A large upper level ridge of warm air over the Great Basin brought persistent east to southeast flow over the region in late August. Strong flow and limited instability largely kept the convection below severe levels.
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 large upper level ridge of warm air over the Great Basin brought persistent east to southeast flow over the region in late August. Strong flow and limited instability largely kept the convection below severe levels. However, weakening of the ridge on the 31st increased instability. This helped severe convection develop over the Inland Empire. Downburst wind intensity was aided by dry air in the low levels and record high temperatures.
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 31, 12:49 PM PST, 46 minutes earlier.
Area Forecast Discussion
National Weather Service San Diego CA
149 PM PDT Thu Aug 31 2017
.SYNOPSIS...
Hot weather will continue through the weekend and into next week
with nearly stationary high pressure over the Western states.
Monsoon moisture will bring chances of thunderstorms each day,
mainly over the mountains. Tropical Storm Lidia, currently just
South of the Baja California Peninsula, is forecast to move
northward, but eventually dissipate as it begins to turn to the
west near Central Baja. Some moisture from this system may move
into Southern California early next week, potentially increasing
the risk of showers and thunderstorms.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
The disturbance that produced isolated showers/tstms this morning
has moved to the northeast, but new showers/tstms are developing
over the mtns and along low-level convergence zones west of the
mtns this afternoon. These convective cells are expected to
continue until this evening. Temperatures are mostly a few degrees
higher than this time yesterday west of the mtns...except for
portions of the Inland Empire where temps are a few degrees lower
than this time yesterday. The shallow marine layer could once
again bring low clouds and patchy dense fog to the immediate coast
of San Diego County, but the offshore flow reduces the chances of
significant coverage.
The high pressure ridge will continue to bring hot conditions
through the forecast period. High temperatures today and tomorrow
will increase by a few degrees over yesterday west of the
mountains as offshore flow develops. Numerical models disagree on
high temperatures for Fri and Sat, but temps are still expected to
be 10 deg F above normal in many areas. Therefore, the Heat
Advisories and Excessive Heat Warnings continue in effect through
10 pm Saturday. Record highs will likely be broken at a few sites
each day through Saturday. High pressure will rebuild aloft over
the Great Basin next week, keeping high temperatures above
seasonal normals through the forecast period.
Monsoon moisture lingering over the region will continue the
chance for showers and thunderstorms over portions of the
mountains and deserts each afternoon through early next week. Some
drying will occur Friday and Saturday, limiting the potential for
afternoon convection in some areas.
Tropical Storm Lidia, currently located about 50 mi south of
Cabo San Lucas, is forecast to track NNW up through central Baja
California then turn more westerly as it encounters the ridge to
the north. Lidia is forecast to degenerate into a post-tropical
depression by Sunday morning while turning westward. However,
associated moisture could move into Southern California. GFS is
more aggressive with a tropical moisture surge Sun-Mon while the
ECM keeps the bulk of the moisture south of the border as it
depicts a stronger ridge holding over the SW United States. Both
models indicate an influx of moisture from the remnants of Lidia,
however the ECM has later timing and significantly smaller
amounts.
&&
.AVIATION...
312025Z...Coast...Through 01/0600, P6SM vis and SCT-FEW clouds AOA
9000 ft msl, except for a chance that BKN-SCT cigs around 300-500 ft
msl and vis 1-4 sm linger near portions of the coast. 01/0600-1500
UTC, patchy low stratus/dense fog may re-develop once again within 5
sm of the coast, with bases around 200-500 ft msl, tops around 600-
800 ft msl, and vis 1/4-5 sm. Confidence in dense fog occurrence and
timing is low for KCRQ, KSAN and very low for KSNA. Forecast marine
layer inversion strength of 11 deg C at 01/1200 UTC. Elsewhere, P6SM
vis and SCT-FEW clouds AOA 9000 ft msl.
Valleys/Mountains/Deserts...SCT-BKN clouds AOA 5000 ft msl and areas
of -SHRA/TSRA developing and increasing in coverage through 01/0300
UTC. Bases 8000-10000 feet msl with CB tops growing to as high as
40000 ft msl. Gusty and erratic winds possible around any
thunderstorm. Showers/thunderstorms gradually decreasing tonight
after 01/0300 UTC with P6SM vis prevailing.
&&
.MARINE...
A shallow marine layer may result in more dense fog tonight into
Friday morning.
Possible remnant moisture from Tropical Storm Lidia may result in
some showers and thunderstorms over the coastal waters on Monday.
&&
.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.
&&
.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Heat Advisory until 10 PM PDT Saturday for Orange County Coastal
Areas-Riverside County Mountains-San Bernardino County
Mountains-San Diego County Coastal Areas-San Diego County
Mountains.
Excessive Heat Warning until 10 PM PDT Saturday for Apple and
Lucerne Valleys-Coachella Valley-Orange County Inland Areas-
San Bernardino and Riverside County Valleys-The Inland
Empire-San Diego County Deserts-San Diego County Valleys-
San Gorgonio Pass Near Banning-Santa Ana Mountains and
Foothills.
PZ...NONE.
&&
PUBLIC...PG
AVIATION/MARINE...HarrisonPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SAN DIEGO CA
926 AM PDT THU AUG 31 2017
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0630 AM LIGHTNING COLTON 34.06N 117.32W
08/31/2017 SAN BERNARDINO CA BROADCAST MEDIA
POWER OUTAGE LIKELY FROM LIGHTNING STRIKE.
&&
JJTThe Santa Ana Mts radar (KSOX), 42 km away, was scanning. The volume nearest the first event began at 21:36 UTC.
3 thunderstorm wind, 2 flood, 1 flash flood, 1 lightning. Local standard time, as filed.
Severe downburst winds toppled multiple large trees and downed seveal strings of power poles in Wildomar.
A downburst from a severe thunderstorm over Temescal Valley produced a 65 mph wind gust at a RAWS station.
Strong downburst winds damages roofs and a deck at a mobile home park in Home Gardens.
Significant street flooding reported by CHP in the Temescal Valley. One vehicle stalled after trying to cross a flooded roadway.
Multiple vehicle stalled on Interstate 215 and Highway 60 due to flooding of lanes.
Lanes #3 and #4 of Highway 60 flooded with 1 foot of water.
A lightning strike damaged a critical power transformer in Colton, cutting power to 95% of the city. An estimated 50,000 customers lost power. Area schools closed due to a lack of power that lingered into the following day.
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: 720695, 720696, 720697, 720736, 720733, 720734, 720727.