Winter Storm Warning 8, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
930 PM PDT Sun Mar 19 2023

.SYNOPSIS...
Light showers will continue tonight into Monday morning as a weak
wave continues to push east. Only a few showers will occur Monday
afternoon, but it will still be cool and mostly cloudy. A strong
storm will move through the region Tuesday through Wednesday and
bring strong winds, periods of moderate to heavy rain and
mountain snow and unseasonably cool weather. A few showers could
linger Thursday, with mostly dry but cool weather Friday through the
weekend.

&&

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

Light showers continued from the mountains west to the coast, with
isolated rainfall totals since this morning of over 1/3" in the
coastal slopes. Otherwise, the light showers will continue into
Monday and then gradually decline with a bit of a gap between the
weak waves moving through the zonal flow tonight/early Monday and
the big storm arriving by Tuesday. What is impressive with the big
storm is how deep the surface low will get along the California
coast with models showing the low somewhere between 985 and 990 MB
moving into the SF Bay Area, with a tight gradient to San Diego's
approx 1007 MB MSLP. This will produce synoptic scale wind at the
coast, with gusts likely topping 50 MPH, and terrain enhanced gusts
to 85 MPH in the mountains. Confidence is high for heavy rain which
could produce some flooding, as well as heavy mountain snow. There
is a chance of thunderstorms as well, which could produce hourly
rainfall rates greater than one-half inch due to the combination of
instability and dynamics. The day shift had a lot of details in the
AFD which can be viewed below.

From previous discussion...

A low pressure system, currently located off the coast of
northern California, will bring us this light shower activity
through Monday. Rainfall amounts will be light, but could wet the
roads for the morning commute on Monday. By Monday afternoon,
areas west of the mountains should dry out a bit, with continued
showers expected over the mountains into the evening. Continued
moist upslope flow could keep some additional showers over the
mountains into Monday night.

In addition to the showers, this weak storm system will bring
increased winds, especially along the desert mountain slopes.
Southerly winds today and tonight will shift westerly Monday
morning as the wind shift passes. South winds today and tonight
could gust up to 40 mph along the desert slopes of the mountains.
Winds should weaken a little on Monday morning across the
San Bernardino mountains and High Desert once the winds turn
west.

All eyes are on the stronger storm system on tap for Tuesday into
Wednesday. This storm is expected to bring periods of moderate to
heavy rain and mountain snowfall beginning Tuesday. There could be
a brief lull in the precipitation Monday night through Tuesday
morning, but during the day Tuesday, heavier and more widespread
precipitation will arrive. The heaviest and most widespread
precipitation is expected Tuesday afternoon and evening with the
peak of the atmospheric river moisture moving through the region.
Periods of rain will continue through Wednesday, but will be
lighter than Tuesday.

Forecast precipitation amounts Tuesday/Wednesday:

Coast/valleys: 1.5 to 2.5 inches
San Bernardino County mtns: 2 to 5 inches, highest along south
facing slopes
Riverside and San Diego mtns: 2 to 4 inches
High Desert: 0.50 to 1 inch
Lower deserts: 0.30 to 0.75 inch

The highest hourly rainfall rates at this time look to be around
one-half inch per hour, but would not be surprised to see higher
rates along the south-facing slopes of the San Bernardino County
mountains due to the strong upslope enhancement with the southerly
winds. Fueling these rainfall rates will be atmospheric river
moisture, which focuses on San Diego County Tuesday before
dropping south. The cold front will pass through sometime Tuesday
afternoon or early evening, with rain activity becoming more
showery Tuesday night through Wednesday. The rain won't totally
end though as lingering instability will contribute to some decent
shower activity, some with heavy rainfall rates, through
Wednesday.

With all of this rainfall that will be occurring, expect
significant runoff, which could contribute to local flooding
issues especially at low-water crossings and in poorly drained
urban areas. Several of our rivers are forecast to reach monitor
stage by Tuesday evening, including the Santa Margarita at Ysidora
and the San Diego River at Fashion Valley. Expect some low water
crossings in Fashion Valley to be inundated with water. Other
areas of concern will be the Whitewater River and Coachella Canal
in the Coachella Valley, as well as the Tijuana River watershed.

