Mesoscale Precipitation Discussion 0833
NWS Weather Prediction Center College Park MD
419 AM EDT Fri Sep 14 2018

Areas affected...middle to upper TX coast

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

Valid 140818Z - 141400Z

Summary...Repeating rounds of heavy rain with 1.5 to 2.5 in/hr
rainfall rates are expected to impact the middle to upper TX coast
through 14Z. Given wet antecedent conditions, flash flooding will
be possible.

Discussion...Regional radar imagery showed a broken line of
thunderstorms extending from offshore Galveston Bay into and
across downtown Houston as of 08Z. Earlier, surface observations
from Wunderground.com indicated 1.5 to 2.5 in/hr rain rates across
Galveston county within this band ending 06Z and 07Z and near 1
in/hr in eastern Harris county as of 08Z. These thunderstorms were
occurring on the northeast side of a disorganized inverted trough
axis propagating slowly westward across the the western Gulf of
Mexico. Enhanced and convergent 850 mb moisture flux was noted on
the northeast side of the inverted trough, with the KHGX VAD wind
plot showing roughly 20 kt of flow, perpendicular to a coastal
trough along the upper Texas coast noted in surface observations
providing a focus for lift. Precipitable water values were 2.3 to
2.5 inches per recent GPS measurements along the middle and upper
Texas coastline with the nocturnal gradient in MLCAPE focused
along the coastline (higher values of MLCAPE over the Gulf).

A slow westward movement is expected of the larger scale inverted
trough axis in the western Gulf of Mexico, with the
confluent/convergent 850 mb flow continuing to focus along the
middle and upper Texas coastline over the next 3-6 hours. The
potential for training and repeating cell motions is high given
Corfidi Vectors are zero to roughly 5 kt oriented opposite to the
low level inflow. 6 hour rainfall totals of 2-4 inches are
expected which may generate flash flooding given much above
average rainfall over the past week for the region.

Otto

ATTN...WFO...CRP...HGX...LCH...

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

LAT...LON   30069482 29909423 29499392 29189425 29039456
            28189571 28139643 28719666 29509602 29919553