A weakening atmospheric river drifted south into the region, bringing areas of heavy rain and gusty winds. This led to debris flows and areas of flooding, which impacted various roads and homes.
Every event with a place to put it; 3 filed by county or forecast zone are drawn at its center, in a lighter ring. Tap one for its entry below.
Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS SGX wrote it up.
A weakening atmospheric river drifted south into the region, bringing areas of heavy rain and gusty winds. This led to debris flows and areas of flooding, which impacted various roads and homes.
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 Oct 24, 7:38 PM PST, 4 hours 22 minutes earlier.
Area Forecast Discussion
National Weather Service San Diego CA
838 PM PDT Sun Oct 24 2021
.SYNOPSIS...
A cold front moving down the coast tonight will bring rain Monday,
mainly in the afternoon and evening. Strong gusty winds will occur
Monday in parts of the mountains and deserts. Tuesday will be a dry,
cool day, followed by warmer weather and weak offshore flow the
second half of this week.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
EVENING UPDATE...
Low clouds are developing just off the coast this evening with
high clouds continuing to move east across the northern areas.
Breezy southwest to west winds are ongoing across the mountains
and deserts with gusts mostly below 35 mph. Winds will continue
overnight, then strengthen Monday morning. A wind advisory remains
in effect for the mountains and deserts except the Coachella
Valley Monday morning through early Tuesday morning.
The Atmospheric River is still aimed at northern California,
soaking the Bay Area into the Sierras with several inches of rain
in the past 24 hours. This feature will slowly sag south and lose
steam as it moves into So Cal tomorrow. Everything remains on
track so no changes were made to the forecast.
PREVIOUS DISCUSSION...
The elephant in the room is the strong atmospheric river currently
moving through northern California. Once it moves into SoCal Monday,
it will have weakened, but CW3E maps still indicate IVT values of
around or slightly over 500 kg/m/s Monday afternoon over our area
based on the ECMWF/GFS/NAM inputs. ECMWF EFI show effectively record
values for both QPF and surface winds in central California today
though nothing outstanding from LA south. Model time-height cross-
sections show most of the moisture at/below 650 MB along with 850 MB
winds from the west/southwest of 20-30 knots, so that will be
favorable for decent orographic precipitation, though without the
instability and dynamics being fairly brief with the cold front,
precip amounts will not be too impressive at the coast. ENS/GEFS
ensemble members are fairly consistent with each other and run-to-
run with mostly around 0.2" at SAN and 0.4" at SNA, though the
latest deterministic HRRR is showing a little bit higher values
around 0.5" in some coastal locations. Orographic precip will be
greatest in the San Bernardino County Mountains as local values
could exceed 2", though most mountain locations, especially
farther south, will be closer to 1". Given that the nearly-
saturated layer only goes up to about 650 mb, it will be difficult
for the deserts to get anything substantial, but the high deserts
could get up to 0.25" with the lower deserts mostly below 0.1".
Timing will mostly be Monday afternoon to early evening, with
Orange/San Bernardino Counties coming before San Diego/Riverside
Counties. Local wind gust potential of around 55 MPH Monday on the
desert mountain slopes which will warrant a wind advisory for
then. Regarding snow, the freezing level mostly remains at/above
10000 ft MSL when the moisture is in place, with only a drop below
at the end of the storm, so little or no snow will fall on any
roadways, even at Onyx Pass.
After Monday night, the trough of low pressure associated with the
atmospheric river will shift east towards the Rockies while broad
zonal high pressure aloft covers much of the eastern Pacific. That
high will amplify and move east over the western US Wednesday
through Friday. Models are having a tough time past then, with some
troughing or possibly a closed low moving by to the north over the
weekend. Some offshore flow at the surface will occur as well
Wednesay and Thursday, but nothing strong. Daytime highs should
increase to above seasonal normals with ENS indicating some 90s in
the valleys and especially lower deserts by Thursday, with a slight
cooling trend over the weekend. Sunshine should be abundant most of
Tuesday through Friday, maybe Saturday.
&&
.AVIATION...
250300Z...Coasts/Valleys/Mountains...Low clouds with bases 1500-2500
ft MSL expected to develop along the coast around 06Z Mon, spreading
inland through 12Z Mon. Higher terrain becoming obscured in clouds
after 12z Mon. RA expected to start in Orange County around 21Z,
with isolated SHRA possible ahead of the main front as early as 17Z
Mon. RA will move from northwest to southeast, reaching San Diego
County around 00Z Tue. Vis will be reduced to 2-4 SM and cigs may
become locally under 1000 ft MSL at times in areas of RA. Dry
conditions expected for all areas after 06Z Tue. Bases 3000-5000 ft
MSL expected overnight Tue.
Strong southerly winds with strong up and downdrafts
along with wind shear near mountains and adjacent mountain slopes
Monday as winds increase to 20 to 30 knots with gusts approaching 50
knots at times. Gusty southwest winds expected at coastal TAF sites
after 21Z Mon.
