184
AWUS01 KWNH 122357
FFGMPD
MSZ000-LAZ000-TXZ000-130500-

Mesoscale Precipitation Discussion 0261
NWS Weather Prediction Center College Park MD
757 PM EDT Sun May 12 2024

Areas affected...Eastern Texas...Central Louisiana...

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

Valid 130000Z - 130500Z

SUMMARY...Exiting shortwave but increasing diffluence aloft will
continue to support a channel of ascent and new development over
area of recent rainfall.  Training profile would support
downstream risk for flash flooding as well as low level flow veers
later this evening.

DISCUSSION...23z surface analysis denotes a surface wave just west
of Houston with a bulge of the warm front across far SE TX before
angling southeast offshore of S LA.  Aloft, GOES-E WV suite
denotes a shortwave crossing northern LA toward the northeast; yet
in its wake, the larger scale height-falls over the Southern High
Plains are broadening the upper-level flow increasing diffluence
aloft for broad scale ascent pattern.  As such, VWP and RAP
analysis denote the 850mb boundary north of but parallel to the
warm front across LA before extending northwest across the Heart
of TX toward Northwest TX with some sort of wave near Dallas, that
is sliding southeastward.  As such, confluent 850mb flow across
Southeast TX (southwesterly over the Middle TX coast and southerly
east of Galveston Bay) ascends over the boundary and tapping
remaining modest instability gradient with MUCAPEs of 1500-2000
J/kg diminishing to less than 500 into northeast TX to central LA.


The deep layer convergence is resulting in new development along
the axis and southeastward.  Deep layer steering has veered a bit
to a more westerly component but will allow for training while
cells continue to redevelop along the boundary through the late
evening/early overnight period.  Slowly the 850mb wave will drop
south into the Piney Woods and flow LLJ will back further
increasing perpendicular ascent for said regeneration.  Moisture
is not going to be an issue with over 2" total PWATs and
downstream 7H RH values over 90% resulting in efficient rainfall
production for vertical development along the instability
gradient.  As such, rats of 1.5"/hr will be common with occasional
uptick to 2"/hr with short-term storm scale interactions with
other cells, increased convergence.   As such, spots of 2-4"
remain possible through 06z and given saturated grounds
(particularly further west in East Texas), increased runoff is
possible to induce localized incidents of flash flooding.

Of note: Greatest uncertainty is the possibility of new
development closer to the apex of the warm front, in the vicinity
of Polk/Trinity/Tyler counties, becoming more surface rooted.
This would disrupt the isentropic ascent channel to the north and
weaken it, while significantly increasing the potential for much
stronger/deeper updrafts and therefore greater rainfall potential
(2+"/hr) and would likely quickly induce flash flooding given
recent heavy rainfall today and the last week plus. However, will
have to watch closely for this evolution though current trends
hint less and less from this coming to fruition, but if it does a
subsequent MPD will be needed.

Gallina

...Please see www.wpc.ncep.noaa.gov for graphic product...

ATTN...WFO...FWD...HGX...JAN...LCH...SHV...

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

LAT...LON   32869597 32829450 32659357 32219201 31859147
            31129164 30979253 30489396 30599465 30789531
            31649622 32409643