Upper level high pressure centered over the Four Corners ushered in moisture from the southeast into SoCal starting on Labor Day. Monsoon thunderstorms impacted the mountains and deserts September 2 through 5th. The event started with a nocturnal MCS over the Coachella Valley early in the morning on September 2nd, dumping 1-1.5 inches over a couple of hours. Even more thunderstorms formed Monday afternoon, especially in the mountains. Hail was reported on I-8 Monday afternoon. Storms continued each afternoon and evening in the mountains through September 5th. Some thunderstorms even reached the San Diego County coast on September 4th. Widespread outflow wind gusts of 30-45 mph were reported on the coast and in the valleys on September 4th.
At the end of September, 24-26, tropical moisture was wrapped into southern California from a closed upper low. This initially brought Santa Ana winds and very warm temperatures to the coast and on 25-26 brought scattered thunderstorms to the San Diego mountains. The flooding rains were confined to the Mojave Desert and Imperial Valley.

Wildfire in SAN BERNARDINO AND RIVERSIDE COUNTY VALLEYS - THE INLAND EMPIRE, CALIFORNIA
From 04-SEP-19 12:00:00 to 14-SEP-19 12:00:00 (PST-8)
Property damage: 100.00K
Crop damage: 0.00K
Reported by: Other Federal Agency

The Tenaja Fire started on September 4th in the town of La Cresta in Riverside County and quickly grew to over 1,900 acres over the course of two days. 1,200 people were evacuated, and many schools closed due to the poor air quality. Two structures were damaged and three firefighters suffered minor injuries. The cause of the fire is still under investigation, but lightning may be the cause. The fire was contained on September 14th.

Debris Flow in SAN DIEGO, CALIFORNIA
From 05-SEP-19 14:00:00 to 05-SEP-19 15:00:00 (PST-8)
Location: BANNER
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Law Enforcement

CHP reported several basketball-sized boulders blocking lanes on highway 78 near Banner.

Heavy Rain in RIVERSIDE, CALIFORNIA
From 02-SEP-19 02:00:00 to 02-SEP-19 06:00:00 (PST-8)
Location: (PSP)PALM SPGS ARPT
Property damage: 0.00K
Crop damage: 0.00K
Reported by: ASOS

Palm Springs Airport ASOS recorded 0.98 inches of precipitation between 0200 and 0600 PST on September 2nd. The heaviest rain was between 0300 and 0400 PST.

Flash Flood in RIVERSIDE, CALIFORNIA
From 02-SEP-19 02:00:00 to 02-SEP-19 06:00:00 (PST-8)
Location: RIMLON
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Broadcast Media

Flash flooding along Date Palm Drive at Los Gatos Road from heavy rain from 0400 through mid-morning on 9/2. At one point, there was roughly 12 inches of water on the road.

Flash Flood in RIVERSIDE, CALIFORNIA
From 02-SEP-19 02:00:00 to 02-SEP-19 14:00:00 (PST-8)
Location: GARNET
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Broadcast Media

North Indian Canyon Drive was closed on the morning of 9/2 due to water over the roadway. Media reports that the road was still closed monday evening.

Strong Wind in SAN DIEGO COUNTY MOUNTAINS, CALIFORNIA
From 03-SEP-19 15:00:00 to 03-SEP-19 16:00:00 (PST-8)
Magnitude: 35.00 EG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Law Enforcement

An electrical cable was dislodged and was draped across the road. A wind gust of 40 mph was reported about 5 miles to the northwest of this location.

Heavy Rain in RIVERSIDE, CALIFORNIA
From 02-SEP-19 14:00:00 to 02-SEP-19 15:00:00 (PST-8)
Location: ANZA
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Official NWS Observations

Over 1 inch of rain fell in Anza between 1400 and 1500 PST on 9/2.

Strong Wind in SAN DIEGO COUNTY MOUNTAINS, CALIFORNIA
From 05-SEP-19 15:00:00 to 05-SEP-19 16:00:00 (PST-8)
Magnitude: 30.00 EG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Spotter reports branches down, dead tree down near Warner Springs. Estimated gusts to 35 mph at the time.

High Wind in SAN DIEGO COUNTY MOUNTAINS, CALIFORNIA
From 04-SEP-19 14:00:00 to 04-SEP-19 15:00:00 (PST-8)
Magnitude: 53.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Official NWS Observations

Wind gust of 53 knots reported in Pine Valley near I-8. A tree was down on the roadway near Pine Blvd. and Cedar Lane. USBP station on I-8 in Pine Valley reported hail and near zero visibility. Spotter John Grisafi reported wind damage at his home.

Heavy Rain in RIVERSIDE, CALIFORNIA
From 05-SEP-19 10:00:00 to 05-SEP-19 15:00:00 (PST-8)
Location: SAUNDERS
Property damage: 0.00K
Crop damage: 0.00K
Reported by: County Official

Around 1 inch of precipitation fell in Pine Cove near the Cranston Burn Scar in 1 hour on 9/5.

Hail in SAN DIEGO, CALIFORNIA
From 05-SEP-19 15:00:00 to 05-SEP-19 16:00:00 (PST-8)
Location: WARNER SPGS
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Spotter reports penny sized hail near Warner Springs.

Heavy Rain in SAN DIEGO, CALIFORNIA
From 05-SEP-19 15:00:00 to 05-SEP-19 17:00:00 (PST-8)
Location: WARNER SPGS
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Heavy rain for 20 minutes ending at 1515 PST with accumulations around 3/4 of an inch.  Road flooding in Chihuahua Valley.