High Wind Watch 6, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
930 PM PST Wed Feb 22 2023

.SYNOPSIS...
Showers will gradually increase tonight, with snow levels locally as
low as 2000 feet. Showers will continue at times through Friday,
then widespread precipitation will occur Friday and Saturday.
Dangerous travel conditions are expected in the mountains Friday
through Saturday due to heavy snow accumulation. Gusty south winds
will also occur Friday through Saturday, though the snow level will
rise to around 5000 feet. Decreasing showers will occur Sunday, then
another cold low pressure system from the northwest may bring more
precipitation and strong winds for the early and middle part of next
week.

&&

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

Forecast Highlights:
*High impact winter storm will create hazardous weather conditions
 for Southern California through the weekend.
*Low elevation snow expected into Thursday morning for portions of
 the Inland Empire, High Desert, and San Diego county foothills.
*Widespread rain, mountain snow, and gusty south winds expected
 again Friday night into Saturday.

Update... Locally gusty winds continued in the mountains and
deserts, especially near Thermal where winds were still gusting to
40 MPH along with blowing dust, and also in some coastal areas.
Winds should continue to decrease somewhat overnight. Current snow
levels are around 2500-3000 feet based on temperatures and dew
points, and that could come down a bit overnight and showers also
increase from the west as precipitable water increases as well as
the top of the saturated layer, to over 700 MB after midnight.

For the Friday-Saturday system, models are converging more on a late
Friday night-Saturday midday time frame with quite a strong surface
low off the coast, after a series of waves moving through the long-
wave trough to bring occasional periods of more moderate rain/snow.
By the end of the weekend, we might have the heaviest mountain snow
totals for a 4-5 day period since 2010. However, one thing that
might negatively impact snow totals is the warm air associated with
the atmospheric river for Friday night/Saturday as 850 MB temps
could increase to 2-5 deg C with 700 MB temps increasing to -5 deg F
which could change some of the snow to rain up to 5000-6000 feet
elevation for at least a short time. Regardless, this is a storm
system that will product substantial precipitation nearly statewide
with many mountain areas encountering heavy snow.

From previous discussion...

Scattered showers this afternoon are expected to become more
widespread into Thursday morning. The cold nature of this system is
expected to bring snow to elevations as low as 2000 feet. Up to 2
inches of snow may accumulate in portions of the Inland Empire
and eastern San Diego County. Snow is expected to accumulate in
portions of the High Desert as well. Snow is expected there by
Thursday afternoon, with up to 4 inches expected. Snow levels will
rise to 3000 to 3500 feet through the day Thursday, with snow
levels closer to 4000 feet expected by Friday morning.

A second low pressure system will drop south Friday and Saturday
bringing additional rounds of precipitation. This system is expected
to move far enough south to bring some tropical moisture in the
mix and associate an atmospheric river with the system. Integrated
water vapor values are expected to peak Friday night into
Saturday morning. Precipitation from this system is expected to be
more widespread and produce periods of heavy precipitation. More
details can be found in the Hydrology section below. Strong south
winds will develop ahead of this system on Friday and Friday
night, especially across the coastal waters and coastal areas and
for the San Bernardino County mountain and desert areas.

Heavy snow and gusty winds will create dangerous conditions in
the mountains. Snowfall is expected on all of the mountain ranges
with the highest accumulations expected in the San Bernardino and
portions of the Riverside County mountains. Top forecast snowfall
totals of 4 to up to 7 feet are expected in the mountains. For the
San Bernardino and Riverside County mountains for Friday into
Saturday, the combination of very heavy snow, low visibilities
from snow, blowing snow, and fog, and strong gusty south winds
will create blizzard conditions at times.

Sunday night and Monday may be mostly dry with another cold low
pressure from the northwest bringing more precipitation by Tuesday
and Wednesday.

&&

.AVIATION...
230450Z...West winds gusting to 20 kt at the coast and 30-40 kt in
the mountains and deserts will gradually weaken overnight. Moderate
up/downdrafts, LLWS, and rotors possible including vcnty KPSP
through 12Z.

SCT-BKN clouds with bases BKN clouds 3000-5000 ft MSL and layers up
to 8000 ft MSL continuing through Thursday evening. Coastal mountain
slopes remaining obscured. ISO-SCT showers continuing from the
mountains westward overnight, locally spreading into the deserts
Thursday afternoon. Brief MVFR CIGS/VIS possible in heavier showers.

