Mesoscale Precipitation Discussion 0167
NWS Weather Prediction Center College Park MD
932 PM EDT Sat Apr 20 2024

Areas affected...South-Central Texas through central Louisiana

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

Valid 210132Z - 210700Z

Summary...Showers and thunderstorms with embedded heavy rain rates
will expand across eastern Texas tonight before slowly advecting
eastward. Rain rates of 1-2"/hr are expected within more intense
convection. This will occur atop soils primed from 1-3" of
rainfall already this afternoon, leading to the potential for
scattered instances of flash flooding.

Discussion...The regional radar mosaic this evening depicts an
expansion of reflectivity above 40dbZ from near the Rio Grande
Valley east-northeast through the Texas Triangle and into central
Louisiana. This convection is expanding in response to impressive
ascent driven by height falls downstream of an approaching
mid-level trough axis, diffluence within the RRQ of a departing
upper jet streak, and robust low-level convergence along a
sfc-850mb front sagging southward towards the Middle Texas Coast.
The associated lift from the overlap of these features is
occurring in favorable thermodynamics noted by PWs as high as
1.9", and SPC/RAP MUCAPE of 500-1500 J/kg. This has resulted in
moderate to heavy rain across much of this area already this aftn,
reflected by MRMS and mesonet observed rainfall of 1 to as much as
3 inches, priming the soils for a slightly greater flash flood
risk overnight.

As the evening progresses, all of the significant forcing should
gradually shift eastward, placing eastern Texas and central
Louisiana in the axis of greatest overlap for heavy rain. Local
backing of the low-level winds into the stationary front will
allow PW advection to persist, and although 850mb wind speeds will
remain modest at 10-15 kts, to temper the effective inflow, this
will be sufficient in the extremely moist column to support
continued convective development with heavy rain. Both the HRRR
and HREF sub-hourly fields suggest rain rates of 1-2" will be
common overnight, and while this may be overdone due to modest
instability, the impressive ascent combined with increasing 0-6km
shear to drive convective organization should offset some of this
to permit at least periodic rain rates of this intensity.
Additionally, Corfidi vectors and mean 0-6km winds aligned to each
other and the low-level front suggest training potential which
could support an additional 2-3" of rain with locally higher
amounts.

FFG remains quite high, but has likely been tempered in some areas
where HRRR 1cm soil moisture is fully saturated from today's
antecedent rain. The 18Z HREF exceedance probabilities for 3-hr
FFG did reach 20-30% just inland from the Middle and Upper Texas
Coast, but it is probable the true potential is greater than that
due to the primed soils. Still, there is some concern that the
most intense rain rates will struggle to linger temporally,
somewhat limiting the flash flood risk. However, where training
can continue, especially over the most saturated soils, flash
flooding remains possible.

Weiss

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

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

LAT...LON   31809342 31709257 31609212 31409177 31159178
            31039216 30829295 30499386 29939483 29519587
            29119689 28799810 28769859 29229886 29779874
            30449822 31019723 31519617 31779456