Flash Flood and Winter Storm in California, March 1 to 2, 2023

Area Forecast Discussion
National Weather Service San Diego CA
900 PM PST Tue Feb 28 2023

.SYNOPSIS...
A trough of low pressure will bring moderate to heavy precipitation
tonight and Wednesday along with heavy snow in the mountains.
Wednesday morning with be cold, wet and windy with falling snow
levels. Fair and somewhat warmer weather is forecast Thursday
through Saturday. Frost is likely in the valleys and also in some
coastal areas Thursday morning.

&&

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

EVENING UPDATE: It was so cold Tuesday...that the high of 30 degrees
at Big Bear was their coldest high temperature ever recorded on this
date. The old record at Big Bear for this date was 33 set in 2015.
In the deserts, Borrego Springs tied their record low high of 59,
which was last set in 2001.

Light rain this evening was moving from Los Angeles County into San
Bernardino, western Riverside and Orange Counties. Rain amounts were
light with only a few hundredths of an inch in Orange County and the
northwestern Inland Empire, but higher amounts were recorded in the
San Bernardino mountain foothills with 1/4" at Lytle Creek (el
2792') through 9 PM.

Wednesday will be a cold and windy day. The bulk of the heavier rain
and mountain snow will arrive with the cold front later tonight and
tomorrow morning. The band of heaviest precipitation will move into
western San Bernardino, western Riverside and Orange Counties
between 3 AM and 6 AM, then race southeast across San Diego County
through 10 AM. Cold instability showers will end by early afternoon
in Orange County, most of the IE and the San Diego County coastal
and valley areas, but continue in the mountains will upslope
orographic flow.

Wednesday night will very cold with clearing skies and decreasing
winds, so much so the Frost Advisories will be needed, perhaps even
Freeze Warnings in some areas.

*From Previous Discussion*

Main upper low associated with the deep West Coast trough will move
through the Sierra and southern Nevada Wednesday then to New Mexico
Thursday. Despite the "Inside Slider" track, there will be ample
moisture in the region with PWs up to around 0.70" near the coast
and strong west flow from the surface to 500 MB and beyond, with
mostly 30-50 knots winds between 850 and 700 MB where it will be
saturated to contribute to orographics, plus there will be at least
a weak atmospheric river. Rainfall amounts have been relatively
consistent among models/ensembles, including run-to-run, with
generally around 0.50" near the coast to 1" in higher coastal
terrain and 2-4" in the mountains, very locally higher. Some precip
will make it into the deserts, especially the high deserts, but
mostly light amounts. There will be two main rounds of precipitation
with timing being mostly late evening to a little after midnight for
the first round (after mostly isolated showers) and then late night
and Wednesday morning for the heavy amounts. Rainfall rates could
reach 0.35" per hour in coastal areas and very locally around 0.50"
per hour on the coastal slopes. However, the snow level will be
worth watching. Some warm air advection will occur late tonight with
the atmospheric river but overall snow levels should not get much
above 5000 feet south and 4500 feet north, and the snow levels will
fall Wednesday, possibly down to 1500 feet around when the
precipitation ends late afternoon, at least in San Bernardino and
Riverside Counties, but only a dusting of snow is expected below
2500-3000 feet. The high deserts could have 1-2" of accumulating
snow, greatest near the foothills. Winds will be a factor, with
gusts over 80 MPH from the crests east to the desert foothills, and
high-resolution model cross-sections show mountain wave driven winds
making it into the Coachella Valley during the day Wednesday with
gust potential to 60 MPH there. Coastal areas will be windy as well
due to the strong MSLP gradients associated with this storm.
Precipitation should end from north to south Wednesday
afternoon/evening. With clearing Wednesday night, there should be
some below freezing temperatures in the valleys and frost even near
the coast, especially since the fairly brief switch to locally gusty
(but not strong) northerly winds will mainly be confined to the
mountains, not the valleys, Wednesday night.

Warmer weather will return Thursday and continue through the
weekend, though temperatures will remain below seasonal normals. But
at least highs will mostly be in the 60s west of the mountains
versus 50s. Ensemble means are trending down a bit with precip for
early next week, but EPS has about 20%-30% of solutions showing at
least some measurable precip around Monday. Most likely we will have
an increase of westerly winds as a wave moves by to the north at the
base of the large Pac NW trough, but a few light showers could occur
as well. For the middle of next week, about half the ensemble
members in the cluster analysis show either zonal flow or even
ridging over our area which would support fair weather then,
but the other half still have troughing which would support more
precipitation and cold weather. Hence, our window of a reasonable
confidence in the forecast ends around Monday of next week.

&&

.AVIATION...
010435Z...SCT-BKN clouds with variable bases 2500-5000 ft MSL and
additional layers up to 10,000 ft MSL through Wednesday evening.
Coastal mountain slopes remaining obscured through Wed evening as
well, then clearing late Wednesday night. Periods of showers tonight
mainly over and west of the mountains, followed by more widespread
precipitation 12-20Z Wed, including the deserts. Showers tapering
off from northwest to southeast after 20Z, ending by 06Z Thu. CIGS
locally lowering to 1000-2000 ft MSL with vis 2-4 SM in showers.
Isolated CIGS down to 800 with vis 1 SM in heavier showers.

Southwest to west winds strengthening this evening with sfc gusts 25-
35 kt at the coast and valleys, and 30-45 kt in the mountains and
deserts with isolated gusts to 60 kt on the desert slopes. STG-SVR
up/downdrafts, LLWS, and rotors likely over and NE/E of the
mountains. Rotors likely VCTY KPSP at times. Winds weakening for the
coastal areas and valleys Wednesday evening.

&&

.MARINE...
Winds and seas will increase tonight creating hazardous conditions
through late Wednesday night. West winds are expected to gust 35 to
40 kt with seas of 10 to 14 feet. Strongest winds are expected from
5 AM through 8 PM Wednesday, then lowering into Thursday morning.
Conditions will improve rapidly Thursday morning and no additional
marine hazards are expected through Sunday.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory from midnight tonight to 6 PM Wednesday 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.

     Winter Storm Warning until 4 PM Wednesday for San Bernardino
     County Mountains.

     Winter Storm Warning until 10 PM Wednesday for Riverside
     County Mountains-San Diego County Mountains.

     Winter Weather Advisory from midnight tonight to 6 PM
     Wednesday for Santa Ana Mountains and Foothills.

     Winter Weather Advisory until 4 PM Wednesday for Apple and
     Lucerne Valleys.

     High Wind Warning from 6 AM to 10 PM Wednesday for Coachella
     Valley.

     High Wind Warning until 10 PM Wednesday for San Diego County
     Deserts-San Gorgonio Pass Near Banning.

PZ...Gale Warning until 4 AM Thursday 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.

&&

EVENING UPDATE...Moede
PUBLIC...Maxwell
AVIATION/MARINE...SS