The month began with a ridge of high pressure over the Great Basin and moderate easterly flow in the mid and upper levels. This allowed monsoon moisture to surge into the southern half of California August 1st-3rd.
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.
The month began with a ridge of high pressure over the Great Basin and moderate easterly flow in the mid and upper levels. This allowed monsoon moisture to surge into the southern half of California August 1st-3rd. Severe storms occurred in the Inland Empire and San Diego County Valleys on the 1st. Storms focused more on the mountains and deserts on the 2nd and 3rd, with reduced intensity overall.
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 Aug 1, 8:36 AM PST, 3 hours 24 minutes earlier.
Area Forecast Discussion
National Weather Service San Diego CA
936 AM PDT Tue Aug 1 2017
.SYNOPSIS...
A couple upper level disturbances will move through Southern
California today through Wednesday, bringing chances for showers
and thunderstorms for all areas, especially over the mountains,
deserts and valleys Wednesday afternoon and evening. Monsoonal
moisture will likely result in afternoon and early evening
thunderstorms over the mountains, deserts and possibly valleys
Thursday. Decreasing moisture will lead to lessening chances for
afternoon thunderstorms over the mountains Friday through Monday,
with night and morning marine layer stratus becoming more
widespread in the coast and western valleys. High pressure aloft
will continue to bring above warm to hot conditions inland through
Friday before gradual cooling takes place this weekend through
early next week.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
A upper level short-wave trough is currently moving through the
region, and has brought areas of showers and thunderstorms overnight
and this morning, with the majority of activity occurring over
Riverside and San Bernardino counties. So far, 868 total lightning flashes
and strikes combined were observed in our forecast area since last
night. Currently, radar shows showers over the high deserts
exiting into L.A. county while a showers and thunderstorms south
of the border and in Imperial county are moving northwest and
could bring another round of active weather late this morning into
the afternoon. Latest SPC mesoanalysis shows about 250-500 J/KG of
MU CAPE across the region, and the morning sounding from Miramar
shows a precipitable water of 1.7 inches. However, most of the
moisture is located above 700 mb, so the showers/thunderstorms
have not produced heavy amounts of rainfall, with most locations
getting a few hundredths of an inch, with the San Bernardino
county mountains having received the most amount of rain (around
0.05-0.20 inches). However, if surface based thunderstorms are
able to develop over the mountains this afternoon, there could be
heavy rain and a higher potential for flash flooding. Meanwhile
on visible satellite, a clear slot is over most of the region,
which should allow good warming to take place, and highs may reach
5-10 degrees above normal today. For the morning forecast update,
increased highs today a little for the inland valleys. Models are
indicating potentially another round of nocturnal showers and
thunderstorms tonight/Wednesday morning for all areas as another
wave rotates into the area from the southeast. Finally, marine
layer stratus is evident about 10-20 miles inland, but clearing
will probably take place before patchy stratus moves inland again
10-15 miles tonight/Wednesday morning.
For Wednesday afternoon/evening, moisture increases further and
models also show another wave moving over the area, in addition to
surface based CAPE greater than 1000 J/KG. The 12Z GFS shows
precipitable water increasing to 2 inches, and the 12Z NAM12
shows it increasing to near 2.2 inches in some locations! With
east to southeast flow aloft of around 10-20 kt, storms may move a
bit too quickly for a high flash flood threat, but at least a
moderate flash flood threat can be expected in the mountains,
deserts and valleys. There's also the chance that some storms
could occur at the coast again.
Although there may be some drying on Thursday, there should still
be enough moisture and instability around for a fairly active
afternoon/early evening thunderstorm day over the mountains,
deserts and valleys, with wind flow turning to the south.
Models show significant drying (precipitable water possibly below
1 inch) taking place Friday as southwest flow moves in due to a
trough developing along the west coast, with continued drying
this weekend through Monday. Thus, starting Friday,
afternoon/early evening thunderstorm chances should be greatly
decreased.