Models and ensembles continue to show unusually strong winds
across the entire area on Tuesday. The ECMWF extreme forecast
index paints the coastal areas in the 0.90 to 0.99 range, which
would translate to a very strong wind event that we only see once
every few years. The strongest winds will be ahead of and with the
cold frontal passage on Tuesday. At the coast, winds of 50 to 60
mph are forecast Tuesday afternoon and evening. If this comes to
fruition, expect numerous impacts including downed trees, and
difficult traveling conditions. We don't typically see winds this
strong, so please prepare by securing loose outdoor objects. The
mountains and deserts will see the strongest winds with gusts
potentially reaching in excess of 70 mph, mainly on the desert
mountain slopes and into the deserts with the cold frontal passage
Tuesday afternoon. A High Wind Watch has been posted for all
areas from Tuesday morning through Tuesday evening.

Last, but certainly not least, let's discuss the snow in the
mountains. Snow levels will initially start out high Monday
afternoon, around 6000-6500 feet, then briefly drop to around
5000 feet Monday night. When the warm AR moisture begins to move
in Tuesday, expect snow levels to rise a bit to around 6000 feet
Tuesday afternoon. Once the AR moisture and cold front pushes
through, snow levels will crash Tuesday evening and night to
around 4000 feet. Because of the uncertainty surrounding the snow
levels during the atmospheric river moisture, snow totals may
fluctuate a bit over the next couple of forecast cycles.

Forecast snow totals Tuesday/Wednesday:

4000-5000 feet: 3 to 8 inches
5000-6000 feet: 8 to 16 inches
6000-7000 feet: 16 to 30 inches
7000+ feet: 30 to 50 inches

A Winter Storm Watch has been posted for the San Bernardino and
Riverside County mountains beginning Tuesday morning and going
through Wednesday afternoon. A Winter Storm Watch is also in
effect for the San Diego County mountains, but beginning a little
later on Tuesday afternoon and going through Wednesday evening.

Models and ensembles have been fairly consistent over the last
couple of days, so confidence in this significant event is high.

Rain and snow will become lighter on Wednesday, but will continue
through the day. Precipitation should mostly wrap-up Wednesday
night. For the end of the week, a few ensemble members are showing
light precipitation at times Friday through the weekend, but those
members are in the minority at this time. The most likely scenario
at this point is some drying for the end of the week, with
continued cool conditions.

&&

.AVIATION...
200400Z...Low clouds with bases 500-2000 ft MSL will
continue to develop this evening and fill in west of the
mountains overnight. Scattered -SHRA west of the mountains
will continue through 18Z Mon. After 18Z, ISO -RA possible through
00Z Tue mainly in the valleys and on the coastal mountain slopes.
Local bases down to 800 ft MSL with 3-5SM vis in heavier showers.
Coastal mountain slopes will be obscured through most of Monday.
Deserts will mostly have BKN-OVC clouds with bases 5000-8000 ft MSL
through Monday with ISO -SHRA at times.

South to southwest winds will continue over the desert mountain
slopes and locally into the deserts, with peak gusts 30-40 kt into
Monday morning. Moderate up/downdrafts and LLWS expected
over/northeast of the mountains. Winds will gradually increase again
after 00Z Tue.

&&

.MARINE...
Locally gusty south winds expected late tonight with gusts around 20
kt, becoming westerly and weakening into Monday morning. A storm
will move through Tuesday into Wednesday, bringing rain, strong
south winds, and large combined seas. Wind gusts will initially
start out around 25-30 kt Tuesday morning, then increase to 35-45 kt
Tuesday afternoon just ahead of a cold front. Winds shift to
westerly behind the front late Tuesday afternoon with gusts around
30-40 kt, then gradually weaken into Wednesday morning. Gusts over
20 kt will continue into Wednesday evening. Combined seas of 9 to 13
ft are forecast Tuesday afternoon through late Thursday morning.
Strong winds and large seas will create hazardous boating conditions.

In addition to strong winds and large seas, there is the potential
for waterspout development late Tuesday morning into Tuesday
afternoon, followed by a slight chance of thunderstorms late Tuesday
afternoon into Wednesday afternoon.

&&

.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...High Wind Watch from Tuesday morning through Tuesday evening for
     Apple and Lucerne Valleys-Coachella Valley-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 Deserts-San Diego County
     Valleys-San Gorgonio Pass Near Banning-Santa Ana Mountains
     and Foothills.

     Winter Storm Watch from Tuesday morning through Wednesday
     afternoon for Riverside County Mountains-San Bernardino
     County Mountains.

     High Wind Watch from Tuesday morning through Tuesday afternoon
     for San Diego County Mountains.

     Winter Storm Watch from Tuesday afternoon through Wednesday
     evening for San Diego County Mountains.

PZ...Gale Watch from late Monday night through Wednesday morning 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...Maxwell (Update)/Connolly (Previous Discussion)
AVIATION/MARINE...CO/Maxwell