Deserts...SCT-BKN high clouds AOA 20000 ft MSL through 00Z Mon.
After 00Z Mon clouds with bases 5000-7000 ft MSL, showers, and
deteriorating conditions expected through Tue AM. Gusty north to
northwest winds expected after 21Z Mon. Gusts 25-30 kt
expected. Strong up and downdrafts near the mountains.
&&
.MARINE...
An approaching storm system will bring gusty southwest winds, that
will transition to northwest winds after 5 PM Mon. Gusts of 20-25
knots expected in the outer waters and closer to 20 knots across the
inner waters Monday afternoon and Tuesday. Combined seas of 6-10 ft
expected, highest in the outer waters. Please see the Small Craft
Advisory and Coastal Waters Forecast for more information.
&&
.BEACHES...
A long period west swell and a long period south swell will result
in high surf and a high rip current risk at all beaches Monday
through early Wednesday. Surf of 6 to 8 feet is expected, with sets
up to 10 feet possible in southern San Diego County. Please see the
High Surf Advisory for more information.
&&
.SKYWARN...
Skywarn activation will not be needed tonight, but may be
requested Monday.
&&
.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory from 10 AM Monday to 2 AM PDT Tuesday for Apple
and Lucerne Valleys-Riverside County Mountains-San
Bernardino County Mountains-San Diego County Deserts-San
Diego County Mountains-San Gorgonio Pass Near Banning.
High Surf Advisory from 11 AM Monday to 3 AM PDT Wednesday for
Orange County Coastal Areas-San Diego County Coastal Areas.
PZ...Small Craft Advisory from 11 AM Monday to 5 AM PDT Tuesday 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...SS/Maxwell
AVIATION/MARINE/BEACHES...COThe Santa Ana Mts radar (KSOX), 15 km away, was scanning. The volume nearest the first event began at 08:03 UTC.
7 debris flow, 5 heavy rain, 3 high wind, 1 flash flood. Local standard time, as filed.
Moderate to heavy rain fell across the region. Santiago Peak reported 1.40 inches of rain in total, with a max hourly rainfall rate of one half inch.
Gusty winds occurred across the region with mesonet Pilot Rock Truck Trail measuring peak wind gust of 66 MPH.
Heavy rain occurred across the area with storm totals mainly 1 to 2 inches. The Lake Arrowhead area received the highest amounts, with stations reporting over 2.5 inches of rain in total. The Lytle creek and Lake Arrowhead areas received some impressive rainfall rates, some measuring near 0.85 inches of rain in one hour.
Moderate to heavy rain occurred across the region with parts of the Idyllwild area receiving amounts near 0.90 inches of rain in total.
The coastal slopes received between 0.50 and 0.90 inches of total rainfall.
Moderate to heavy rain fell across the region with areas of 0.50 to 0.80 inches total.
Gusty winds occurred across the region with mesonet Toro Peak measuring peak wind gust of 86 MPH.
Gusty winds occurred across the region with mesonet Volcan Mountain measuring a peak wind gust of 61 MPH.
Rocks on Highway 330 lanes and at least 1 large boulder reported. Heavy rainfall of 0.5 to 0.7 in 1 hour occurred in this area. Location at the highway 330 passing lane.
Rocks in both lanes on Highway 18, which has steep rocky terrain between Big Bear Dam and Snow Valley. Heavy rain of 0.4 to 0.7 inches in 1 hour occurred in the area. Caltrans and CHP reported on Quickmap.
Both lanes of Highway 18 covered in mud reported at Big Bear Dam.
A large boulder was in a lane of Highway 18 at Gregory Lane reported on Quickmap via CHP Caltrans. Heavy rain band moved through this area with rain rates of 0.70 inch per hour.
Hospital Road and State Route 173 roadway damage reported from heavy rain. 1.18 inches rainfall in 3 hours reported nearby.
Large boulder in lane of Highway 330 at Live Oak Drive per CHP Caltrans Quickmap.
About 1000 cubic yards of debris, mud and rock removed from Highway 38 near Valley of the Falls Road with some rock slides near Mountain Home Village. This prompted the closure of the highway from Valley of the Falls Road to Sugar Pine Road from 530 pm on the 25th to 430 am on the 26th.
Large amount of water flow near Potato Canyon and Oak Glen Roads, possibly on Birch Creek Road. A vehicle was swept off roadway and rescue was performed. Heavy rainfall of 0.4 to 0.6 in 1 hour occurred across the El Dorado burn scar and San Bernardino Mountains. Highway 38 was closed for several hours through the evening while Caltrans removed mud and debris from the highway. Total rainfall in the area was about 0.75 to 1.00 inch. County Public Works removed 250 cubic yards of debris from Birch Creek. Survey of the flood and debris flow was done by California Geological Survey jointly with WCM Alex Tardy.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in California in 2021Every state in 2021This day in weather historyWarnings in California, October 2021
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: 986979, 986978, 986924, 986926, 986928, 986927, 986976, 986977, 986990, 986991, 986992, 986987, 986988, 986989, 988227, 986984.