Mesoscale Precipitation Discussion 0396
NWS Weather Prediction Center College Park MD
428 AM EDT Mon Jun 28 2021
Areas affected...upper TX coast into southwestern LA
Concerning...Heavy rainfall...Flash flooding possible
Valid 280827Z - 281330Z
Summary...The potential for flash flooding will be increasing
along parts of the upper TX coast into southwestern LA through
14Z. Localized rainfall rates of over 3 in/hr will be possible
along with 6-hr totals of 3-6 inches.
Discussion...Loops of regional radar and satellite imagery through
08Z showed a low to mid-level vorticity max located just off the
upper TX coast. This feature was embedded within a very moist
environment with precipitable water values of 2.0 to 2.2 inches
per recent GPS data. The southwestern side of low-level ridging
over the eastern U.S. was allowing for an increased 850-700 mb
height gradient over LA into far eastern TX, supporting locally
stronger winds of 15 to 25 kt between Houston and Lake Charles.
Low level winds are stronger than the 850-300 mb mean wind but the
two vectors are more favorably aligned from near Galveston Bay and
eastward to support training of heavy rain. This was observed
earlier across western Cameron Parish where KLCH estimates showed
one hour rainfall between 3 and 4 inches ending 0730Z. Veering
winds will support the training potential to extend west of
Galveston Bay by 12Z.
SPC mesoanalysis and RAP analysis data from 07Z showed a gradient
in instability along the Coastal Plain with 2000-3000 J/kg MUCAPE
just offshore of the coast transitioning to less than 1000 J/kg
inland. Low level onshore flow will continue to support locally
heavy rainfall within about 75 miles of the coast. Vertical shear
is lacking for organization, but the potential for narrow axes of
heavy rain is present and will likely increase across parts of the
upper TX coast into southwestern LA as the low to mid-level
vorticity center moves inland over the next 3-6 hours. While not
expected to be widespread across the discussion area, localized to
scattered heavy rainfall rates, possibly in excess of 3 in/hr, are
expected to increase the threat for flash flooding through sunrise.
Otto
ATTN...WFO...HGX...LCH...
ATTN...RFC...LMRFC...WGRFC...NWC...
LAT...LON 30789442 30759387 30659352 30479316 30259281
30029252 29849244 29589242 29409248 29379286
29279339 28589501 28559595 28709625 29129641
29609640 30159610 30479559 30739490