FOUS30 KWBC 130749
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
249 AM EST Wed Nov 13 2024

Day 1
Valid 12Z Wed Nov 13 2024 - 12Z Thu Nov 14 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE DEEP SOUTH...

...Southeast U.S and Ohio Valley...

A 1034mb surface ridge is currently analyzed over the Great Lakes 
and adjacent Ontario Province with an expected translation eastward
over the next 24 hrs leading to an increasing return flow pattern 
over the Southeastern U.S. Current v-vector anomalies pinpoint the 
strongest return flow between sfc-850mb bisecting areas from LA 
into MS and Western AL leading to ample moisture being pulled 
poleward as far north as the Great Lakes. PWAT anomalies on the 
order of +2 to +3 deviations will be co-located with the strongest 
v-vector component leading to a broadening area of elevated
moisture and accompanying theta-E's indicative of greater
instability, especially the closer you are to the Gulf. Across the
Plains, a potent shortwave trough will shift eastward with
increasing mid and upper level ascent across areas downstream
within the Low to Mid-Mississippi Valley area. The combination of
the pattern evolution will aid in increased convective development
and coverage, especially as we work through the late-morning time
frame and beyond during the period. 

Heavy rain potential will be on the rise with the introduction of
the thunderstorm risk with rainfall totals from the latest HREF
blended mean now sufficiently into the 1.5"+ territory with
associated CAMs signaling isolated totals >4" during the time frame
of convective impact. This has ramped up probabilistic signals with
the neighborhood probs now upwards of 40-70% across portions of
South-Central LA through much of MS with the highest probs located
over the MS Delta over to Jackson. Organized convective pattern is
most likely to occur over this area thanks to the approach of a
cold front upstream stemming from surface low development over the
Southern Plains, moving northeast through the Mississippi Valley
after nightfall. Hourly rates within the heaviest convection across
the Deep South is likely to range from 1-2"/hr on average, but some
non-zero prospects for 2.5-3"/hr persists within the CAMs and
expressed through the probability fields on the 00z HREF output.
Typically these signals would warrant a MRGL risk for excessive
rainfall, but considering the soils across parts of LA and MS still
recovering from the onslaught of heavy precip from a few days
prior, the antecedent environment is a bit more conducive for flash
flood prospects. In coordination with the local WFOs within LA and
MS, a Slight Risk was added to encompass areas where heavier
convection is most likely considering the pattern evolution, as
well as covering the areas where top layer soil moisture anomalies
lie above the 70th percentile as reported by the updated NASA SPoRT
data.

The remainder of the Southeast and points up into the Ohio Valley
will maintain the previous MRGL risk forecast with only a minor
expansion to the north and east to account for some of the trends
of increasing heavy rainfall potential through Northern KY as the 
system remains fairly progressive in its forward propagation. 
Limited instability will also cap the risk for significant rainfall
rates, but instances of >1"/hr are certainly plausible considering
the prime mid-level evolution and approaching cold front providing
surface based forcing. Totals will be closer to the 1-2" range for
most, but 2-3" is within reason in the heaviest rain cores. 

...Pacific Northwest...

Atmospheric River from this evening will continue to unfold across
the Pacific Northwest with the IVT pulse orienting further south
through the morning and early afternoon with heavy rain impacting
areas of Southwest OR through much of Northwestern CA with the
heaviest rain focused at the immediate coast into the interior
terrain aligning from the Klamath Mountains down towards the King
Range south of Eureka. The forecast has been very consistent on the
threat the past several days with little deviation in the expected
outcome with totals ranging from 2-4" at the immediate coast from
WA down through Northern CA with 3-6", local maxima of 8" during
the 48-hr period of impact over places like the Olympic Range in WA
state and the Klamath Range down in Southwestern OR. The premise of
the MRGL risk was mainly for 3 or 6 hour FFG exceedance intervals,
along with some 24 hour FFG potential, mainly located over the
Olympics for the latter index. Hourly rates will be majority sub- 
FFG exceedance, however some 1-1.5"/hr rates during the height of 
the IVT surge Wednesday morning over the coastal mountain ranges 
could be sufficient for isolated flooding prospects before becoming
more of an accumulative rainfall concern over time. The one area 
where the heavier precip is not anticipated but still a relative 
concern remains in the area located south of Mount Shasta and 
points west where remnant burn scars maintain locally lower FFG 
indices leading to a heightened threat for flash flooding where the
scarring is located. This is the reason for the inland extent 
across Northern CA and to maintain consistent messaging with the 
local WFO for the threat. 

Kleebauer


Day 2
Valid 12Z Thu Nov 14 2024 - 12Z Fri Nov 15 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer


Day 3
Valid 12Z Fri Nov 15 2024 - 12Z Sat Nov 16 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer

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