Graphic for MPD #0749

Mesoscale Precipitation Discussion 0749
NWS Weather Prediction Center College Park MD
506 AM EDT Mon Sep 03 2018

Areas affected...Southeast TX...Southwest LA

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

Valid 030905Z - 031505Z

Summary...Bands of showers and thunderstorms are expected to
organize over the next several hours with extremely heavy rainfall
rates. Flash flooding will become likely with particular focus on
the urbanized corridor from Beaumont/Port Arthur down to the
Houston/Galveston area.

Discussion...The latest GOES-16 WV/IR satellite imagery along with
local VWP data suggests a weak closed mid level low center over
southeast TX that is centered northwest of Houston. The 00Z hires
model suite, including numerous runs of the HRRR throughout the
night, have been emphatic on developing organized convection
around the southeastern flank of the system as it drifts northwest
and inland across southeast TX. So far, the system has been
struggling to do this overnight as it has only intermittently
produced a few showers and thunderstorms.

However, the expectation is that this will change going through
the morning hours, and especially after 12Z, as the flow in the
850/700 mb layer is expected to become more confluent and in a
position to become better aligned with convergent boundary flow
associated with a surface trough along the coast. This will allow
for a much better connection to offshore instability which will be
pooling along the middle and upper TX coastline. Meanwhile, the
flow aloft is expected to quite diffluent given the upper level
anti-cyclone over the region. The juxtaposition of all of this
around the southeast flank of the mid level low center should
allow for the development of some organized southwest/northeast or
south/north oriented bands of very heavy showers and thunderstorms
with extremely high rainfall rates along the middle and upper TX
coast and including adjacent areas of southwest LA. Some focused
bands will also be possible closer in to the stronger mid level
vorticity center farther inland.

PWATS across the region are very high and up near 2.5 inches. This
environment is conducive to extremely efficient warm rain
processes that will favor rainfall rates as high as 3 to 4
inches/hr within the more organized convective bands. The concern
through especially the mid to late morning hours will be for these
bands to potentially train over the same area and settle in close
or over some of the more urbanized environments, including
Beaumont/Port Arthur on down to the Houston/Galveston area. Even
areas farther south down the coast toward Palacios will be
potentially at risk of seeing organized banded convection with
extremely heavy rainfall potential.

Based on the environment and rainfall output from the latest CAM
guidance, expect the potential for locally as much as 4 to 8
inches of rain through 15Z. These amounts will cause flash
flooding and especially over any of the aforementioned urbanized
areas which will include adjacent suburbia. This situation will
need to be monitored very closely through the morning hours.

Orrison

ATTN...WFO...CRP...FWD...HGX...LCH...SHV...

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

LAT...LON   32079609 31919495 31409356 30849280 30419253
            30019236 29659232 29479262 29589295 29669345
            29429431 28969493 28759526 28439590 27929666
            28019707 28649708 29109659 29389620 30039582
            30569603 31259707 31889701


Last Updated: 500 AM EDT Mon Sep 03 2018