A weak atmospheric river brought widespread precipitation and mountain snow to Southern California ahead of a storm system. Moisture transport with the atmospheric river was 200-300 IVT.
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The episode as NWS SGX wrote it up.
A weak atmospheric river brought widespread precipitation and mountain snow to Southern California ahead of a storm system. Moisture transport with the atmospheric river was 200-300 IVT. At the start of the atmospheric river (warm sector), snow levels were 7000 feet, then once the cold front passed through, snow levels lowered to 5700 feet. Two day precipitation totals ranged from a few hundredths of an inch along some coastal areas, up to 0.80 in the valleys. The highest precipitation totals were over the mountains, where up to 2.8 of rainfall occurred. An inch of snow was reported in Big Bear and Green Valley Lake.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS San Diego, CA wrote its last area forecast discussion before the first event at Mar 22, 8:41 AM PST, 1 hour 19 minutes earlier.
Area Forecast Discussion
National Weather Service San Diego CA
941 AM PDT Sun Mar 22 2020
.SYNOPSIS...
Tranquil weather will prevail much of the day ahead of a Pacific
storm off the coast. This storm system will bring rain and
mountain snow to the area, beginning late this afternoon into
early this evening over Orange County and mid to late this evening
for San Diego County. Moderate to brief heavy rain will occur
mainly this evening to about midnight before precipitation
gradually tapers off throughout the night. A break in the
precipitation will occur late Monday through Tuesday with more
showers set to arrive Wednesday through Thursday as a trough from
the north arrives. Dry weather will return Friday through next
weekend, likely lasting through next week. Temperatures will warm
late this week.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
...Update...
Plenty of sun this morning under some high cirrus cloudiness
streaming eastward. Most of the day will be nice ahead of the
Pacific storm system.
The storm system off the California coast will move inland
overnight with a weak Atmospheric River impacting the area. This
will result in a round of moderate to brief heavy rainfall. HRRR
and WRF show timing of rain onset after 4-5 PM for Orange County,
western Riverside and San Bernardino Counties and mid-late
evening for San Diego (after about 9 PM). Rain will last longest
across the northern areas, especially along the coastal mountain
slopes where moist, upslope flow will enhance rainfall.
Rainfall rates could reach near 0.50"/hr, which could result in
minor urban flooding, especially in potential isolated
thunderstorms over Orange, western Riverside and San Bernardino
Counties late this afternoon and evening. If isolated
thunderstorms develop, Orange County would be the most likely
area (marginal CAPE but closer to the upper low), so added this to
the forecast. Did include far western Riverside and San
Bernardino Counties as well, in line with SPC.
Snow will begin at high elevations late this afternoon/evening
above 7000 feet, but the snow level will lower overnight to
5000-5500 feet. A Winter Weather Advisory remains in effect for
the San Bernardino and Riverside County mountains above 6000 feet.
No significant changes were made to the forecast this morning. See
previous discussion below for more forecast details.
...Previous Discussion (Issued at 357 AM PDT Sun Mar 22 2020)...
An upper level low pressure system was located about 600 nm due
west of San Diego early this morning. This system is forecast to
gradually open up into a trough as it moves ENE over Central
California through Monday and into the Great Basin by late Monday.
Moderate moisture transport with this system over the LA Basin is
forecast to occur with PWAT values forecast of around 1.2 inches.
This system is forecast to have weak atmospheric river IVT values
of around 300 per the GFS Ensemble Mean but per the Operational
GFS mostly misplaced with the upper level energy and dropping
south of the border through Monday morning.
The trajectory of the upper low will bring the best forcing over
the LA Basin. Our SGX northwest areas including Orange, Western
Riverside and SW San Bernardino Counties will be in best position
to receive the healthier precipitation rates and accumulation. The
latest high-res guidance is moderately higher especially over
those aforementioned northwestern areas. The latest national blend
guidance was a fine compromise of the WPC guidance and the
heavier totals from the latest 36 hour HRRR. Of note the high-res
local WRF brings a bit heavier totals to the coastal slopes of San
Diego County however the trajectories really don't seem as
favorable.
