FOUS30 KWBC 010038
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
838 PM EDT Thu Oct 31 2024

Day 1
Valid 01Z Fri Nov 01 2024 - 12Z Fri Nov 01 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
LOUISIANA AND MISSISSIPPI...

A Marginal risk remains from portions of central LA into southern
and central MS. Localized cell mergers and training may result in a
few instances of flash flooding over portions of these areas.

PWs around 2" and MLCAPE of 1000 j/kg are supportive of efficient
rainfall with a few instances if 2-3" per hour rainfall expected
where cells briefly train. Some cell merging and training is
expected over portions of southwest to south central MS through=20
the evening hours where a slowing line of convection is=20
intersecting cells moving northward within an axis of southerly=20
flow. Guidance indicates that the axis of enhanced convergence that
is driving this threat should weaken within a few hours, with=20
convective intensity also decreasing. However the MCV that is=20
evident over central LA may help sustain this convergence a bit=20
longer than the HRRR indicates...and do think isolated 2-4" amounts
within a couple hours are possible.

Instability is a tad higher over LA, and convergence near the
remnant MCV may linger longer here. While differing in location
from run to run, recent HRRR runs indicate the potential for a few
slow moving convective cells/clusters over central LA tonight. In
the HRRR these cells are anchoring near the instability
gradient/boundary and are able to persist long enough to produce
localized amounts in excess of 6". Confidence in only average at=20
best, but the HRRR scenario is plausible given the setup and=20
ingredients in place. The main question will be whether the MCV=20
stays pronounced enough to support continued low level moisture=20
transport/convergence into the instability gradient.

Dry antecedent conditions have resulted in high FFG values over=20
the region. Thus flash flood issues should generally stay very=20
localized in nature and focused within any more sensitive urban=20
areas where excess runoff from these heavy rates is more likely.=20
However if the HRRR scenario of 6"+ amounts over portions of LA=20
pans out overnight, then a more robust flash flood threat could=20
evolve, so will need to monitor trends through the overnight hours.

Chenard


Day 2
Valid 12Z Fri Nov 01 2024 - 12Z Sat Nov 02 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN PLAINS AND ADJACENT SOUTHERN ROCKIES...

2030Z Update...
Introduced a targeted Slight Risk area in southeast New Mexico
where an uptick in model QPF and model-derived ingredients overlap
with an area that was soaked less than 2 weeks ago and remains
hydrologically sensitive...with the expectation that the risk is
best late in the period and continuing into the Day 3 period. The
maximum Areal average rainfall amount looks to be around an
inch...which normally wouldn't warrant a Slight (especially with
the given the high flash flood guidance being so high). But with
the trends of increasing precipitable water, thetae and model QPF
over over and near the site of the flash flood...felt a Slight was
appropriate here while a Marginal remained more appropriate
elsewhere from the plains of southeast New Mexico to western
Oklahoma.

Bann

0830Z Excessive Rainfall Discussion...

Another shortwave trough will move inland across the West on Friday
as ridging stays in place over much of the Southeastern US. Warm
Gulf moisture will be advecting northward into the southern High
Plains ahead of the trough. Meanwhile, the strongest instability
(1000+ J/kg of SB CAPE) will likely be in place over West Texas
and eastern New Mexico. Areal coverage of thunderstorms is forecast
to increase during the overnight hours (after 00z) into portions
of southeastern NM and the TX Panhandle, propagating eastward into
portions of western OK Oklahoma and far southern Kansas. Maintained
the Marginal Risk from prior cycles, given PWs increasing to 1.0"+
(above the 90th percentile, per SPC sounding climatology). While
most CAMs still do not reach out to the timeframe of interest, the
ones that do (the FV3 and CMCreg) suggest the potential for
localized 3"+ totals, as well as support from the downscaled ECMWF.
Decided to maintain the Marginal for this cycle, given considerable
uncertainty and dry antecedent soil conditions.

Churchill/Campbell


Day 3
Valid 12Z Sat Nov 02 2024 - 12Z Sun Nov 03 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN PLAINS...

2030Z Update...
Few changes were needed given fairly decent run to run agreement
with the scenario of a much more potent longwave trough digging
into the Southwest US that results in more organized convection
capable of producing heavy rainfall. Opted to not to jump all the
way to the full extent suggested by the ML ERO guidance...but felt
an expanded areal coverage was warranted based on the placement
suggested by spaghetti plots of QPF from the ensembles as well as
the global models combined with the idea that the rainfall on Day 3
will function to prime the soils for more runoff/flooding potential
on Day 4 and beyond. Will continue to watch how much overlap there
is and upgrade if necessary. Until then...thinking is that the risk
from Saturday into early Sunday is a high-end Slight Risk from
parts of Oklahoma into far north Texas.

Bann


0830Z Excessive Rainfall Discussion...

A much more potent longwave trough is progged to dig into the
Southwest US on Day 2, as leading shortwave impulses are expected
to result in much more organized convection over much of the
Southern Plains. Continued southerly low-level flow ahead of the
impulses should result in PWs pushing 2" (near record levels, per
SPC sounding climatology) with ample forcing (left exit region of
100+ kt subtropical jet streak) for organizing convection. The WPC
QPF calls for areal average totals of 2-3", which is still well
below the 90th percentile of the PQPF (~4-5"). Given strong
ensemble support, the inherited Slight Risk was expanded a bit
(with the expectation of verification on the higher-end of the
Slight Risk probability spectrum from Northwest TX/Big Country into
south-central OK). Given very dry antecedent conditions, a Slight
Risk was deemed sufficient (but will need to be reevaluated for
upgrade potential with future updates).

Churchill


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!5_UoLTmQYR0fuAD228hQet4eWbUzO9WiC5fyKCKwxcHw=
fxofNhFm06fWd8KK9UIOapAq20XbRJXjHNI_w7wmcIEChU0$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!5_UoLTmQYR0fuAD228hQet4eWbUzO9WiC5fyKCKwxcHw=
fxofNhFm06fWd8KK9UIOapAq20XbRJXjHNI_w7wm6vDvbpI$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!5_UoLTmQYR0fuAD228hQet4eWbUzO9WiC5fyKCKwxcHw=
fxofNhFm06fWd8KK9UIOapAq20XbRJXjHNI_w7wmFbu04ek$=20

=3D =3D =3D
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