Weather Prediction Center issues Mesoscale Precipitation Discussion #678 concerning HEAVY RAINFALL...FLASH FLOODING POSSIBLE730
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Mesoscale Precipitation Discussion 0678
NWS Weather Prediction Center College Park MD
539 AM EDT Sun Jul 21 2024
Areas affected...Upper Texas Coast, Southern Louisiana
Concerning...Heavy rainfall...Flash flooding possible
Valid 210938Z - 211500Z
Summary...Showers and thunderstorms will increase over the Gulf of
Mexico and lift northward onshore Louisiana and the Upper Texas
Coast this morning. Rainfall rates of 2-3"/h are likely, which
through slow storm motions could produce 2-4" of rain with locally
higher amounts. Flash flooding is possible.
Discussion...The regional radar mosaic this morning shows showers
and thunderstorms slowly expanding across southern Louisiana,
while the GOES-E IR imagery indicates cooling cloud tops
indicative of additional convection of the northern Gulf of
Mexico. These thunderstorms are expanding in response to a
shortwave lifting northward from the Gulf, which is providing
additional lift to an area already beneath a mid-level weakness
and tail of an upper jet streak. A modest 10-15 kt 850mb LLJ
measured via regional VWPs is lifting northward, drawing PWs over
over 2 inches and MLCAPE of 2000-3090 J/kg onshore and into a
stationary front analyzed by WPC. The overlap of ascent into these
robust thermodynamics is driving the expanding convection this
morning, while warm cloud depths above 14,000 ft and a deep layer
of moist-adiabatic lapse rates support the efficient warm rain
processes driving radar-estimated rain rates of 1.5"/hr.
Although thunderstorms are likely to remain scattered through the
late morning as reflected by recent CAMs including the HRRR,
RRFSp1, and experimental RRFSp3/p4, there is good agreement in an
expansion in coverage, especially along the immediate coast. This
will be driven by persistent moisture advection into the
frictional convergence boundary along the coast, and aided by the
shortwave lifting northward. While there is some uncertainty as to
how far inland the convection can track through the morning due to
modest instability over land, both the HREF and REFS ensembles
indicate 2"/hr rainfall rate probabilities rising to 30-40%,
suggesting locally 3"/hr rates are possible. These rates
themselves could overwhelm soils, even in locations that are
typically less susceptible due to swampy soils and higher FFG.
However, the setup is conducive to backbuilding and training as
Corfidi vectors increasingly veer to the SW and collapse. This
indicates that cells will regenerate over the Gulf into the better
instability and lift northeast, producing effective cell motions
of less than 5 kts in some areas. Where this net motion results in
longer duration training, 2-4" of rain with locally 5" is possible
as reflected by HREF and REFS probabilities for 3" (5") of 30-50%
(10-20%). Additionally, the 6-hr PMM from both ensembles indicates
locally 4-6" of rainfall near the coast.
While this portion of the Gulf Coast, especially outside of urban
areas, can generally handle heavy rain, some parts of the coast
have experienced well above normal rainfall of 150-300% in the
last 7 days according to AHPS. This has resulted in high soil
moisture anomalies, leading to slightly more favorable conditions
for rapid runoff and flash flooding. While instances of flash
flooding will be more likely should convection train across urban
areas, anywhere that experiences training of these intense rain
rates could have at least isolated impacts this morning.
Weiss
...Please see www.wpc.ncep.noaa.gov for graphic product...
ATTN...WFO...HGX...LCH...LIX...
ATTN...RFC...LMRFC...WGRFC...NWC...
LAT...LON 30449335 30439200 30379105 30249030 30068951
29808901 29358887 28968905 28828976 28949052
29219138 29339227 29429309 29339389 29129446
29069492 29269524 29779509 30209440
Source:
Weather Prediction Center issues Mesoscale Precipitation Discussion #678 concerning HEAVY RAINFALL...FLASH FLOODING POSSIBLE---------------
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