Graphic for MPD #0076
Mesoscale Precipitation Discussion 0076
NWS Weather Prediction Center College Park MD
1230 PM EST Mon Mar 04 2024

Areas affected...Much of Louisiana & Adjacent SW Mississippi and E
Texas...

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

Valid 041730Z - 042330Z

SUMMARY...Strong, slow moving rotating cells capable of very
intense rainfall rates (2-2.5"/hr) pose short-term totals possibly
inducing localized flash flooding.

DISCUSSION...Regional RADAR and GOES-E imagery suite depicts broad
area of widely scattered thunderstorms starting to sprout through
low level cloudy conditions across Louisiana.  Solid
warm-advective regime with southeasterly surface veering quickly
to south and south-southwest into the 850-700mb layer is providing
ample theta-E advection in combination with solid low level
moisture flux convergence bisecting LA from SE to NW.  GOES-WV
suite and VWP/RAP analysis denotes solid mid to upper level
divergence/diffluence just west of apex of mid-level ridging over
south-central TX, providing excellent evacuation as well as solid
lapse rates for the upper 60s to lower 70Fs surface moisture
values into any developing updrafts.  CIRA LPW does note that
850-500mb moisture is lagged a bit further south across the
northwestern Gulf of Mexico, but will continue to advect
throughout the afternoon rising from total PWAT values of 1.3"
currently per GPS (1.5" at the coast) eventually toward 1.75" by
evening hours. 

Cell mode will be key toward rainfall efficiency and given bulk
shear values, cell rotation and therefore isallobaric moisture
influx should load the lower portion of the cells for the
potential for efficient rainfall production.  12-15z HRRR
solutions suggest discrete cells with capability  of 1.25-1.5" in
15-30 minutes are possible with the rotating thunderstorms.  Cells
across eastern Texas and northern Louisiana will have slight
faster cell motions and are probably going to be more
scattered/discrete in nature, even with Bunkers right moving
propagation vectors being a bit faster than further south from
I-10 to the Gulf, this may limit some of the overall totals
further north, but isolated 2-3" over a short-duration may still
pose localized flash flooding concerns even though soils have been
dry (as noted by NASA SPoRT LIS 10-40cm soil moisture below 50%,
in the 5-10 percentile for the season).  If hard enough,
infiltration will be difficult and almost hydrophobic given the
high rates. 

Further south, propagation vectors due to rotation may be very
slow, perhaps even support less than 10kt southeasterly
propagation with some near stationary motions anchoring along the
deeper layer moisture convergence/WAA axis in the realm of
possibility between 21-00z. While most of the guidance and
observations suggest these stronger cells are likely to be further
south into the swamps...there are ample areas at or south of I-10
with urban ground conditions that may quickly flood if the cells
stall or even potentially back-build/redevelop upstream along that
moisture convergence axis.  As such, highly focused but higher
totals in excess of 4" are possible with 12z HREF denoting some
spots of 30-50% of 5" exceedance by 06z from BTR to MSY and into
the Chandeleur Islands.  This may occur after the valid time of
this MPD, so will continue to watch trends closely for any
subsequent required updates.

All in all, the discrete nature to the cells requiring stronger
cell rotation to support moisture influx as well as slower cell
motions while also occurring across urban areas leaves the risk of
flash flooding to localized and considered possible given the
lower confidence in precision of cell track/location this
afternoon into evening.  

Gallina

ATTN...WFO...JAN...LCH...LIX...SHV...

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

LAT...LON   32989272 32919173 32639125 31919074 31329031
            30888973 30238900 29588884 29028908 29008986
            29879117 30919304 31799379 32799360
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Last Updated: 1230 PM EST Mon Mar 04 2024