&&

.MARINE...
West-northwest winds have lowered this evening, currently gusting to
around 20 kt with seas of 10.5 ft. Conditions will continue to
improve overnight, though occasional and isolated gusts to around 20-
25 kt possible with heavier showers. Southerly winds strengthen
early Friday and seas rapidly rebuild to produce hazardous boating
conditions again through Friday night. Winds could gust as high as
35-40 kt. Winds will shift from southerly to westerly and weaken
considerably as a front moves through the coastal waters Saturday
morning. No additional marine hazards are expected Sunday and
Monday.

&&

.BEACHES...
Rough surf with sets to 8 feet will gradually lower overnight as a
short period west swell diminishes. A High Surf Advisory remains in
effect through 3 AM Thursday.

&&

.HYDROLOGY...
Showers are expected to increase in coverage this evening into
Thursday. Snow levels near 2000 to 2500 feet, will rise to 3000 to
3500 feet by Thursday afternoon. Rainfall and liquid equivalent
through Thursday night is expected to range from around one-half
inch near the coast to 2 to 3 inches in the mountains with locally
greater amounts.

Friday into Saturday, another low pressure system from the northwest
will bring more widespread and heavier precipitation to the area.
Global models are showing a weak atmospheric river with IVT values
above 400 arriving by Friday afternoon or evening (there remain some
timing differences between the GFS and ECMWF). Due to the slightly
warmer nature of this system at its onset, snow levels will rise to
5000 to 5500 feet Friday night, before falling to 4000 feet Saturday
afternoon. For Orange and southwestern San Bernardino counties,
additional rainfall and liquid equivalent is expected to range from
2 to 3 inches near the coast to 4 to 6 inches in the mountains and
locally to 10 inches on south facing mountain slopes with 1 to 2
inches in the high desert. For San Diego and western Riverside
Counties, additional rainfall and liquid equivalent of 2 inches near
the coast to 2 to 3 inches in the mountains is expected with one-
quarter to one-half inch in the deserts. Rain rates of 0.50 to 0.75
inches per hour are expected, with the highest rainfall rates on the
coastal slopes of the San Bernardinos. Heaviest rainfall is expected
Friday night into Saturday morning.

Low snow levels will initially limit runoff. As we move to the end
of the week and weekend, soil moistens and snow levels rise
increasing the potential for runoff and flooding. Normally dry river
and stream channels will likely have flowing water by Friday
morning, if not sooner, and mainstem river levels will be elevated.
Excess water flowing into the Whitewater Basin, coupled with
moderate rainfall rates Friday night and Saturday, could send
significant flow through the Coachella Canal, impacting low water
crossings in the Coachella Valley. Low water crossings through
Mission Valley, especially along the San Diego river, will likely be
impacted as well by Saturday.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Advisory until 3 AM PST Thursday for Orange County
     Coastal Areas-San Diego County Coastal Areas.

     Flood Watch from Friday afternoon through Saturday evening for
     San Diego County Coastal Areas-San Diego County Mountains-
     San Diego County Valleys.

     Wind Advisory until 6 PM PST this evening for Orange County
     Inland Areas-San Bernardino and Riverside County Valleys-
     The Inland Empire-San Diego County Valleys.

     Winter Weather Advisory until 10 AM PST Thursday for San
     Bernardino and Riverside County Valleys-The Inland Empire-
     San Diego County Valleys.

     Flood Watch from Friday afternoon through Saturday evening 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 Gorgonio Pass Near Banning-Santa Ana Mountains
     and Foothills.

     Winter Storm Warning until 3 PM PST Saturday for Riverside
     County Mountains-San Bernardino County Mountains-San Diego
     County Mountains.

     Winter Storm Warning until 6 PM PST Thursday for Santa Ana
     Mountains and Foothills.

     Winter Weather Advisory from noon Thursday to 10 AM PST Friday
     for Apple and Lucerne Valleys.

PZ...None.

&&

PUBLIC/HYDROLOGY...Maxwell (Update)/CO (Previous Discussion)
AVIATION/MARINE/BEACHES...SS