FOUS30 KWBC 120805
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
305 AM EST Thu Dec 12 2024

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

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

Kleebauer


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

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHWEST CALIFORNIA AND SOUTHWESTERN OREGON...

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 MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHWEST CALIFORNIA INTO THE CENTRAL SIERRA FOOTHILLS...

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
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