FOUS30 KWBC 130011
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
711 PM EST Thu Dec 12 2024

Day 1
Valid 01Z Fri Dec 13 2024 - 12Z Fri Dec 13 2024

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

Pereira

Day 2
Valid 12Z Fri Dec 13 2024 - 12Z Sat Dec 14 2024

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

20z Update: Upgraded to a Slight risk across portions of northwest
CA with this update. Event total rainfall from Friday through
Saturday morning is forecast to be 3-5" across the Slight risk
area, with isolated amounts of 5-7" within the most favored
southerly facing slopes. Rainfall rates should increase Friday as
the core of IVT moves into the coast. By Friday night into
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 12z 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. The
forecast rainfall combined with these locally higher rates indicate
some flooding is probable. Still not expecting this to be a high
end event given the overall progressive nature of the system and
IVT peaking just around 750 kg/ms, but some creek and stream
flooding, along with a few landslides, are possible.

Chenard

...Previous Discussion...

The overall forecast maintained a general continuity in the
expected synoptic evolution of a weak atmospheric river transpiring
across the Northwest CA coast, extending northward into Southwest
OR. IVT signatures between 700-1000 kg/ms will advect into the
coastal plain with some protrusion inland on the initial surge as
the southwest to northeast alignment of the pulse relegates a
majority of the moisture into the coast and adjacent coastal ranges
that will garner the greatest QPF just given the upslope component
of the wind field. A widespread 2-4" with local max of 5-5.5" is
forecast over the period with the general maxima likely within the
coastal King Range as is customary with IVT pulses exhibiting a
southwestern angle of approach across Northern CA. Rainfall rates
on the order of 0.25-0.5"/hr will be most common within these bands
of precipitation, however a few instances of >0.5"/hr might be
realized during the period between 00-12z Saturday when the primary
surge is forecast amongst the 00z deterministic suite. This
correlates well with the anticipated IVT maximum making headway
into the coastal plain which could lead to localized flooding
concerns in the expected zone(s) of impact. As of this juncture,
the heaviest rainfall will lie north of Santa Rosa with the primary
areas of interest likely positioned from the King Range up US101 to
the southern half of the Siskiyou in southwestern OR. With this
alignment, very little change was necessary from the previous MRGL
risk issuance, thus the forecast was relatively unchanged as the
MRGL risk was sufficient in consensus.

Kleebauer

Day 3
Valid 12Z Sat Dec 14 2024 - 12Z Sun Dec 15 2024

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

20z Update: The atmospheric river described in the day 2
discussion will continue to impact portions of northern CA
Saturday morning. Thus the Slight risk was continued into this
period as well. The strong mid level shortwave mentioned in the
day 2 discussion will move onshore Saturday morning, with this
resulting in some weak instability and also likely a temporary
slowing of the IVT axis. These factors should result in some
higher rainfall rates from 12z-18z Sat over the Slight risk area,
with flooding of creeks, streams and low lying flood-prone areas
possible, along with the continued risk of smaller landslides. The
higher rates should quickly diminish as the shortwave moves east,
with the risk of heavy rain over by 18z. Thus this Slight risk is
really only for the first few hours of this day 3 period.

Chenard

...Previous Discussion...

Atmospheric River setup from the previous period will translate
into D3 with a greater inland protrusion of the IVT pulse as the
upper level trough and associated surface low pivot onshore with
an expected wind shift that will mute heavier rain prospects along
the coast and eventually inland as we move into the second half of
the period. The threat is currently very marginal in nature within
the confines of the inland topography with the eastern foothills
of the Sierra's being the furthest east expanse of any heavy rain
threat, but even that is bordering on the lower end of a risk
threshold. This period is more of the transition point in the
pattern as the event will see a general decay in total rainfall and
overall rates as the system moves inland. There is a further south
push of the IVT pulse in the early portions of Saturday with some
modest dynamical element involved as the 500mb vorticity evolution
does bring some significant forcing towards the North- Central CA
coastal plain with impacts to places like Santa Rosa down into the
Bay area. Thankfully the pattern is progressive, but a quick 1-2"
is within the realm of possibility for a mainly urban zone which
could bring about some localized flooding prospects before the
disturbance moves inland and we get a quick end to the chances.

A MRGL risk continues for a good portion of Northwest CA with the
risk area now extending down into the Bay area compared to the
previous D2 alignment. A MRGL risk is also located at the base of
the Sierra foothills for locations below 5500ft.

Kleebauer

Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!7BOu_NBb7LacvtO76ILGsizezEwK5amsLgti264fkYbb=
QyDhD0-QHXLMLecs7n2EIbv5dq2rASZz-d8AOnqGdZ_Ddaw$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!7BOu_NBb7LacvtO76ILGsizezEwK5amsLgti264fkYbb=
QyDhD0-QHXLMLecs7n2EIbv5dq2rASZz-d8AOnqGyxjJaZw$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!7BOu_NBb7LacvtO76ILGsizezEwK5amsLgti264fkYbb=
QyDhD0-QHXLMLecs7n2EIbv5dq2rASZz-d8AOnqGIzNKZok$=20

=3D =3D =3D
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