FOUS30 KWBC 231558
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1058 AM EST Sat Nov 23 2024

Day 1
Valid 16Z Sat Nov 23 2024 - 12Z Sun Nov 24 2024

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

...16Z update...

While the IVT plume has weakened considerably over the past 24
hours and will continue to do so through today as it progresses
southward into southern California, an isolated threat for=20
runoff/flooding will continue along the foothills/lower elevations=20
of the Sierra Nevada. Moisture and instability look to remain=20
limited (0.50 to 0.75 inch PWATs and pockets of CAPE in excess of=20
500 J/kg...via recent RAP guidance) but W to SW unidirectional=20
flow may support a few isolated low-topped showers capable of=20
training with 0.5+ in/hr rates. This potential will maintain a low-
end flash flood threat across very wet antecedent soils and area=20
burn scars. The Marginal Risk was extended northward to include the
Park burn scar in the northern Sacramento Valley.

Otto

...previous discussion follows...


A cold front and the remnants of the long duration, multi-day
atmospheric river that has been impacting the West Coast will be
dropping south across the central Sierra Nevada early this morning.
As the deeper layer moisture flux parameters/IVT magnitudes
continue to weaken and thin out, the rainfall rates will be
continuing to drop off. However, there still may be a sufficient
level of upslope flow/orographic ascent in the foothills of the
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
scattered pockets of heavy shower activity going through midday to
impact some of the western slopes of the central Sierra Nevada.
This could result in some isolated quick- hitting 0.25" to 0.50"
rainfall amounts, but these convective cells are forecast to be
progressive. Overall, the additional storm total amounts based off
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,
the Marginal Risk area over portions of the Sierra Nevada will be
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
northwest CA into far southwest OR. A front slowing near the coast
and a weak low to mid level wave riding along it will support a
modest uptick in IVT and plentiful low level convergence. Moisture
is a limiting factor for heavy rainfall with the better moisture
plume situated well south and not really getting drawn into this
system. Thus this is not expected to be a significant atmospheric
river, and normally would not expect a system of this nature to
pose a flood risk. However given the significant rainfall over the
past few days, this additional rainfall may be enough to cause some
localized additional flooding as the area will be more
hydrologically sensitive than normal.

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

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
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
for flooding with limited moisture and dropping IVT
values...however this area remains more hydrologically sensitive
than normal following the significant atmospheric river event the
past few days. Additional rainfall over far northwest CA into
southwest OR is generally 1-3" in the guidance, with isolated
totals of this magnitude also possible over the western slopes of
the northern Sierra Nevada.

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

Chenard

Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!68bx-209Y76qEJf32dRiJ4wbPKprQDIrwqTv97O7cDDS=
h6E36hWexb7B1MhKQI7oCeM13iWVdNhSqCoYLDX1JQkYEXY$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!68bx-209Y76qEJf32dRiJ4wbPKprQDIrwqTv97O7cDDS=
h6E36hWexb7B1MhKQI7oCeM13iWVdNhSqCoYLDX1XIRtxE8$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!68bx-209Y76qEJf32dRiJ4wbPKprQDIrwqTv97O7cDDS=
h6E36hWexb7B1MhKQI7oCeM13iWVdNhSqCoYLDX1RDfQQqI$=20

=3D =3D =3D
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