From Dumas Walker@21:1/175 to All on Sun Dec 21 08:58:16 2025
FOUS30 KWBC 210820
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
320 AM EST Sun Dec 21 2025
Day 1
Valid 12Z Sun Dec 21 2025 - 12Z Mon Dec 22 2025
...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL IN EFFECT FOR
PORTIONS OF THE SIERRA NEVADA IN NORTHERN CALIFORNIA...
A significant surge in Pacific moisture will overspread much of
northern California during this period within the reinforced
atmospheric river and will lead to widespread heavy rainfall.
Within the Moderate Risk area over the portions of the Sierra
Nevada range there are several hours of >50% HREF probabilities of
at least 0.50"/hr rates between 12Z Sun and 03Z Mon, while HREF
probabilities of >8" within the 24 hour Day 2 period are >60% for
much of the area. There will likely be many areas where flash
flooding may arise, along with sloughing and debris flows. For
additional details please refer to WPC MPD #1267.
Campbell/Hurley
Day 2
Valid 12Z Mon Dec 22 2025 - 12Z Tue Dec 23 2025
...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL IN EFFECT FOR
PORTIONS OF THE SIERRA NEVADA IN NORTHERN CALIFORNIA...
The atmospheric river will move southward during this period as
will the heavy rainfall footprint spread further down the Sierra
Nevada Range and along the coastline. Much of the same area of the
Sierra Nevada is projected to receive additional 2-3 inches over
the water logged area from Day 1. A Moderate Risk was maintained
along with expanding a touch to the south. Multi-day accumulations
will likely be well into the double digits by this time and leading
to several instances of rapid runoff, sloughing and debris flows
along the foothills/adjacent locations.
Campbell
Day 3
Valid 12Z Tue Dec 23 2025 - 12Z Wed Dec 24 2025
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL IN EFFECT FOR
PORTIONS OF THE SIERRA NEVADA IN NORTHERN CALIFORNIA, ALONG THE
COASTLINE OF CENTRAL CALIFORNIA AND FOR SOUTHERN CALIFORNIA....
Another reinforcing surge of Pacific moisture will be moving
onshore northern California while the southward moving impulse will
lead to a rapid uptick in rainfall in southern California. The
latest guidance is depicting another 1 to 4 inches for northern and
central California and 2 to 4+ for the higher terrain of southern
California. A Slight Risk covers much of the coastline from central
to northern California and into the northern part of the Sierra
Nevada. Additionally, another Slight Risk is in place for the
favored terrain of southern California.