A moisture plume with a weak atmospheric river arrived on December 5th and brought rain to the region. On December 6th, a strong closed upper level low pressure brought an unstable air mass into Southern California with showers and thunderstorms, especially Orange and San Diego Counties.
Every event with a place to put it. 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 moisture plume with a weak atmospheric river arrived on December 5th and brought rain to the region. On December 6th, a strong closed upper level low pressure brought an unstable air mass into Southern California with showers and thunderstorms, especially Orange and San Diego Counties. Rainfall of 1 inch per hour amounts occurred in Costa Mesa and downtown San Diego as well as adjacent cities. The main storm system on December 6th brought widespread rain to the entire region. All mountains and the coast and valleys areas received 1-3 inches and some spots receiving over 4 of rainfall over higher terrain. The most significant mud and debris flows occurred at recent Holy burn scar areas, and many reports of flash flooding across the region. Another strong cold front brought widespread rain on Christmas Day though the impacts were minimal. Snow fell as low as 4000 feet with mountains receiving 1 to 4 inches of snow including the San Diego mountains. Burns Canyon in the high deserts of San Bernardino County had a wind gust to 70 mph.
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 Dec 5, 9:17 PM PST, 1 hour 43 minutes earlier.
Area Forecast Discussion
National Weather Service San Diego CA
917 PM PST Wed Dec 5 2018
.SYNOPSIS...
A low pressure trough will move slowly over SoCal Thursday and
Thursday night, further destabilizing the atmosphere and
generating scattered showers. Strong and gusty east winds will
continue overnight across much of inland San Diego County.Clouds
and showers will also keep temperatures below average. Some snow
will fall in the mountains above 6,500 feet. Dry and warmer Friday
through the weekend as a ridge builds over the southwest. A weak
trough will follow early next week with cooler weather.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
Radar has shown a marked decrease in areal coverage of rain this
evening, but scattered showers continue to develop off the coast
and drift in a N-NE direction, and remote rain gages indicate light
precip is accumulating along the coastal mountain slopes in San
Bernardino County. The 00Z Miramar sounding indicated a PW of 1.10
inch and had several weak inversions below 10K FT, with near
saturation in the column. Peak winds below 10K FT were 25 KT from
the SE and easterly winds prevailed at the sfc. The NWS sfc analysis
had a 6 MB offshore gradient form Amboy in SE San Bernardino County,
to San Nicolas Isl. East wind gusts were peaking at 35-45 MPH over
higher spots inland in San Diego County at 9 PM PST. This wind may
continue into Thursday morning.
The rain today was widespread and beneficial, since it fell at a
light to moderate rate. Through 9 PM PST...Rain reports ranged from
0.01, to just under one inch, with the bulk of the heaviest rain
in San Diego County. A rain summary is headlined on our home page.
Several inches of snow have fallen above 6,500 feet as well, but
this evening temperatures are above freezing all the way to near
7000 FT in the San Bernardino Mts, probably closer to 8000 FT for
the mountains farther to the south. A Winter Weather Advisory
remains in effect for the San Bernardino Mountains above 6,500 feet
through 10 PM PST Thursday.
So far, most of the random shower activity this evening has been
light, but farther to the northwest (Western LA county) moderate
showers have trained in some areas, resulting in rates at or above
1/2 inch/hr. This has been isolated, but highlights the potential
for us late tonight or on Thursday as the trough axis approaches.
The chance of off and on light rainfall is very high tonight into
Thursday, but the odds of excessive rates hitting debris susceptible
burn areas is small based on available guidance. Therefore no
forecast changes are planned.
From previous discussion...
The 1.25" PW bullseye aimed at San Diego County this afternoon will
drift south into northern Baja this evening, and that will be the
end of the steady rain. Behind this band we will transition to
scattered showers in the trailing convectively unstable airmass.
Shower coverage may not be very widespread tonight, but there will
still be scattered showers around. Coverage will be a little more
widespread Thursday afternoon with greater instability associated
with a weak shortwave rounding the base of the cold upper trough off
the coast. 500 mb temps fall to -22C and there may be a thunderclap
or two as the atmosphere destabilizes with heating.
Projecting rain amounts and rain areas in a weather pattern where
the precipitation is highly variable is like trying to herd cats.
It's not easy. We anticipate scattered lighter showers tonight due
to the lack of any strong upper level shortwave. Shower coverage
will increase Thursday afternoon when instability increases and
convective cells will likely be stronger. Periods of heavier showers
will occur just about anywhere in the area.
Over the weekend high pressure aloft will build over southwest.
Surface high pressure over the Great Basin will bring dry offshore
flow and warmer days Saturday and Sunday. Early next week a trough
over the Pacific will move southward along the West Coast for cooler
weather Monday and Tuesday. The GFS and EC have disparate solutions
with the GFS deeper and colder with the trough and a chance for
rain, while the EC is more progressive and farther north for drier
weather.
&&
.AVIATION...
