Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
756 PM EST Mon Dec 22 2025
Day 2
Valid 12Z Tue Dec 23 2025 - 12Z Wed Dec 24 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CALIFORNIA...

2100 UTC update: No changes made to the previous

Previous discussion...

The trajectory of the atmospheric river will become more S/SW to
N/NE as it shifts southward during this period as the trough
offshore in the Pacific also digs to the south. In contrast to
the air mass containing the atmospheric river, which will be warm
and moisture-laden, this contrast will support cyclogenesis along
the greatest gradient/boundary between the cold air mass within the
trough and the much warmer air mass in the ridge/atmospheric
river. As the low deepens the onshore flow will align parallel to
the coastline thus significantly increasing the areal extent heavy
rainfall and the higher terrain snow accumulations. The onshore
flow associated with the low itself will continue very heavy rain
and mountain snow across northern California, while the trailing
cold front moves into southern California starting Tuesday night.

The areas of greatest impact will be into the northern Coastal
Ranges and the Transverse Ranges. Both these ranges are
sufficiently aligned perpendicular to the south-southwesterly flow
to have a significant local upslope contribution to rainfall rates.
In the Transverse Ranges, local burn scars will also worsen impacts
from resultant flooding over and downstream of any burn scars north
and west of the Los Angeles Basin. The renewed round of heavy rain
will be in areas getting hard hit by the current atmospheric river
and will continue with the deepening low offshore into the
northern coastal ranges. A Slight Risk was maintained for the
Oregon border down through Los Angeles.

Campbell/Wegman


Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt