FOUS30 KWBC 240816
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
316 AM EST Sun Nov 24 2024

Day 1
Valid 12Z Sun Nov 24 2024 - 12Z Mon Nov 25 2024

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

Locally heavy rain will be possible today and especially tonight=20
over portions of northern CA into far southwest OR as an offshore=20
shortwave and attendant wave of low pressure arrives near the=20
coastal ranges. Generally the IVT magnitudes out ahead of this=20
surface wave will be modest with values increasing to as high as=20
250 to 400 kg/m/s. However, with a front nearby and favorable=20
upslope flow/orographic ascent involving the coastal ranges, there=20
will likely be some concentrations of heavier shower activity with=20
rainfall rates capable of occasionally reaching into the 0.25" to=20
0.50"/hour range. Some pockets of heavier rain may also make it=20
into the far northern Sacramento Valley and adjacent foothills of=20
the northern Sierra Nevada and Shasta/Siskiyou ranges. The 00Z HREF
guidance supports some spotty 1 to 3 inch rainfall amounts going=20
through early Monday morning, and thus this is not expected to be a
particularly heavy event. However, given the significant rainfall=20
and lingering hydrological impacts from the most recent atmospheric
river, these additional rains may result in at least some=20
localized runoff problems and flooding.

Orrison

Day 2
Valid 12Z Mon Nov 25 2024 - 12Z Tue Nov 26 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS=20
OF CALIFORNIA AND SOUTHWEST OREGON...

Showery conditions will likely continue across portions of=20
northern CA and southwest OR Monday into Monday night as a deep=20
layered low and shortwave energy moves inland. Certainly not a=20
classic setup 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 upwards of 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 move into portions of=20
southern/central CA...generally from portions of San Luis Obispo and=20
Monterey counties into the western slopes of the southern/central=20
Sierra Nevada. IVT values are forecast to be around 500-600 kg/ms,=20
which at this latitude is starting to approach climatological max=20
values for late November. PWs are also forecast to increase to=20
around the 99th percentile for late November, and thus expecting=20
plenty of moisture and upslope flow to allow for decent rainfall=20
totals. Not expecting any instability, so rainfall intensity will be=20
solely driven by the aforementioned moisture transport and upslope=20
enhancement...and these factors alone are unlikely to support=20
rainfall rates high enough for true flash flooding. However we will=20
have an extended period of time with elevated IVT values, and so a=20
prolonged steady rainfall could eventually result in some localized=20
minor flood concerns. Overall expecting 1-3" of rain over the=20
coastal ranges in San Luis Obispo and Monterey counties (with=20
localized totals towards 4" possible)...with totals ranging from=20
near 1" in the eastern valley locations to 2-4" over the western=20
Sierra below snow levels.=20

Chenard

Day 3
Valid 12Z Tue Nov 26 2024 - 12Z Wed Nov 27 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL AND SOUTHERN SIERRA NEVADA...

The atmospheric river event described in the day 2 discussion
continues into Tuesday over portions of the central and southern=20
Sierra Nevada. There is a bit more model spread by this time with=20
regards to the persistence of the IVT and strength of the mid/upper
forcing. A majority of the 00z deterministic models have trended a
bit weaker and faster with the shortwave energy pushing across CA=20
and into NV...and this trend would result in less additional QPF=20
over central CA on day 3. However the 00z GEFS and recent NBM=20
probabilities suggest there is still a chance for a slower/stronger
wave resulting in more robust rainfall amounts.

This will be the 2nd consecutive day of steady rainfall for these=20
areas, and so do expect some increased hydrologic sensitivity that=20
could result in at least some localized minor flooding issues. When=20
summing the two days of rainfall together, the latest WPC=20
deterministic QPF is calling for around 2" in Fresno to Visalia,=20
increasing to 3-6" in the foothills/western slopes of the southern=20
and central Sierra Nevada. Given the magnitude of IVT forecast even=20
even forecasting 1-2" spilling over to the east of the crest towards=20
Bishop and Independence. These forecast values average around a 2yr=20
ARI for 48 hr amounts, also supportive of a localized minor flood=20
threat. As mentioned above, there is both higher and lower end=20
potential with this forecast rainfall. The higher end scenarios=20
could even result in Slight risk level impacts, while the lower end=20
outcomes would be a minimal flood threat. So will need to continue=20
to monitor trends, and for now still think the Marginal risk is=20
appropriate.

Chenard

Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!_ykx03faeA26MpvhXzhN9LhF94lV23wAHtLVeaAIt3qC=
0-r-fw78UU5ci5znFcXZtZZgDP3BLcCpADnFuaZ5BaASJAk$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!_ykx03faeA26MpvhXzhN9LhF94lV23wAHtLVeaAIt3qC=
0-r-fw78UU5ci5znFcXZtZZgDP3BLcCpADnFuaZ55Z-2y5k$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!_ykx03faeA26MpvhXzhN9LhF94lV23wAHtLVeaAIt3qC=
0-r-fw78UU5ci5znFcXZtZZgDP3BLcCpADnFuaZ5hIu6i8I$=20

=3D =3D =3D
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