FOUS30 KWBC 230748
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
248 AM EST Sat Nov 23 2024

Day 1
Valid 12Z Sat Nov 23 2024 - 12Z Sun Nov 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SIERRA NEVADA...

A cold front and the remnants of the long duration, multi-day
atmospheric river that has been impacting the West Coast will be=20
dropping south across the central Sierra Nevada early this morning.
As the deeper layer moisture flux parameters/IVT magnitudes=20
continue to weaken and thin out, the rainfall rates will be=20
continuing to drop off. However, there still may be a sufficient
level of upslope flow/orographic ascent in the foothills of the=20
central Sierra Nevada for a few more hours early this morning of
spotty 0.25" to 0.50"/hour rainfall rates. Some weak post-frontal
instability is also still forecast which could allow for some=20
scattered pockets of heavy shower activity going through midday to=20
impact some of the western slopes of the central Sierra Nevada.=20
This could result in some isolated quick- hitting 0.25" to 0.50"=20
rainfall amounts, but these convective cells are forecast to be=20
progressive. Overall, the additional storm total amounts based off=20
the 00Z HREF guidance may reach as high as 1 to 2 inches.

Given the very wet antecedent conditions, the additional rains may
foster some additional localized runoff/flooding concerns. Thus,=20
the Marginal Risk area over portions of the Sierra Nevada will be=20
maintained.

Orrison

Day 2
Valid 12Z Sun Nov 24 2024 - 12Z Mon Nov 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NORTHWEST CALIFORNIA INTO FAR SOUTHWEST OREGON...

Locally heavy rain will be possible Sunday into Sunday night over=20
northwest CA into far southwest OR. A front slowing near the coast=20
and a weak low to mid level wave riding along it will support a=20
modest uptick in IVT and plentiful low level convergence. Moisture=20
is a limiting factor for heavy rainfall with the better moisture=20
plume situated well south and not really getting drawn into this=20
system. Thus this is not expected to be a significant atmospheric=20
river, and normally would not expect a system of this nature to=20
pose a flood risk. However given the significant rainfall over the=20
past few days, this additional rainfall may be enough to cause some
localized additional flooding as the area will be more=20
hydrologically sensitive than normal.=20

We are still seeing some spread in model QPF output over this=20
period. If some of the more aggressive solutions pan out then we=20
could see 1-3" of rainfall is spots with hourly totals peaking=20
around 0.3-0.4" in the typically more favored upslope areas.=20

Chenard

Day 3
Valid 12Z Mon Nov 25 2024 - 12Z Tue Nov 26 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF=20
NORTHERN CALIFORNIA INTO FAR SOUTHWEST OREGON...

Showery conditions will likely continue across portions of northern
CA and southwest OR Monday into Monday night as a deep layered low
and shortwave energy moves inland. Certainly not a classic setup=20
for flooding with limited moisture and dropping IVT=20
values...however this area remains more hydrologically sensitive=20
than normal following the significant atmospheric river event the=20
past few days. Additional rainfall over far northwest CA into=20
southwest OR is generally 1-3" in the guidance, with isolated=20
totals of this magnitude also possible over the western slopes of=20
the northern Sierra Nevada.

A more significant plume of moisture/IVT will actually move into=20
portions of southern/central CA...generally from portions of San=20
Luis Obispo and Monterey counties into the western slopes of the=20
southern Sierra Nevada. QPF has been trending up with this system,=20
although still seeing some model spread. The latest model consensus=20
suggests a possible 2-3" of rain along the coastal ranges and the=20
southern Sierra below snow levels. Not seeing any instability with=20
this system and large scale forcing is only modest, so hourly=20
rainfall rates are generally not expected to exceed 0.5". Given=20
this, and the fact that most of this area has not seen significant=20
rainfall recently, will continue with no areas in the excessive=20
rainfall outlook for now.

Chenard

Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!6yeO_D40cNklhUiMlxIM4a7rsIHA0yVsXyWBCuurEtH4=
85DTi92n3jEwspeJtV6NBSTQjrc5sQyL8zRYWuHrkmJbmcI$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!6yeO_D40cNklhUiMlxIM4a7rsIHA0yVsXyWBCuurEtH4=
85DTi92n3jEwspeJtV6NBSTQjrc5sQyL8zRYWuHrzrOP1S8$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!6yeO_D40cNklhUiMlxIM4a7rsIHA0yVsXyWBCuurEtH4=
85DTi92n3jEwspeJtV6NBSTQjrc5sQyL8zRYWuHrXKMA4zM$=20

=3D =3D =3D
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