832
AWUS01 KWNH 092201
FFGMPD
MSZ000-LAZ000-TXZ000-100400-

Mesoscale Precipitation Discussion 1146
NWS Weather Prediction Center College Park MD
459 PM EST Sat Nov 09 2024

Areas affected...south-central LA and surrounding portions of
Upper TX Coast and southwest MS

Concerning...Heavy rainfall...Flash flooding likely

Valid 092200Z - 100400Z

Summary...Deep tropical moisture will remain support of high
rainfall rates (of 1-2"/hr) with additional 6-hr totals of 3-6"+
likely. Locally significant flash flooding is expected to
continue, and overall coverage of flooding is likely to become
more numerous to widespread.

Discussion...A wide band of rainfall with embedded heavier showers
has persisted over the past several hours along a surface trough
and ahead of a stalling cold front. Deep layer tropical moisture
remains in place over southwestern LA and surroundings (in
association with southwesterly low to mid level moisture transport
from Tropical Storm Rafael), as indicated by precipitable water
levels of 2.1-2.3 inches (per CIRA advected layered precipitable
water imagery and RAP mesoanalysis). This tropical moisture is
well above the 90th percentile (per experimental ALPW sfc-850mb
and 850-700mb percentiles and LCH sounding climatology), and much
more typical of late August to early September. While instability
remains the primary limiting factor (limiting peak rainfall rates
to 1-2"/hr), 250-500 J/kg of SB/ML CAPE has been sufficient for
sustained heavy rainfall. Going forward, convection may continue
to slowly shift to the southeast (depsite the front stalling near
the LA/TX border), as SB CAPE has risen to 500-1000 J/kg over
south-central LA and southwest MS. Deep layer shear has also
increased a bit (to 25-35 kts), which should continue to be
supportive of at least embedded organized updrafts (with
diffluence aloft also improving with the right-entrance region of
a broad 75 kt jet streak at 250 mb). And with deep layer flow as
low as 15 kts, slow storm motions will continue to support a
mesoscale setup capable of locally significant flash flooding
(particularly over areas that have already received as much as
3-6" of rain over the past 6-12 hours).

Updated CAM guidance (18z HREF suite) remains supportive of
additional significant to extreme rainfall totals over the next 6
hours (HREF 40-km neighborhood probabilities for 5" exceedance
threshold, also corresponding to the 10-yr ARI, as high as 50%),
suggesting the potential for additional localized amounts of 3-6"+
(through 04z). Much of this rainfall may occur near the already
hard hit region in and around Alexandria, though propagation
towards higher instability (south and east) appears to be a more
favorable outcome (as suggested by several HRRR runs, as well as
the ARW/ARW2/FV3 members of the HREF). Locally significant (to
even extreme) flash flooding remains possible, conditional upon
the rainfall axis remaining stalled over southwestern and central
LA. Should the rainfall axis eventually shift towards the south
and east, then additional numerous (to possibly widespread)
instances of flash flooding will shift in tandem.

Churchill

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

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

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

LAT...LON   32009189 31859111 31459090 30629139 30049208
            29699262 29599388 29619424 29939427 30429390
            30979347 31309313 31809265 31969237