267
AWUS01 KWNH 132354
FFGMPD
FLZ000-ALZ000-MSZ000-LAZ000-140530-

Mesoscale Precipitation Discussion 0268
NWS Weather Prediction Center College Park MD
753 PM EDT Mon May 13 2024

Areas affected...Central Gulf Coast

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

Valid 132353Z - 140530Z

Summary...Organized showers and thunderstorms will traverse a
stationary front to the east this evening, with additional storms
possibly developing and training across the same areas. Rainfall
rates of 2"/hr will accompany these thunderstorms, producing rain
totals of 2-3" with locally higher amounts. Flash flooding is
possible.

Discussion...The regional radar mosaic this evening shows a
cluster of thunderstorms associated with a wave of low pressure
moving across south-central Louisiana. This cluster is advecting
east along a stationary front analyzed by WPC, and is producing
radar-estimated rain rates of 2-3"/hr, resulting in local mesonet
rainfall of more than 2 inches around Lake Charles where several
FFWs are currently in effect. The thermodynamic environment
remains primed for heavy rain producing convection due to a ribbon
of 3000+ SBCAPE lifting northward from the Gulf of Mexico
downstream of the surface wave, combined with PWs of 2-2.2 inches,
near the daily max PW according to the SPC sounding climatology.
Weak mid-level lapse rates around 6 C/km and warm cloud depths
approaching 13,000 ft are supporting these efficient warm-rain
processes, and this is likely to continue as subtly sharpened and
backed low-level flow pushes the 850mb winds to 20 kts from the
south, driving continued thermodynamic transport onshore.

There is some uncertainty as to how the next several hours will
evolve downstream of the current convection, and high-res CAMs are
struggling to resolve the current activity. However, the recent
HRRR does appear to be handling the setup a bit better than other
CAMs, but is about 2 hours too slow with its progression. Using
this as a proxy, it appears that the pinched southerly flow will
have the dual effect of surging higher instability just far enough
onshore to persist heavy rain rates, while also leading to some
increased ascent through the isentropic upglide atop the
stationary front. Together, this will likely produce multiple
rounds of convection moving E/NE from southern LA through the
western FL Panhandle. Rain rates within this convection should
exceed 2"/hr at times, which is reflected by HREF 3"/3hr
probabilities of 40-60%. Although the lead cluster near the
surface low may continue to race east, development along its
outflow to the SW could efficiently train back onshore behind it
as mean winds and Corfidi vectors align, leading to additional
rounds of heavy rain moving across the same areas.

Rainfall across this area has generally been 1-3" in the last 24
hours, and well above normal the past 7 days, except from New
Orleans southward through southeast LA, and this is reflected by
anomalously moist 40cm soil percentiles from NASA SPoRT in many
areas. While there is some uncertainty as to how far north this
renewed heavy rain will occur, and whether it can lift to impact
the most sensitive soils, these rates could cause instances of
flash flooding even over the drier soils, especially if they
impact urban areas. For this reason flash flooding is deemed
possible during the next several hours.


Weiss

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

ATTN...WFO...JAN...LCH...LIX...MOB...SHV...TAE...

ATTN...RFC...LMRFC...SERFC...NWC...

LAT...LON   31819164 31759118 31629075 31429018 31228932
            31158880 31138768 31138679 31088619 30778607
            30478625 30378687 30368721 30228809 30108846
            29568866 29168892 29078936 29078978 29078978
            29289118 29419187 29499247 29739292 30219296
            30619289 31279260 31639222