River Flood Warning 3, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
130 PM PST Sun Jan 15 2023

.SYNOPSIS...
A powerful Pacific jet will continue to feed disturbances onto the
California Coast through Tuesday. The most consequential will arrive
late tonight and Monday morning with widespread rain, mountain snow,
and strong winds. A lower snow level will allow for heavier snow
accumulations in our mountain communities through Tuesday morning
as progressively colder air is drawn southward. Scattered rain,
snow, and ice pellet showers are likely to persist through Tuesday.
The weather looks dry for Wednesday, but it will remain cool.

&&

.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...

A few breaks have managed to develop in the extensive cloud
deck flowing east off the Pacific. Moderate onshore sfc pressure
gradients are supporting peak westerly wind gusts of 45-55 MPH,
mainly over the San Diego County Mts at midday. At 1 PM PST, radar
still showed scattered, mostly light showers continuing to
drift east into the forecast area. Over the past hour, all sites
reporting rain showed less than one-tenth inch. A variety of wind,
winter, and flood advisories are in effect. Please check our webpage
for the latest updates and storm totals.

A powerful EastPac jet will continue to send waves onto the West
Coast for the next few days, destabilizing our weather. The next in
the series arrives late tonight along the Central Coast. A well
defined sfc low will send a warm front onto our coast just after
midnight, followed by a cold front Monday morning. These will tap
an AR offshore briefly, increasing the coverage and intensity of
rain and high elevation snowfall through early Monday morning.
Expect some strong southerly winds ahead of the cold front late
tonight as well, before turning westerly post frontal. The rain will
turn more showery again on Monday, favoring the peninsular ranges
for heaviest additional accumulations. Expect impacts for the early
morning commute on Monday.

For Monday night through Tuesday, the coldest in this series of
disturbances will arrive, with maximum destabilization of the
atmosphere, fueling continued shower development and lower snow
levels. Moisture will be more limited though, so the flooding threat
will diminish. The final wave in this series will move over the
region Tuesday night, and should bring an end to the showers. A weak
ridge builds in behind for dry weather on Wednesday.

The models are beginning to show more variability into Friday, with
some ensemble solutions developing another cold wave, now in NW
flow aloft. If it should amplify, there could be more showers by Thu
afternoon or Friday, but confidence is low and moisture will be
limited. Confidence is higher for the cool conditions to linger.

&&

.AVIATION...
152030Z...Coast/Valleys...Scattered -SHRA expected through this
afternoon. SCT-BKN clouds with bases 2000-5000 feet MSL to continue
through this evening. SHRA becoming more widespread after 06Z
Monday, with the heaviest SHRA 11-18Z. Ceilings dropping to 700-1500
feet with reduced vis of 1-3 SM in heavier SHRA. South winds to
increase at the coast after 06Z Monday, with peak gusts 25-40 kts.
Winds decreasing after 18Z Monday.

Mountains/Deserts...SCT SHRA/SHSN continuing in the mountains today,
with areas of fog and terrain obscurations. SHRA/SHSN becoming more
widespread after 02Z Monday. Strong west winds will occur through
this evening, with peak gusts 40-50 kts. Winds will turn
south/southwest after 06Z Monday, with gusts to 50 kts, strongest on
the desert mountain slopes. Areas of strong up/downdrafts and LLWS
expected over/east of the mountains.

&&

.MARINE...
The large long-period west swell will continue to generate combined
seas of 6-8 feet. Southwest to west winds of 15-20 kt with local
gusts to 25 kt and rough seas will occur through much of today.
Stronger southerly winds and high combined seas will develop late
tonight and persist through Monday as another storm and a large west
swell move in, with hazardous winds and seas persisting through
Tuesday. The Small Craft Advisory has been extended into Tuesday
evening.

&&

.BEACHES...
The long-period west swell from 270-280 degrees will continue to
generate high surf of 5 to 8 feet today. A slight decrease in surf
is possible on Monday before another west swell (though with a
slightly shorter period) produces more elevated to high surf Monday
night through Tuesday, mostly remaining below 10 feet. The surf will
gradually lower Tuesday night and Wednesday, with no high surf
expected the rest of the week.

&&

.HYDROLOGY...
Low pressure is headed for the Central Coast once again and will
send more rain our way later tonight. For now, expect scattered
showers for the remainder of the day into the evening. They will be
moving quickly, so peak rainfall rates are likely to be below 1/4
inch per hour. Runoff from previous rains will continue local
flooding of low-water crossings and some creeks and streams.

Just after midnight, we should begin to see an increase in the
coverage of the rainfall. Upslope will again augment rates and
amounts onto the coastal slopes, initially the San Bernardino County
Mountains, then along the peninsular ranges. Winds will turn from
southerly to westerly after frontal passage early Monday morning,
and there could be some local heavier rainfall rates to one-half
inch per hour. Scattered showers will follow for the remainder of
the day into Tuesday, with snow showers in the mountains. The snow
level will lower on Monday, after a brief rise again overnight. We
are expecting it will be around 6200 feet later tonight, falling
to around 4000 feet by Tuesday morning.

Additional rainfall forecast through Tuesday night...
Mountain coastal slopes.............3-5 inches (heaviest spot totals
most likely in San Bernardino County)
Mountain (north and east slopes)...0.50-3.0 inches
Coasts and Valleys.................1-2 inches
Deserts............................0.10-0.50 inch

The San Diego and Santa Margarita Rivers will continue to hit
Monitor Stage with crests expected on Monday. See the CNRFC, or our
webpage for details. Water is flowing through the Coachella Canal as
well, with low-water crossings in the Valley barricaded. This is
likely to continue into Tuesday. the Mojave River has rare flow
as well through Victorville and points north. With the expected
rainfall on top of saturated ground and ongoing minor flooding,
expect urban and small stream flood issues to continue, especially
in poorly drained areas through Monday. Runoff from higher
elevations will become less of an issue after Monday morning as snow
levels fall.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Advisory until 10 PM PST Tuesday for Orange County
     Coastal Areas-San Diego County Coastal Areas.

     Wind Advisory from midnight tonight to 7 AM PST Monday for
     Orange County Coastal Areas-Orange County Inland Areas-San
     Bernardino and Riverside County Valleys-The Inland Empire-
     San Diego County Coastal Areas-San Diego County Valleys-
     Santa Ana Mountains and Foothills.

     Winter Storm Warning until 4 AM PST Tuesday for Riverside County
     Mountains-San Bernardino County Mountains.

     Wind Advisory until 10 AM PST Monday for San Diego County
     Mountains.

     Winter Weather Advisory from 10 AM Monday to 4 PM PST Tuesday
     for San Diego County Mountains.

PZ...Small Craft Advisory until 6 PM PST Tuesday for Coastal Waters
     from San Mateo Point to the Mexican Border and out to 30 nm-
     Waters from San Mateo Point to the Mexican Border Extending
     30 to 60 nm out including San Clemente Island.

&&

PUBLIC/HYDROLOGY...10
AVIATION/MARINE/BEACHES...Connolly