FOUS30 KWBC 132007
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
307 PM EST Fri Dec 13 2024

Day 1
Valid 16Z Fri Dec 13 2024 - 12Z Sat Dec 14 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
NORTHWEST CALIFORNIA...

...16Z Update...

The inherited Slight Risk along the Coastal Range of Northern
California remains on track with just a few minor adjustments to
the threat areas based on incoming 12Z CAMs. Guidance has continued
to trend slightly more amplified with the approaching upper-level
shortwave trough tonight. NAEFS shows the IVT will exceed 750
kg/m/s upon approach, topping the 99th climatological percentile
near Eureka, CA for 00Z Saturday then southward into Sonoma and
Napa counties by 06Z tonight. The HREF suggests low-chance
neighborhood probabilities (10-30%) for localized amounts
surpassing 8" through 12Z Saturday. In addition, the ECMWF EFI
sports a >0.8 signal for significant QPF vs climatology for early-
mid December from the Coastal Range north of the Bay Area to the
Shasta/northern Sierra Nevada. The EFI signal indicates there is
the potential for an extreme event, but note it does not account
for other non-meteorological factors. Of note, this AR is likely
to be fairly progressive and soils have recovered some over the
past two weeks. For these reasons, the Slight Risk was maintained
for this forecast cycle as this should reduce the areal extent of
the flash flood threat. That said, some localized considerable
impacts cannot be ruled out, especially in areas where excessive
rainfall amounts above 8" potentially come to fruition tonight and
into early Saturday morning.

Mullinax


--Previous Discussion--

Maintained the Slight Risk area over portions of northwest
California with only minor adjustments needed based on latest
deterministic and ensemble guidance. Rainfall should be growing in
both areal coverage and intensity today as a plume of moisture is
directed from the eastern North Pacific into northwest California.
Event total rainfall amounts through early Saturday morning are
still forecast to be in the 3 to 5 inch range across the Slight
Risk area. By late tonight/early Saturday morning the heavier rain
will shift southward as a strong mid level shortwave approaches the
region. This feature will act to increase large scale ascent,
while also cooling the mid levels enough to allow for some weak
instability to develop within the IVT axis. The 13/00Z HREF
neighborhood probabilities of rainfall exceeding 0.5" in an hour
are over 50% across most of the Slight risk...both across the most
favored upslope areas and also within a narrow southward shifting
axis of stronger convergence. This does not appear to be a high end
event given the progressive nature of the system...although some
creek and stream flooding and a few mudslides will be possible.

Bann

Day 2
Valid 12Z Sat Dec 14 2024 - 12Z Sun Dec 15 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
COASTAL CALIFORNIA FROM SANTA ROSA INTO SAN FRANCISCO AND SAN JOSE...

20z Update: Main change was to shift the Slight risk a bit
southward along the coast into the San Francisco to Oakland to San
Jose urban corridor. Concerned about a 1-3 hour window of high
rainfall rate potential right around 12z as a strong mid level=20
shortwave moves ashore resulting in steeper lapse rates and some=20
weak instability. HREF probabilities show an almost 100% chance of
0.5"/hr rainfall, with ~30-40% chance of 1"/hr...and the GFS and=20
ECMWF also have a narrow band of heavier rainfall. Even though=20
event total rain may only be 1-3" over these areas, much of that=20
should fall in a just a couple hours...so thinking urban flood=20
impacts are becoming increasingly likely. HREF probabilities of=20
exceeding 3 hr FFG (which is 1-1.5") are as high as 40-60% over=20
this area.

Chenard

...Previous Discussion...

Rainfall associated with an atmospheric river will be on-going as
the Day 2 period begins at 14/12Z...with the plume of deepest
moisture continuing to be directed in to parts of northwest
California although the axis of heaviest rainfall rates/amount will
be gradually be easing eastward in time with the main shortwave
while being shunted southward ahead of the surface cold front.
southward with time. The higher rates should quickly diminish with
the departure of the shortwave moves...with the risk of heavy rain
over by 18z. Thus this Slight risk is really only for the first few
hours of this day 2 period.

Bann

Day 3
Valid 12Z Sun Dec 15 2024 - 12Z Mon Dec 16 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.|

Chenard


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!9dEXjPpf1-dK9bcDViyER1b77EvhqkwN-YuhYJZn8RA5=
tdsrxPBIpQ60KVceOb_m4cTIkybnnLOkYVX4cimC2m-1W7w$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!9dEXjPpf1-dK9bcDViyER1b77EvhqkwN-YuhYJZn8RA5=
tdsrxPBIpQ60KVceOb_m4cTIkybnnLOkYVX4cimCYVieo3E$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!9dEXjPpf1-dK9bcDViyER1b77EvhqkwN-YuhYJZn8RA5=
tdsrxPBIpQ60KVceOb_m4cTIkybnnLOkYVX4cimCXRxdwc0$=20

=3D =3D =3D
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