060500Z...Occasional SHRA will occur overnight, with most CIGs 4000-
6000 ft MSL, only locally below 3000 ft MSL. Areas of VIS 3-5 miles
will occur. SHRA will continue Thu with a slight chance of TS with
little change in CIGs or VIS. Local snow above 7000 feet MSL will
occur, along with terrain obscurations in the mountains in
clouds/fog/precip. Local east and southeast winds 15-25 kt with
gusts around 30 kt will continue through Thursday.
&&
.MARINE...
Southeast winds 15-20 kt with gusts to 25 kt will continue through
tonight, creating hazardous boating conditions. Winds should mostly
decrease Thursday morning. There is a small chance of thunderstorms
late tonight through Thursday. No hazardous marine weather is
expected after Thursday.
&&
.BEACHES...
There is a slight chance of thunderstorms Thursday. A building long-
period WNW swell (5 feet/20 seconds in the outer coastal waters)
late Thursday will peak Friday through early Saturday. The direction
is from 290 degrees, so the surf will not be too high, but local
surf over 6 feet will be possible, mainly on the beaches of southern
San Diego County. Strong rip currents and dangerous swimming
conditions are likely.
&&
.SKYWARN...
Skywarn activation may be needed on Thursday. Rain and snow reports
are welcome!
&&
.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Winter Weather Advisory through 10 PM PST Thursday
for San Bernardino County Mountains.
PZ...Small Craft Advisory through 4 AM PST Thursday for the
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...10/Moede
AVIATION/MARINE/BEACHES...MaxwellThe San Diego radar (KNKX), 15 km away, was scanning. The volume nearest the first event began at 06:58 UTC.
7 flash flood, 4 heavy rain, 4 debris flow, 3 flood, 1 funnel cloud. Local standard time, as filed.
Broadcast media reported flooding in Sorrento Valley caused road closure on Carmel Valley Road.
John Wayne Airport reported 2.56 inches of rainfall in about 2 hours on December 6th. This is a 1 in 100 year return interval. Cars in nearby Costa Mesa were seen completely submerged completely or stalled in deep water up to the windows.
Public reported flash flooding along Highway 55 at Paularino Ave. in Costa Mesa. Reported that approximately 1 ft of water was along the right side of the road.
Spotter reported flash flooding on Orangethorpe Avenue at Raymond Street in Anaheim.
Mud and debris flowed right next to Rice Canyon Elementary School on December 6th due to heavy rain in the burn scar area.
Mud, water and debris impacted Grand Ave near Lakeshore Drive in Lake Elsinore due to heavy rain in the Holy fire burn area around 1400 PST on December 6th. Some areas of Grand Ave. reported mud 1 to 3 feet deep covering streets and yards.
Mud and debris in the road on Grand Ave. near Via Lakistas in Lake Elsinore due to heavy rain.
Mud and debris forced the closure of Temescal Canyon Road on December 6th due to heavy rain.
Pacific Coast Highway (Route 1) was closed between 3rd and 9th street in South Laguna Beach due to flash flooding.
Victor Valley News Group reported that Del Gado Road between Anacapa Road and Palmdale Road was closed due to flash flooding. Three vehicles were stranded but no injuries were reported.
Notable heavy rainfall rates at the Holy Fire burn scar. Coldwater Canyon received 0.35 of rainfall in 30 minutes. McVicker Canyon received 0.35 in 15 minutes. USGS 30-minute burn scar rainfall thresholds were exceeded in 60 canyons, and the 1-hour thresholds were exceeded in 5 canyons.
Broadcast media reported a funnel cloud in Escondido around 1500 PST on December 6th.
Oceanside Airport reported 0.50 of rainfall in 20 minutes between 1503 and 1533 PST on December 6th. This is a 1 in 20 year return interval.
A foot of water was reported in downtown Oceanside with multiple cars stalled. Oceanside reported 2.5 of rainfall in 2 hours.
Heavy rain resulted in flooding near the Shoppes at Carlsbad. Five businesses reported flooding damage. A roof collapsed at a childcare center in the shopping center.
The San Diego River at Fashion Valley went from 5 ft to 12 ft (minor flood stage) between 1700 and 2200 PST on December 6th. Avenida del Rio and Fashion Valley Road were both flooded out December 6th and 7th.
Broadcast media reported that the Alpha Project Bridge Shelter in East Village San Diego closed for a week due to flooding from the heavy rain.
Flash flooding was observed throughout downtown San Diego as a result of the heavy rain. Eight cars were seen floating on Laurel and Pacific Street in downtown requiring rescues with two cars having occupants.
San Diego International Airport reported 0.84 of rainfall betweem 1751 and 1851 PST on December 6th.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in California in 2018Every state in 2018This day in weather historyWarnings in California, December 2018
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: 798600, 793931, 793932, 793933, 798732, 798739, 798763, 798766, 798778, 793934, 798774, 798891, 798681, 798894, 798895, 798682, 798794, 798892, 798680.