell motions are analyzed as zero to ten knots based on the
vertical wind profile over the region. Steering flow for the MCVs
will take them generally eastward into the instability axis, with
uncapped mixed-layer CAPE values of several hundred J/kg in the
Hill Country increasing to near 2000 J/kg at the coast.
Additionally, a surface frontal zone increasing in strength was
already leading to an expansion of new convection extending
northeast from San Antonio toward Bastrop.

Instability will gradually be consumed, and without sustained low
level inflow, there is a limit to how much rain will fall at any
given location. But the extremely efficient, tropical environment,
coupled with broad lift and sufficient instability, should yield
some rapid accumulations of 3-plus inches of rain. The 08Z HRRR
and 00Z NSSL WRF were doing particularly well depicting this.
Flash flooding appears likely, especially in the more prone Hill
Country where FFG is also lower. But some lowered FFG also exists
near and north of Corpus Christi where models depict a secondary
maximum of rainfall this morning.

Burke

ATTN...WFO...BRO...CRP...EWX...HGX...

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

LAT...LON   30859845 30799756 30119692 29219676 28519625
            27899670 27229734 27779811 26749923 27750023
            28370046 29289965 29899971