958
FOUS30 KWBC 091922
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
222 PM EST Sat Nov 9 2024

Day 1
Valid 16Z Sat Nov 09 2024 - 12Z Sun Nov 10 2024

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

...16z Update...

Little changes needed for this update, as the new suite of 12z CAMs
remain supportive of relatively small High Risk over southwestern
and central portions of LA. HREF 40-km neighborhood probabilities
for 8" exceedance remain high (between 40-80%), with 100-yr ARI
exceedance probabilities as high as 30-40%. The deep tropical
moisture ushered in as a result of Tropical Storm Rafael is more
typical of late August to early September with precipitable water
values near 2.2" (well above the 90th percentile and near record
levels for early November, per LCH sounding climatology). See MPD
#1145 (and subsequent MPDs) for more information on the near term
threat. Only relatively minor adjustments to the Slight, Moderate,
and High Risk contours for this update (based on the latest trends).

Churchill


...Previous Discussion...

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
LOWER MISSISSIPPI VALLEY AND TENNESSEE VALLEY...

20z Update: A Marginal risk was maintained from the central Gulf
Coast northeastward into KY. Flash flooding may be ongoing at 12z
Sunday across portions of LA, although guidance indicates
convection should be on a weakening trend by late tonight into the
morning hours. At the moment our post 12z QPF does not support
anything more than a Marginal risk over LA, however given what
should be saturated conditions after todays and tonight`s rainfall,
if convection is persisting longer than forecast than an upgrade
in the ERO may be needed through Sunday morning. We were able to
trim back the northeastern portion of the risk area over PA/OH/WV.
While QPF of 1-1.5" is still forecast, limited instability should
keep rainfall rates below 0.5" in an hour over these areas. Not
seeing any FFG exceedance probabilities in the HREF given these
weaker expected rates and dry antecedent conditions.

The Marginal risk over the Pacific Northwest was removed with this
update. The front Sunday night into Monday morning is expected to
be progressive in nature limiting rainfall totals and impacts. Some
brief 0.5" per hour rates are possible along the OR coast, but the
quick movement should limit impacts. All the impacted WFOs were in
agreement on dropping the risk.

Chenard

...Previous Discussion...

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.

Roth


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

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

The Gulf Coast Marginal was dropped as rainfall intensity/coverage
should be decreasing by this time. Still a non-zero chance of a
localized flash flood near the coast, however the latest
deterministic and ensemble guidance suggests FFG exceedance chances
are too low to justify an area at this time.

The Marginal over the Western U.S. was also dropped in coordination
with impacted local WFOs. The first system will exit the area
Monday morning, with just post frontal showery conditions expected
behind it through Tuesday morning. While weak instability behind
the front may support some briefly heavy rates, the coverage and
duration of any heavier rainfall should be too short to cause
impacts with antecedent soil and streamflows running either
average or below average for early November. This rainfall will
help increase saturation ahead of the day 4/5 atmospheric
river...which is expected to be a stronger and more persistent
system. Thus Marginal risks will continue for the day 4 and 5
period.

Chenard


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt