FOUS30 KWBC 161931
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
231 PM EST Sat Nov 16 2024

Day 1
Valid 16Z Sat Nov 16 2024 - 12Z Sun Nov 17 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE COASTAL
RANGES OF THE PACIFIC NORTHWEST...

Conventional GOES satellite imagery along with ALPW data shows an
offshore atmospheric river beginning to gradually impinge on the
Pacific Northwest. An offshore warm front will gradually begin to
overspread the region by this evening which will coincide with
strong warm air advection and moisture transport. The latest 12Z
HREF guidance suggests a 40 to 50+ kt low-level jet arriving with
an orientation gradually becoming more orthognal to the coastal
terrain of western WA including the Olympic Peninsula and extending
southward down into northwest OR. A cold front will then begin to
approach and gradually cross western WA by 12Z/Sunday. This
atmospheric river event will be progressive, but there will be a
corridor of IVT values locally exceeding 500 kg/m/s tonight across
these coastal ranges based off the 06Z GFS which seems reasonable
considering the elevated CIRA-LVT parameters offshore. Some
rainfall rates associated with the combination of strong warm air
advection and orographic forcing should reach into the 0.25" to
0.50"/hour range between 00Z and 12Z, and this will help facilitate
some rainfall totals of 2 to 3 inches by 12Z for the windward
slopes of the coastal ranges. Given the expected rates and totals
by early Sunday morning, and thus earlier arrival of this event, a
Marginal Risk of excessive rainfall has introduced for this period.

Orrison


Day 2
Valid 12Z Sun Nov 17 2024 - 12Z Mon Nov 18 2024

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

20z Update: Main change with this update was a minor east and=20
southeast expansion of the Slight risk over north central TX into=20
central OK. Model agreement is pretty good on the large scale, but
as is typically the case, smaller scale convective processes will=20
likely drive where the heaviest rainfall axis ends up. This is=20
less predictable and warrants a slightly larger Slight risk area.=20
If there is a model error with convective QPF placement, more=20
often than not the heavier QPF is south and east of the forecast.=20
This is why we were a bit more generous on the east and southeast=20
side with the Slight area...with the 12z HREF guidance also=20
supporting the southward expansion.

Chenard

...Previous Discussion...

A southern stream cutoff low is expected to eject into the Plains
and be accompanied with very impressive IVT, PWs and forcing as it
moves across Texas and Oklahoma by Sunday evening into the
overnight hours. This should 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, but would expect isolated swaths as high as 4-6"
within this setup. 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 a minor expansion 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.

Campbell


Day 3
Valid 12Z Mon Nov 18 2024 - 12Z Tue Nov 19 2024

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

20z Update: Only minor changes to the inherited Marginal risk
areas. We tightened up the Plains Marginal to try to keep it
focused along/ahead of the low track where weak instability may=20
briefly enhance rainfall rates. There is some chance we'll need a=20
Slight risk over portions of OK into south central KS, as a flash=20
flood threat may be ongoing at 12z Monday (a continuation of the=20
day 1 Slight risk). However for now will maintain the Marginal and=20
allow for a look at the overnight HREF guidance.

There is also a chance we'll need a Slight risk over portions of
the central Gulf Coast for Monday night. Convection should develop
and grow upscale during this time, although the stronger model=20
signal is after 12z Tuesday (and we do have a day 4 Slight risk=20
covering that threat). Certainly plenty of moisture moving in with
a deep connection to tropical moisture from the remnants of=20
Sara...just a question with regards to how quickly we see a more=20
robust uptick in forcing and convective organization. This is more=20
likely by Tuesday (day 4), thus will keep this day 3 risk at=20
Marginal for now and continue to monitor.

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 with minor adjustments for this
issuance.

...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.

Campbell


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!5ObbgveIoPJkmw1DtI_DwVoqJnBtuGlSCBXGhNYGeq9V=
4IGmEwSeswDP0XOS4Qbz7QEVkWD2t8Uw8v602G35fAkIlOA$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!5ObbgveIoPJkmw1DtI_DwVoqJnBtuGlSCBXGhNYGeq9V=
4IGmEwSeswDP0XOS4Qbz7QEVkWD2t8Uw8v602G35eIYGMAw$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!5ObbgveIoPJkmw1DtI_DwVoqJnBtuGlSCBXGhNYGeq9V=
4IGmEwSeswDP0XOS4Qbz7QEVkWD2t8Uw8v602G35BbmZwEc$=20

=3D =3D =3D
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