FOUS30 KWBC 090748
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
248 AM EST Sat Nov 9 2024

Day 1
Valid 12Z Sat Nov 09 2024 - 12Z Sun Nov 10 2024

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL IN AND NEAR IMPERIAL
CALCASIEU IN LOUISIANA...

An upper level low will move faster towards the northeast. As
Rafael moves west over the central Gulf, a plume of moisture with
precipitable water values of 1.5-2" gets drawn north ahead of the
surface low's strong cold front. The front will likely be the
dominant forcing for storms in this region as it stalls, which have
already begun to evolve in recent radar imagery. The southern
areas of Louisiana and Mississippi will have the greater moisture.
MU CAPE rises to 1000 J/kg or so across parts of LA/MS/TN. Areas
further north have soils which are more saturated/flash flood
guidance is lower. The front should progress slow enough that
training storms tracking north along the front could still cause
flooding problems, especially over portions of LA, MS, and TN.
Hourly rain totals to 2.5" and local amounts of around 8" due to
cell training appear likely based on the 00z HREF probabilities of
8" totals. There is much better agreement on the placement when
compared to this time yesterday. Portions of Central LA have
received 300%+ of their average seven day rainfall, so soils should
have some sensitivity. Coordination with the LCH/Lake Charles LA
forecast office led to the increase to a High Risk.

Roth


Day 2
Valid 12Z Sun Nov 10 2024 - 12Z Mon Nov 11 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
PACIFIC NORTHWEST, LOWER MISSISSIPPI VALLEY...TENNESSEE VALLEY...
AND UPPER OHIO VALLEY...

Central Gulf Coast northeast towards southwest Pennsylvania...
Moisture and instability from the Gulf gets advected from the
vicinity of Raphael northeast across portions of the Central Gulf
Coast through the Tennessee and Upper Ohio Valleys. The forward
progress of the front across much of the East should keep any
excessive rainfall concerns minimal -- some of the risk area from
eastern AR to the east-northeast just south of the OH river
accounts for recent rains/some soil saturation. The guidance
remains unclear as to whether heavy rainfall gets ashore or not.
Precipitable water values should rise above 1.5", particularly in 
the southern portion of the area near the coast, which should be 
enough for heavy rain concerns. It appears that enough instability 
could be available along the immediate Gulf Coast for convection 
with heavy rainfall. Hourly rain totals to 2" with local totals to 
5" appear achievable along or near the immediate Gulf coast. 
Whenever better agreement amongst the guidance on the future of 
Rafael as well as the placement any rainfall maximum near the coast
occurs, a Slight Risk could prove useful at that later time. To 
the north, hourly totals up to ~1" with local totals to 2" appear 
to be the maximum potential.

Pacific Northwest...
Precipitable water values rise to 0.75-1" with a front moving into
the area. Inflow from the Pacific rises to 50-60 kts from the
south-southwest near the front. In the wake of the front,
instability moves in due to colder air aloft. Hourly rain totals
exceeding 0.5" appear likely here, which would be most problematic
in burn scars. Left the Marginal Risk area intact though simplified
the area's appearance.

Roth


Day 3
Valid 12Z Mon Nov 11 2024 - 12Z Tue Nov 12 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
PACIFIC NORTHWEST & CENTRAL GULF COAST...

Central Gulf Coast...
Precipitable water values north of Rafael are expected to remain in
the 1.5-2" range in the vicinity of a slowly progressive front near
the Central Gulf Coast. While much of the guidance keeps rainfall
amounts minimal, the 00z NAM showed local amounts of 3"+, though
it gets there by allowing Rafael to get much closer to the Gulf
Coast than other model guidance. Considering that heavy rainfall
is possible over the previous couple of days, figured a new
Marginal Risk was a sensible precaution in case the NAM is close to
correct and/or soils are saturated by that time.

Pacific Northwest...
Precipitable water values of 0.5-0.75" and MU CAPE of a few hundred
J/kg are expected in a post-frontal environment. Inflow off the
Pacific is expected to remain around 40 kts. Hourly rain totals
exceeding 0.5" remain possible, which would be most problematic in
burn scars. Left the inherited Marginal Risk area intact though
simplified the area's appearance.

Roth

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