Overall precip amounts have trended up with this forecast; one-
half to one inch for the Northern Inland Empire and Orange County,
only around one-half inch for the Southern Inland Empire, with
one to two inches over the coastal slopes of the San Bernardino
Mountains. Amounts in Riverside and San Diego Counties trail off
with around one-quarter inch for coastal San Diego County, perhaps
one-half inch for inland San Diego County and up to one inch for
the mountains of San Diego County.
As for snowfall, a winter weather advisory remains in effect for
the mountains of SBD and Riverside Counties above 6000 feet. Snow
levels will initially be very high, around 7000 feet or higher
this afternoon, slowly decreasing tonight to around 6000 feet, and
by Monday morning as low as 5000 feet in the San Bernardino
Mountains, although the precipitation will be much more scattered
by then. Snowfall totals will be heaviest above 7000 feet where as
much as 5-10 inches and isolated totals around 15 inches will be
likely. Amounts will only be a few inches, however, between
6000-7000 feet. Winds will also be gusty especially in the San
Bernardino Mountains through tonight, with southwest gusts around
40 mph likely at times.
The upper level trough will scoot east of California on Monday,
with lingering showers isolated over the mountains mainly
throughout the day on Monday, but mainly dry elsewhere.
For Tuesday and Wednesday, a brisk zonal flow with weak embedded
chunks of energy could allow a few light showers to occur however
POPs remain very low, between 10-20% during this timeframe. A
colder upper low pressure system is still expected to drop south
out of the Pac NW Wednesday and Wednesday night into California.
This system looks to bring meager moisture transport, but vigorous
upper level energy will bring scattered showers, and much lower
snow levels around 3000 to 4000 feet, so we could see some light
snowfall along the mountain passes. In addition, winds will become
strong at times during this period.
&&
.AVIATION...
221530Z...Coast/Valleys/Mountains...SCT-BKN clouds high clouds with
bases AOA 15000 ft MSL. Clouds becoming BKN and bases lowering to
around 2000-3500 ft MSL. -SHRA developing after 22Z in Orange
County/SW San Bernardino County, spreading south and east after 00Z-
03Z Mon, continuing through Monday morning. Bases and vis locally
lower in areas of -SHRA. Higher terrain becoming obscured in
clouds/precip this evening and overnight. South to southwest winds
with surface gusts to 15-20 kt along the coast after 00Z Mon. Gusty
southwest winds up expected over the mtns onto desert slopes through
Mon AM. Areas of mod UDDFTS possible over and east of the mtns.
Deserts...SCT-BKN clouds with bases AOA 10000 ft MSL and
unrestricted vis through this evening. Isolated -SHRA possible in
the High Desert 00-12Z Mon.
&&
.MARINE...
Late this afternoon into Monday afternoon, scattered light to
moderate showers are expected. Periods of light to moderate showers
are expected again Wednesday morning into Thursday evening. West-
northwest winds gusting to 20-25 kt possible Thursday afternoon into
Friday morning, strongest in the outer coastal waters.
&&
.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.
&&
.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Winter Weather Advisory from 3 PM this afternoon to 1 PM PDT
Monday for Riverside County Mountains-San Bernardino County
Mountains.
PZ...NONE.
&&
PUBLIC...Gregoria (Update)/Brotherton (Prev Discussion)
AVIATION/MARINE...COThe national radar mosaic for the moment the first event began.
1 winter weather, 1 flood. Local standard time, as filed.
Up to 3 inches of snow fell in the San Bernardino Mountains, with the highest amounts at Mountain High and Eagle Point (Big Bear).
Indian Canyon Road just south of I-10 was closed for 3 hours due to roadway flooding. This was caused by runoff from the Whitewater Channel due to heavy rainfall in the San Bernardino Mountains.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in California in 2020Every state in 2020This day in weather historyWarnings in California, March 2020
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 882502, 882458.