AWUS01 KWNH 061154
FFGMPD
LAZ000-061501-

Mesoscale Precipitation Discussion 0981
NWS Weather Prediction Center College Park MD
753 AM EDT Fri Sep 06 2024

Areas affected...Central LA coastline

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

Valid 061153Z - 061501Z

Summary...Periods of 3-3.5"/hr rainfall rates along the eastern
flank of a meso-low will drive a risk of additional flash flooding
along the Central LA coastline through 15z.

Discussion...Recent radar mosaic depicts an emerging mesoscale
circulation near Vermilion Bay with a slow northeastward track,
attached to a stationary front analyzed along the Gulf Coast. East
of this low, periods of 3-3.5"/hr rainfall rates are noted within
the warm sector of the front along the coastline.

RAP forecasts suggest this boundary will slowly lift inland over
the next several hours, ushering in 1000-2000 J/kg MUCAPE to
maintain periods of very heavy rainfall close to the coastline in
the presence of 1) 2.5-2.6" PWATS and 2) enhanced deep moisture
convergence east of the low. The last 3-4 runs of the HRRR appear
to have initialized the location of the and strength of the
circulation better, and suggest very localized rainfall totals
upwards of 4-6" are possible through 15z. Given 1-3 hour FFGs
generally in the 2-3" range, additional flash flooding may result
along the immediate coastline.

Asherman

ATTN...WFO...LCH...LIX...

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

LAT...LON 29999151 29799048 29378997 29079019 29039093
29359144 29529181 29849188
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AWUS01 KWNH 131111
FFGMPD
MSZ000-LAZ000-131635-

Mesoscale Precipitation Discussion 1153
NWS Weather Prediction Center College Park MD
611 AM EST Wed Nov 13 2024

Areas affected...South-central Louisiana...

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

Valid 131110Z - 131635Z

SUMMARY...Slow moving, efficient cells capable of 2.5"/hr rain
rates. Training profile and upstream redevelopment pose risk of
localized flash flooding with 3-4" totals possible.

DISCUSSION...The remnant low level circulation of Rafael has been
shearing north to south through a deep axis, but has retained a
few convergent low-level bands and fairly robust core of anomalous
deep layer moisture. Aloft, large scale ridge over-top the
circulation is also being stretched/amplified along/ahead of
height-falls coming out of the central Plains. As a result, weak
850-500 DPVA is providing ascent and low level wind response to
increase surface to boundary layer moisture convergence with 25kts
of 850mb flow from the southeast cyclonically converging at the
apex/downshear of the wave just south of central LA coastline. A
core of enhanced inability also exists through this moisture
transport axis with 2000-2500 J/kg of SBCAPE to provide strength
to vertical development over the next few hours. Within this axis,
CIRA LPW shows core of enhanced moisture with 1-1.1" in the
sfc-850, and over .5" in the 850-700mb layer. Both are near the
99th to maximum percentile for climatology in November.

Current GOES-E SWIR/10.3um EIR along with RADAR denotes initial
convective development near Marsh Island lifting north through the
best convergence/confluence axis. Cells are initially weaker with
1-1.5"/hr rates, but with increasing DPVA/strengthening of the
winds strengthening convergence, coverage of cells capable of
2-2.5"/hr are likely to become more numerous as winds strengthen
to 20-30kts by 13-14Z. Deep layer moisture is confluent into
increasing FGEN/moisture axis, deformation zone that extends into
central LA and up the Lower MS River Valley/Delta Region. As
such, a favorable upstream convergence should support upstream
development for potential training/repeating into a slowly (0-5kt)
eastward drifting moisture gradient. Modest effective bulk shear
in the 25-30kt range, suggest some weak rotation to the updrafts
could further slow forward propagation resulting in localized axis
of 2-4".

While current trends suggest most cells will remain near the coast
in the near-term, mid-morning could see further downstream motions
toward areas of recent heavy rainfall and saturated upper soil
depths...while FFG value have rebounded, 0-40cm saturation ratios
remain over .65 which is the 90th-95th percentile, suggesting
increased runoff in more likely north of I-10 into central LA.
Still, coastal regions may be more receptive and scattered flash
flooding/rapid inundation is considered possible through 16z.

Gallina

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

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

LAT...LON 31769219 31579159 30609114 29649066 29069024
28929090 29369197 29489241 29539295 29939321
31079309 31539277

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