FOUS30 KWBC 210756
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
256 AM EST Sat Dec 21 2024

Day 1
Valid 12Z Sat Dec 21 2024 - 12Z Sun Dec 22 2024

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

Kleebauer


Day 2
Valid 12Z Sun Dec 22 2024 - 12Z Mon Dec 23 2024

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

Kleebauer


Day 3
Valid 12Z Mon Dec 23 2024 - 12Z Tue Dec 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA AND SOUTHWEST OREGON...

The next, more prominent atmospheric river is forecast to impact
the NorCal coast into southwestern OR by the end of the D2 into D3
time frame with a better aligned IVT pulse anticipated for areas
that saw significant rainfall totals over the past 7-10 days. The
heaviest rain will be confined to the immediate coast with a
southwest to northeast oriented moisture flux expected leading to
upslope flow a bit orthogonal to the coastal terrain extending from
the King Range up through the Siskiyou in southwest OR. Current
forecasted totals within the ensemble suite are between 2-4" with
some deterministic output between 4-5" at max. This is generally
within the confines of a low-end MRGL risk threshold when assessing
historical precedence for these types of events. The main
difference between this event and the previous was the primary
hydrometeor will come in the form of rainfall, even inland as lower
heights will be confined offshore and a flood of warmer air in the
boundary layer will protrude inland. Only the highest elevations of
the Northern Sierra and Mount Shasta will have a higher threat for
winter ptypes, so this points some of the more sensitive areas of
northern CA into play where burn scar remnants are located. 

The inherited MRGL from the D4 was relatively maintained for the
areas outlined in CA, but the prospects for flash flooding were
pretty low given the QPF signature for locations north of
southwestern OR, so decided to pull back the northern extension
along the OR coast. The MRGL extends inland to include those more
sensitive areas within the complex terrain, even extending to the
foothills of the Northern Sierra where heavier precip will make its
way by the end of the forecast cycle and likely carrying beyond to
the D4 period. 

Kleebauer
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