Graphic for MPD #0077

Mesoscale Precipitation Discussion 0077
NWS Weather Prediction Center College Park MD
452 PM EST Mon Mar 04 2024

Areas affected...portions of southeast LA & southwest MS

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

Valid 042151Z - 050351Z

Summary...Showers and thunderstorms are expected to continue to
cause heavy rainfall as a cold pool develops and moves somewhat as
the rain area as a whole shifts east-northeast with time.  Hourly
rain totals to 2.5" with local amounts to 4" could lead to flash
flooding.

Discussion...Heavy rains have fallen across portions of central
LA, with radar estimates of 4"+ west of Simmesport.  Showers and
thunderstorms have been forming within a region of decent 850 hPa
confluence across LA ahead of an upper level shortwave moving into
northwest LA.  Surface observations show a forming cold pool
across areas near the southwest MS/LA border, with upper 60s
temperatures in the cold pool and mid to upper 70s south of the
boundary.  CIN is developing; MU CAPE of 1000-2000 J/kg exists in
this area per SPC mesoanalyses.  Effective bulk shear is near 40
kts, which has supported occasional mesocyclones.  Precipitable
water values of ~1.5" exist here, per GPS data.  The convective
area appears to be forming on the cool side of the 5C isotherm at
700 hPa.

The mesoscale guidance has a strong signal for 3"+ through 04z
over portions of LA and southwest MS, but show a bit of dispersion
on location and on areal extent of the 3"+ area anticipated. 
Recent RAP runs give as suggestion as to the evolution expected. 
The area of 850 hPa confluence shifts east with time, which should
move the heaviest rainfall slowly eastward.  Normally the guidance
can be too poleward with convective activity, which is probably
true with the 12z Canadian Regional guidance.  However, RAP runs
show warming at 700 hPa moving in from the southwest which implies
some northeast shift with time due to a possible mid-level capping
inversion nosing in.  The movement of the 850 hPa confluence zone
eastward implies an east-northeast shift with time.  Since CIN has
already developed, convection is likely to remain elevated and if
it follows the 5C isotherm at 700 hPa northeast, the outflow
boundary/possible section of the polar warm front would follow
suit/retreat somewhat in that direction.  The 18z HREF
probabilities of 3"+ seemed to be a reasonable starting point for
future expectations; generally followed the 15% outline for the
MPD area since the guidance dispersion is likely underplaying the
potential.  Due to mesocyclone formation, short periods of cell
training, and cell mergers, hourly rain totals to 2.5" and
additional local amounts to 4" remain possible, which could lead
to widely scattered flash flooding concerns, particularly across
urban areas.

Roth

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

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

LAT...LON   31749064 31368990 30888946 30468936 29848957
            29719039 30299142 30749237 31329219 31699132
           
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Last Updated: 452 PM EST Mon Mar 04 2024