Look for high temperatures 5-10 degrees above normal to continue
through Friday with a strong ridge to the north (centered near
northwest Nevada and northern California) extending down into
Southern California. Once the trough develops, we should start to
see gradual cooling this weekend into early next week, with highs
falling below normal by Sunday. The marine layer should be able to
re-establish itself late this week into early next week as the
monsoon moisture goes away, so expect night and morning marine
layer stratus along the coast and western valleys during that
time. Before Friday, the night and morning stratus along the coast
will likely be very patchy.
&&
.AVIATION...
011500Z...Stratus with bases 1000-1400 ft MSL and tops to
1800 ft MSL hanging on near portions of the coast. Clearing is
expected at the terminals, though low clouds could remain confined
to some immediate coastal locales this afternoon. Low cloud coverage
will be significantly less tonight, but CIGS near 1000 ft MSL after
02/06Z will be possible for a few hours, before scattering out
after 15Z Wed.
Isolated thunderstorms will be possible this afternoon through
tonight, but low chances at any location. On Wednesday, chances
of thunderstorms increase for the mountains and deserts during the
afternoon. Brief moderate to heavy rain and gusty, erratic outflow
winds are possible with these storms. Otherwise SCT-BKN clouds at
or above 8000 ft layered to 25000 ft.
&&
.MARINE...
A chance of showers and thunderstorms will continue through
Wednesday evening. These storms may produce gusty outflow winds,
locally heavy rain, and cloud-to-water lightning. A Marine Weather
Statement remains in effect through 11 PM Wednesday.
&&
.BEACHES...
A Beach Hazards Statement remains in effect for elevated surf and
strong rip currents through this evening. South swells will produce
surf of 4-6 feet with max sets to 7 feet in Orange county and far
northern San Diego County. Surf will range from 2-4 feet elsewhere.
Isolated thunderstorms are possible at the beaches through
Wednesday evening.
&&
.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.
Skywarn activation may be necessary on Wednesday.
&&
.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement through this evening for Orange County
Coastal Areas-San Diego County Coastal Areas.
PZ...NONE.
&&
PUBLIC...Harrison
AVIATION/MARINE/BEACHES...GregoriaThe San Diego radar (KNKX), 24 km away, was scanning. The volume nearest the first event began at 19:58 UTC.
5 flash flood, 5 thunderstorm wind, 2 lightning, 2 flood, 1 heavy rain. Local standard time, as filed.
A thunderstorm produced heavy rain with subsequent runoff that raised El Capitan Reservoir, a reservoir with a capacity of 112,800 acre feet, by 3 inches overnight.
High waters sent flows over the roadway near the intersection of State Routes 79 and 371. Vehicles were trapped in rocks and debris and the roadway was closed by CHP at 1320PST.
Severe downburst winds knocked down branches that blocked roadways in Alpine and Harbison Canyon. A spotter estimated winds at 65 mph plus.
Severe winds downed a large tree (diameter > 2 ft) on a home in San Bernardino. A brief funnel cloud was also reported.
A severe downburst struck Temecula and Murrieta, winds overturned a big rig, downed trees, and damaged traffic signals.
Downburst winds toppled power lines and downed multiple large trees blocking several roadways in Perris.
A mud and debris flow from a recent fire flowed over Grand Ave. and Plumas St., both roads were closed.
An intense thunderstorm produced intense street flooding in southern Corona. Flow pushed debris, branches, and trash cans down local streets. The flow along Colt Dr. was strong enough to damage and shift parked cars.
Power lines downed by strong outflow winds.
Lightning strikes produced power outages in Palm Springs.
An intense thunderstorm produced widespread street flooding in Palm Springs. Flood water inundated Palm Spring High School with 1-1.5 inches of water, damaging 10 classrooms.
Lightning strikes produced power outages in Desert Hot Springs.
Heavy runoff through a normally dry wash flowed over Highway 78 two miles east of Yaqui Pass, forcing a closure of the highway.
Video via social media of 4-6 inches of water flowing over the road near the intersection of Pipes Canyon Rd. and Gamma Gulch Rd.
A trained spotter reported a debris flow from Sheep Creek Wash flowing over Lone Pine Canyon Rd.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in California in 2017Every state in 2017This day in weather historyWarnings in California, August 2017
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: 719295, 719290, 718886, 719306, 718887, 718888, 719289, 719301, 718889, 719296, 719299, 719297, 719291, 719294, 719293.