FOUS30 KWBC 180005
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
705 PM EST Sun Nov 17 2024

Day 1
Valid 01Z Mon Nov 18 2024 - 12Z Mon Nov 18 2024

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

0100Z Update..
Maintained the high-end Slight risk area in place with few minor
modifications as the 18Z HREF and 12Z RRFS/REFS continues to
support a locally heavy rainfall event overnight. Rainfall rates
so far have been modest although some enhanced rates were starting
to appear on radar as of late afternoon. Signals still point to=20
increasing rates becoming more widespread during the mid- to late-
evening hours as instability increases in proximity to a deepening
surface low and associated cold front across the Texas High=20
Plains. In addition to the increasing instability with time...the=20
potential for training of cells/repeat convection will also be=20
increasing. That sets the stage for swaths of 2 to 4 inch rainfall=20
amounts with the associated risk of flash flooding tonight and into
the early morning hours on Monday.

Bann

1600Z Update...

The 12Z HREF guidance along with 06Z RRFS/REFS output continues to
support a locally heavy rainfall event across areas of the southern
Plains by this evening and extending into the overnight period. There
has been some shift farther west with some of the swaths of heavy
rainfall potential across west TX as some of the individual CAMs
are supporting the heavier rainfall/convection along the immediate
cold front and in close proximity to the deepening surface low
that will be evolving across the TX High Plains tonight. However, a
complication to the rainfall forecast is the downstream evolution
of convection along the front ahead of the surface low involving
areas of northwest TX through southwest to central OK. This is
where strong moisture convergence and forcing may foster a threat
for some training areas of convection that become aligned in a
southwest to northeast fashion. Overall, still expecting some
swaths of 2 to 4 inch rainfall totals, with isolated potential for
heavier amounts and especially across northwest TX into southwest
OK where the better convective training threat will exist. Given
the latest CAM guidance, the Slight Risk area has been expanded
somewhat to account for the varying modes of convective evolution
that are expected this evening and overnight.

Elsewhere, no changes have been made to the previous D1 outlook
including the Marginal Risk area across western OR where there
continues to be some lingering, but modest atmospheric river
activity.

Orrison

Previous Discussion...

...Southern Plains...

A southern stream cutoff low is expected to eject into the Plains
today with very impressive IVT and accompanying anomalous PWs and
forcing as it moves across Texas and Oklahoma by this evening into
the overnight hours. This looks to be enough to overcome the
limited forecast instability and result in deep convection and
heavy rainfall rates across portions of the Southern Rockies and
the adjacent Southern/Central Plains. Guidance continues to suggest
2-4" of rain (per latest HREF and WPC QPF), but would expect
isolated swaths as high as 4-6" within this setup (indicated by
some of the heaviest CAMs). The overall progressive nature of the
system and limited instability will cap the upper bound of rainfall
totals, but this system is dynamic enough with plentiful moisture
to suggest at least localized flash flooding is likely. A Marginal
Risk area remains in effect for central/eastern New Mexico,
northern Texas, most of Oklahoma and southern Kansas. The inherited
Slight Risk area saw only minor adjustments to reflect the latest
trends and WPC QPF and spans from northern Texas/panhandle to
western Oklahoma.

...Coastal Oregon and northwest California...

A fairly progressive, weak/moderate atmospheric river will be
ongoing across the region. Total rain of 2-4" (through 12z Mon)
and hourly rainfall rates possibly approaching 0.5" within a narrow
southward shifting axis suggest that some minor flooding issues
could arise along coastal areas/mountains. A Marginal Risk area
remains in effect along the coastline.

Churchill/Campbell


Day 2
Valid 12Z Mon Nov 18 2024 - 12Z Tue Nov 19 2024

...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE GULF COAST AND THE PLAINS/UPPER MIDWEST...

