Mesoscale Precipitation Discussion 1273
NWS Weather Prediction Center College Park MD
250 PM EST Wed Dec 24 2025

Areas affected...Southern and Eastern CA

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

Valid 241950Z - 250600Z

SUMMARY...Heavy rainfall with dangerous and locally
life-threatening flash flooding impacts will continue to impact
portions of southern and eastern CA going into the evening hours
as strong atmospheric river activity continues.

DISCUSSION...GOES-W visible satellite imagery shows a significant
atmospheric river bringing heavy rain across large areas of
southern CA including the Los Angeles Basin, the adjacent high
terrain of the Transverse Range, the southern Sierra Nevada and
also adjacent interior desert valley locations. A deep
full-latitude trough offshore of the West Coast continues to shift
gradually eastward and is allowing for a cold front to advance
inland through southern CA.

This continues to help focus deep and very moist south-southwest
flow up out of the eastern tropical-Pacific and across the region
with enhanced IVT magnitudes of near 1000 kg/m/s aimed into
portions of Los Angeles, Orange and San Bernadino Counties. This
also includes the southwest facing slopes of the San Gabriel and
San Bernadino Mountains where enhanced orographic ascent coupled
with deep layer forcing and frontal convergence is yielding high
rainfall rates of locally over 1"/hour.

Over the next several hours, the cold front will continue to
settle south and east, which will allow for heavy rain to arrive
over the Peninsular Range of southern CA while also overspreading
interior areas of eastern CA including some of the desert
locations adjacent to the high terrain of the southern Sierra
Nevada.

The greatest short-term rainfall impacts and flash flooding
concerns will likely be over Kern, Ventura, Los Angeles, Orange,
and San Bernadino Counties, with eventually areas of Riverside and
San Diego Counties getting into heavy rainfall later this
afternoon and evening. Some of the rates for the aforementioned
southwest facing slopes of the high terrain will continue to be
locally near or over 1"/hour which is supported by the
experimental WoFS guidance which has a particular focus over the
next 6 hours across eastern parts of Los Angeles County, southwest
parts of San Bernadino County, and also Orange County.

Additional rainfall totals of 3 to 5+ inches going through late
this evening will be possible for the orographically favored high
terrain of southern CA, with as much as 1 to 3 inches elsewhere
including portions of eastern CA near the southern Sierra Nevada.
Some interior valley locations away from the terrain may even see
as much as 1 inch of rain.

Dangerous and locally life-threatening flash flooding is expected
going into the evening hours across southern CA which include
concerns for not only urban flash flooding, but also mud and
landslide activity, and debris flows near and adjacent to the more
sensitive burn scar locations. Areas of flash flooding will also
be a concern for eastern CA where heavy rains here also contribute
to enhanced runoff. This will include some open dry wash areas.

Orrison

ATTN...WFO...HNX...LOX...PSR...SGX...VEF...

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

LAT...LON   37091767 36531652 35781566 35011537 34351558
            33801566 33051589 32591620 32511726 33451806
            33651851 34151951 34821971 35921915 36921862