AWUS01 KWNH 091614
FFGMPD
LAZ000-TXZ000-092200-

Mesoscale Precipitation Discussion 1145
NWS Weather Prediction Center College Park MD
1112 AM EST Sat Nov 09 2024

Areas affected...southwestern and central LA and surrounding
portions of Upper TX Coast

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

Valid 091600Z - 092200Z

Summary...Deep tropical moisture will support very high rainfall
rates of 2-3"/hr with 6-hr totals of 3-6"+. Isolated to scattered
instances of flash flooding are likely (and may locally be
significant).

Discussion...A stalling cold front is combining with deep layer
tropical moisture transport from the southeast (in association
with Tropical Storm Rafael) to result in significant moisture flux
convergence into western LA and surrounding portions of the Upper
TX coast. Precipitable water levels are indicated to be as high as
2.2 inches (per CIRA advected layered precipitable water analysis
and RAP mesoanalysis), which is well above the 90th percentile
(per experimental ALPW sfc-850mb and 850-700mb percentiles and LCH
sounding climatology) and more typical of late August to early
September. Although instability is somewhat limited (500-1000 J/kg
of ML CAPE), it remains sufficient for convection and is unlikely
to be eroded with steady low to mid level moisture transport
coinciding with peak diurnal heating. Likewise, deep layer shear
of 20-30 kts will be supportive of organizing updrafts, while
10-15 kts of deep layer flow will keep storm motions near a crawl
(particularly so closer to the coast). All in all, a concerning
mesoscale setup that will be supportive of locally significant
flash flooding.

Updated CAM guidance (12z HREF suite) is increasingly supportive
of significant rainfall totals over the next 6 hours (HREF 40-km
neighborhood probabilities for 5" exceedance, near the 10-yr ARI,
threshold being near 15%), suggesting the potential for localized
amounts of 3-6" (with rainfall rates of 2-3" due to the deep
tropical moisture and slow storm motions). With much of this same
area already having received 3-6" over the prior 6 hours (mainly
areas west of Alexandria, as well as near Winnie, TX), the
associated 4.0" 6-hr FFGs are likely underrepresentative of the
flash flood threat. Given these antecedent conditons AND the
possibility for rainfall amounts to overperform (given the ARW2
solution of localized 6"+ totals), isolated to scattered instances
of flash flooding are considered likely, and locally this flash
flooding may be significant. The threat for significant flash
flooding is also to likely extend beyond the next 6 hours, so a
subsequent MPD is expected (with the threat likely becoming even
more significant later this evening).

Churchill

...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=
DZ3fjg!_9GOqR31doEVg5fnc5xBLOKROmNKwmylL07Vg5HTjGA4LzWucYoszOgIEinx0Dto94TB=
hUNxkmu2h2KiO5ZQ46z5h_E$  for graphic product...

ATTN...WFO...HGX...JAN...LCH...SHV...

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

LAT...LON   32279211 32069174 31219173 30399213 29919256=20
            29559297 29489403 29509452 30089430 30329421=20
            30769397 31419340 31809308 32219254=20

=3D =3D =3D
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