20z Update: A Slight risk was added across portions of southeast
Louisiana into coastal Mississippi including New Orleans and
Biloxi. The higher QPF with this event is still forecast to fall on
day 3 over areas further east. However, while heavy QPF coverage
may not be as high on day 2, the ingredients will still be there
for intense rainfall rates over southeast LA. The 12z HREF is not
overly aggressive, but given the setup and ingredients would not be
surprised to see an upward trend with the 00z guidance. Overall,
the environment in place is conducive for a conditional flash flood
threat over urban locations within the Slight risk. A slower
evolution (as indicated by the ECMWF and AIFS) would bring Baton
Rouge more into play, but will let the Marginal risk cover that for
now and keep the Slight focused over the corridor from New Orleans
to Biloxi where the probability of some training convection is
higher Monday night.

Chenard

...Previous Discussion...

...Central U.S. ...

The precipitation shield will shift eastward as the cold front
advances further into the Plains. Moderate to locally heavy
rainfall is expected from northern Texas to southern Minnesota with
areal average of 1 to 3 inches. The higher amounts will likely
concentrate over eastern Kansas, eastern Nebraska and western Iowa.
A Marginal Risk area was maintained once again with only minor
adjustments for this issuance (not enough confidence for 2"+ totals
to introduce a Slight Risk).

...Gulf Coast...

A plume of enhanced tropical moisture will likely advect northward
through the Gulf of Mexico well in advance of tropical cyclone
Sara; which will increase the potential for heavy rain and
localized flooding concerns along portions of the Gulf Coast. A
Marginal Risk area is in effect from southeast Texas to southern
Mississippi (changed very little from inherited with 3" localized
totals possible).

Churchill/Campbell


Day 3
Valid 12Z Tue Nov 19 2024 - 12Z Wed Nov 20 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST AND CENTRAL GULF COAST...

20z Update: Only minor changes to the inherited risk areas, as the
forecast generally looks on track. Expecting a backbuilding
convective complex to push eastward across the central Gulf Coast
Tuesday. Anticipate rainfall rates with this complex will be
intense enough to overcome the high FFG on an isolated to scattered
basis. 3 to 5" totals remain most likely, although localized
swaths exceeding that are probable. The progressive nature of the
system will put a cap on the upper bound of rainfall
magnitudes...although still think locally significant flooding is a
possibility given the intense rates and potential for backbuilding
into the strong southerly low level inflow.

Chenard

...Previous Discussion...

...Central Gulf Coast and Southeast...

By Tuesday significantly more tropical moisture will likely advect
northward through the Gulf of Mexico in association with the
remnants of Tropical Storm Sara; which will increase the potential
for heavy rain and localized flooding concerns along portions of
the Gulf Coast. An inherited Slight Risk area was maintained for
southern AL and surrounding portions of FL/GA/MS/LA, given the
potential for localized 3-5" totals (per ensemble guidance and WPC
QPF).

...Coastal Washington, Oregon, and northwest California...

Another weak/moderate atmospheric river is expected for region for
Day 3. Total rain of 2-4" (similar to Day 1) are expected with
hourly rainfall rates possibly approaching 0.5" once again suggest
that some minor flooding issues could arise along coastal
areas/mountains. An inherited Marginal Risk area remains in effect
along the coastline.

Churchill


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!62lEdMHwnE96Cm0kU3YiHu58H0Gs-gcTa0klBnEw4whu=
TXPS_wTumNa_ZZiuapx5kI_erPbzLh3AiOOtX2DRm5CY47o$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!62lEdMHwnE96Cm0kU3YiHu58H0Gs-gcTa0klBnEw4whu=
TXPS_wTumNa_ZZiuapx5kI_erPbzLh3AiOOtX2DR9EXZrLs$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!62lEdMHwnE96Cm0kU3YiHu58H0Gs-gcTa0klBnEw4whu=
TXPS_wTumNa_ZZiuapx5kI_erPbzLh3AiOOtX2DRe6On2qg$=20

=3D =3D =3D
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