FOUS30 KWBC 231941
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
241 PM 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
runoff/flooding will continue along the foothills/lower elevations
of the Sierra Nevada. Moisture and instability look to remain
limited (0.50 to 0.75 inch PWATs and pockets of CAPE in excess of
500 J/kg...via recent RAP guidance) but W to SW unidirectional
flow may support a few isolated low-topped showers capable of
training with 0.5+ in/hr rates. This potential will maintain a low-
end flash flood threat across very wet antecedent soils and area
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...

...1930 UTC Update... No changes made to the Day 2 ERO based on=20
the 12Z guidance. This includes the array of HREF probabilistic=20
data, which continues to depict spotty elevated probabilities of=20
0.50+ in/hr rates during day 2 in the Marginal Risk area, and the=20
south- north axis of 250-500 kg/ms IVT aligns along the NorCal=20
coast.

Hurley

...Previous discussion...
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.

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 ACROSS MUCH OF
THE CALIFORNIA CENTRAL VALLEY AND WESTERN SLOPES OF THE SIERRA,
ALONG WITH PORTIONS OF NORTHERN CALIFORNIA INTO FAR SOUTHWEST=20
OREGON...

...1930 UTC Update...
Have made a fairly sizable southward expansion to the Marginal Risk
area on Day 3, to now encompass the CA Central Valley and adjacent
Sierra foothills. This was based on the latest guidance trends,
which are fairly consistent in showing a developing modest, W-SW to
E-NE oriented AR coming ashore south of the Bay area Mon and Mon
night. IVTs of 400-600 Kg/ms are expected to push ashore per both
the GFS and ECMWF, with a corresponding TPW anomaly of 2.5-3
standard deviations above normal. Global QPFs, along with the NAM
and GEM regional guidance, have come up, especially across the
western Sierra slopes given the strengthening upslope enhancement.
24 hour rainfall totals of 2-5+ are noted from the models, again
with the heaviest totals across far eastern portions of the Central
Valley into the western slopes of the Sierra.=20

Hurley

...Previous discussion...
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 values...=20
however this area remains more hydrologically sensitive than normal
following the significant atmospheric river event the past few=20
days. Additional rainfall over far northwest CA into southwest OR=20
is generally 1-3" in the guidance, with isolated totals of this=20
magnitude also possible over the western slopes of the northern=20
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!6z9vjm7pjQIcLdQ478pgh90Aj50GhPAKMBW_CBl2raji=
6PHnC0OZNHKwbPuZD8U_q1tJSBQ_QC_D0SFT4Xl_U8LLCvg$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!6z9vjm7pjQIcLdQ478pgh90Aj50GhPAKMBW_CBl2raji=
6PHnC0OZNHKwbPuZD8U_q1tJSBQ_QC_D0SFT4Xl_ltnxbe0$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!6z9vjm7pjQIcLdQ478pgh90Aj50GhPAKMBW_CBl2raji=
6PHnC0OZNHKwbPuZD8U_q1tJSBQ_QC_D0SFT4Xl_xnmhDZw$=20

=3D =3D =3D
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