Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 282055
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
155 PM PDT Thu Mar 28 2024

.SYNOPSIS...
A cold Pacific storm moving down the West Coast will bring
widespread rain and mountain snow to Southern California this
weekend. Steady precipitation Saturday morning will transition to
showers and isolated thunderstorms Saturday night and Sunday.
Lingering showers Monday will be followed by dry and warmer weather
Tuesday and Wednesday.

&&

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

An atmospheric river ahead of a trough deepening down the West Coast
Friday will bring a period of steady moderate to heavy rain to
Southern California Saturday. Showers and isolated thunderstorms are
forecast Saturday night and Sunday in the cold, unstable airmass
behind the cold front. A Flood Watch has been issued from late
Friday night through Sunday evening. The Weather Prediction Center
has a risk of Excessive Rainfall and increased flood potential from
the coast to the mountains this weekend.

A strong shortwave diving south out of the Gulf of Alaska tonight
will deepen a longwave trough down the West Coast Friday. This
trough taps into a conveyor belt of moisture over the Pacific that
will make landfall over the Southwest Friday night and Saturday.
Moderate to heavy rain over Santa Barbara Friday evening will move
south across the LA basin Friday night, and then spread south into
Orange, San Bernardino, Riverside and San Diego Counties Saturday
morning. This is when steady moderate to heavy rain is likely from
the combined effects of the 500 mb positive vorticity advection,
lifting along the surface cold front and the orographic enhancement
over the mountains. Rainfall rates may exceed 0.50"/hr in the AR
Saturday morning.

After the AR passes Saturday afternoon, the cold core low behind the
trough will settle over the Southwest. A second trailing vort max
will help fuel scattered showers and a thunderstorms in the unstable
airmass Saturday night and Sunday.

A Winter Storm Watch has been issued for the mountains above 5,000
feet this weekend. Snow levels Friday night and early Saturday will
rise to 6,000 feet in the warm air advection ahead of the cold
front, then fall to between 4,500 and 5,000 feet Saturday night,
and hover between 5,000 and 5,500 feet Sunday. Snow accumulations
will be greatest in the San Bernardino Mountains above 7,500 where
over a foot of snow is expected.

Showers will linger into Monday in the broad cyclonic flow trailing
the upper low which will dig all the way to central Baja Monday
evening. Ridging slowly builds over the Southwest Tuesday through
Thursday for warmer days.

&&

.HYDROLOGY...
Rain and mountain snow will begin as early as Friday night in Orange
County and the San Bernardino Mountains, then move south across the
Riverside and San Diego Counties Saturday morning. On Saturday night
and Sunday, showers will be fairly widespread with locally heavy
rain rates in any thunderstorms in the cold unstable airmass behind
the trough.

Forecast Rain Totals Through Monday:

Coastal and Valley........1.00"-2.00"
Mountains.................2.00"-3.50" locally 5.00" in the SBD mtns
Highs Deserts.............0.50"-1.00"
Lower Deserts.............0.25"-0.50"

The snow level near 6,000 feet late Friday night and Saturday
morning will fall to 4500 feet Saturday night, then fluctuate
between 5000 and 5,500 feet Sunday.

Projected Snow Totals Through Monday:

4500-5500 ft...0-3"
5500-6500 ft...3-6"
6500-7500 ft...6-12"
Above 7500 ft..12+"

The current forecast for the San Diego River at Fashion Valley has
the river rising on Saturday, but remaining below the low-water
crossings. With additional rainfall on Sunday, the river could rise
sufficiently to flood the low-water crossings late Sunday.

&&

.AVIATION...
282045Z...Coast/Valleys... Clear conditions will continue across the
area for the remainder of the daylight hours. Low clouds will push
ashore and inland after 01Z in San Diego County and after 06Z in
Orange County. Bases will be around 1500 feet MSL but will rise to
over 2000 feet MSL and fill most of the coastal basin overnight into
Friday. Partial and intermittent cigs near the coast. Slow erosion
of the ceilings along the coast ranges is expected to last to 17-20Z
Friday.

Mountains/Deserts...Increasing westerly winds after 23Z with gusts
of 25-35 kt, locally up to 55 kt. MOD up/downdrafts and turbulence
expected over and east of the mountains. Surface winds should
decrease after 09Z. Winds are expected to increase again after 21Z
Friday.

Otherwise...occasional high clouds above 20000 feet MSL today and
tonight.

&&

.MARINE...
No hazardous marine conditions are expected through Friday. A Small
Craft Advisory has been issued for Strong southerly winds and choppy
conditions Friday night and Saturday morning. A distinct wind shift
from south to west will occur with a front moving through the
coastal waters Saturday morning, with weaker winds and less chop
thereafter. There is a slight chance of thunderstorms Saturday night
through Sunday.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Flood Watch from late Friday night through Sunday afternoon for
     Orange County Coastal Areas-Orange County Inland Areas-
     Riverside County Mountains-San Bernardino County Mountains-
     San Bernardino and Riverside County Valleys-The Inland
     Empire-San Diego County Coastal Areas-San Diego County
     Mountains-San Diego County Valleys-Santa Ana Mountains and
     Foothills.

     Winter Storm Watch from Friday evening through Sunday evening
     for Riverside County Mountains-San Bernardino County
     Mountains.

     Winter Storm Watch from Saturday afternoon through late Sunday
     night for San Diego County Mountains.

     Wind Advisory from 2 PM to 11 PM PDT Saturday for Apple and
     Lucerne Valleys-Coachella Valley-San Diego County Deserts-
     San Gorgonio Pass Near Banning.

PZ...Small Craft Advisory from 11 PM Friday to 5 PM PDT Saturday 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...Moede
AVIATION/MARINE...Suk


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