Mesoscale Precipitation Discussion 0166
NWS Weather Prediction Center College Park MD
404 PM EDT Sat Apr 20 2024

Areas affected...South-Central through Eastern Texas

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

Valid 202003Z - 210200Z

Summary...Showers and thunderstorms expanding across portions of
eastern Texas this afternoon will intensify through the evening
and begin to train to the northeast. Rainfall rates of 1-2"/hr,
possibly higher at times, are expected within this strengthening
convection. This will result in 2-3" of rain with locally higher
amounts up to 5", possibly resulting in instances of flash
flooding.

Discussion...The regional radar mosaic this afternoon indicates a
rapid expansion of showers and thunderstorms across eastern Texas
from near the LA/TX border southwest through portions of
south-central Texas. Rainfall rates within this fresh convection
have been estimated via local WSR-88Ds to be up to 1"/hr already,
with regional mesonet observations measuring as much as 1/3 of an
inch in 20 minutes. This development is being fueled by a
combination of synoptic ascent through increasing upper diffluence
in the tail of a 130kt upper jet streak, modest PVA in the
vicinity of weak mid-level impulses moving through the flow across
TX, and increasingly impressive low-level convergence along the
front. Additionally, some isentropic upglide is occurring as the
925-850mb flow surges out of the Gulf of Mexico and atop the
stationary boundary. This deep layer ascent is working upon a
favorable thermodynamic environment for heavy rain characterized
by PWs as high as 1.8", around the 90th% according to the SPC
sounding climatology, and MUCAPE above 3000 J/kg.

As the afternoon progresses, the high-res guidance is very
consistent depicting a strong signal for an expansion of heavy
rainfall across this area. Recent runs of the HRRR, ARW, and ARW2
all indicate that an axis of heavy rain will develop along and
just north of this stationary front, likely responding to the
persistent mesoscale ascent occurring there in the otherwise
pronounced synoptic lift. Mean 850-300mb winds are progged to
remain progressive at 20-30 kts, but will also be favorable
aligned with the Corfidi vectors, suggesting an enhanced training
threat as storms build back into the greater instability and lift
ENE. With rain rates likely peaking above 2"/hr at times as
reflected by both HREF neighborhood probabilities and the HRRR
sub-hourly fields, this will likely cause stripes of 2-3" of rain,
with locally as much as 5" possible noted by HREF 54"/6hr
neighborhood probabilities reaching 20-30%.

FFG across the region is quite high at generally 3"/1hr and
4"/3hrs, which is responsible for capping HREF FFG exceedance
probabilities at just 20-30%. However, there is an axis of higher
40cm soil moisture noted via NASA SPoRT from just west of Houston
up towards the ArkLaTex. While flash flooding is possible in most
areas where these heavy rain rates train most efficiently,
instances are most likely should the rain fall across these more
saturated soils.

Weiss

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

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

LAT...LON   31959462 31709384 31169358 30499374 29949427
            29549529 29079646 28319777 28089802 28039844
            28229869 28789868 29449846 30149790 30929717
            31699595