AWUS01 KWNH 250803
FFGMPD
CAZ000-251800-

Mesoscale Precipitation Discussion 1274
NWS Weather Prediction Center College Park MD
302 AM EST Thu Dec 25 2025

Areas affected...portions of northern, central and southern CA

Concerning...Heavy rainfall...Flash flooding likely

Valid 250759Z - 251800Z

SUMMARY...Another round of heavy rain will approach central and
southern CA tonight into Christmas morning. Peak hourly rainfall
of 0.5 to 1.0 inches is expected, with isolated hourly totals over
1 inch. Due to recent heavy rainfall and areas of ongoing
flooding, renewed areas of flood/flash flooding are likely from an
additional 1 to 3 inches of rainfall (locally higher) through ~18Z.

DISCUSSION...GOES West water vapor/infrared satellite imagery from
07Z showed a mid-level vorticity max near 39N 127W with a
southward extending trough axis, advancing toward the northeast. A
surface low was associated with the vorticity max containing an
attached occluded/cold front extending to the south. Infrared
imagery and lightning data showed a line of showers/thunderstorms
just ahead of the front, advancing closer to the central CA coast.
This feature was located ahead of a broader closed low aloft, with
steepening lapse rates in the 850-500 mb layer edging eastward
central to northern CA.

While the primary moisture axis from Wednesday has moved into the
lower CO River Valley, layered PW imagery from OSPO showed a
secondary moisture axis just ahead of the offshore cold front with
blended TPW values of 0.8 to 0.9 inches. Instability along the
coast and within the Sacramento/San Joaquin Valley was less than
500 J/kg and should stay that way over the next 6-12 hours with
some fluctuations up/down over time via recent RAP forecasts. RAP
guidance also showed 850-700 mb winds from the south to
south-southwest increasing into the 60-65 kt range from near San
Francisco Bay to the northern Sacramento Valley ahead of the front
which will likely be accompanied by a band of
showers/thunderstorms sweeping inland containing peak hourly
rainfall of 0.5 to 1.0 inches. The band will impact central to
northern coastal locations first, followed by inland locations
across the Sacramental Valley into the Sierra Nevada foothills.
The band of heavy rain will also begin to advance southward down
the Santa Lucia Range into the western Transverse Ranges between
12Z and 18Z. Post-frontal low level axes of convergence could
support additional narrow bands of showers/thunderstorms with
brief training across central to northern CA locations with peak
hourly rainfall near 0.5 inches.

Through ~18Z, peak additional rainfall of 1 to 3 inches is
expected (locally higher) for the Coastal Ranges into the Sierra
Nevada foothills (below snow levels of ~5000 ft). This additional
rainfall, coupled with brief high rates, should allow for areas of
renewed flooding/flash flooding across urban/terrain and low-lying
locations through 18Z. Due to saturated soils, the potential for landslides/debris flows will exist, including any sensitive burn
scar locations.

Otto

ATTN...WFO...EKA...HNX...LOX...MFR...MTR...STO...

ATTN...RFC...RSA...NWC...

LAT...LON 41222225 40912174 40102152 39872122 39182064
38632033 37621963 36411861 36311931 36131974
35721981 35191938 34581808 34011806 33771847
33841954 34132068 34802148 35772178 37082281
38992416 40242462 40682433 40842312

$$
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