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Severe flash flooding hits Southern California, millions under Flood Watch

ventura county flood december 21 2023

Southern California is under a dangerous flood threat with heavy rainfall impacting Los Angeles and San Diego areas, and moving eastward. Flood Watch is in effect for more than 20 million people from Southern California to central Arizona.

As the storms continue on Friday, December 22, 2023, Southern California is grappling with a severe flood threat that has been persisting throughout the week. The Los Angeles and San Diego areas have been particularly impacted, with the severe weather conditions expected to move eastward.

Over 20 million residents across Southern California and central Arizona remain under Flood Watch as cumulative rainfall totals have surpassed 250 mm (10 inches) in several mountain areas as of December 22 — Rocky Butte, for example, registered a total of 408.2 mm (16.07 inches) in 5 days, while Ventura County experienced a month’s worth of rain in just an hour.

The NOAA’s Weather Prediction Center (WPC) has classified a swath of Southern California in a Level 3 out of 4 risk zone for flash flooding due to the overall rainfall amounts and potential for bursts of thunderstorm-triggered heavier rain, especially over recent wildfire burn-scar areas.

For many communities, this period marks the heaviest rainfall event since the remnants of Hurricane “Hilary” impacted the region in August.

LocationRainfall (mm/inches)
Rocky Butte (SLO County)408.18 mm (16.07 inches)
Old Man Mountain (Ventura County)353.06 mm (13.90 inches)
El Deseo (SBA County)283.82 mm (11.17 inches)
KTYD Tower (SBA County)282.70 mm (11.13 inches)
Doulton Tunnel (SBA County)269.74 mm (10.62 inches)
East Camino Cielo (SBA County)265.43 mm (10.45 inches)
Matilija Canyon (Ventura County)261.88 mm (10.31 inches)
Upper Romero (SBA County)260.35 mm (10.25 inches)
San Marcos Pass (SBA County)234.70 mm (9.24 inches)
Juncal Dam (SBA County)225.30 mm (8.86 inches)
5-day rainfall totals to 04:00 PST (12:00 UTC) on December 22, 2023.

Significant flash flooding has occurred in the Oxnard area, the largest city in Ventura County, after 80.8 mm (3.18 inches) of rain fell in one hour on December 21.

The city’s overall storm totals approached 152.4 mm (6 inches) at the city’s civic center by Thursday evening, December 21. Local residents have been severely affected, with about 60 homes flooded and emergency evacuations, including from a senior living center.

The Ventura County Fire Department reported a significant increase in emergency calls during the peak of the storm, with approximately 275 calls received within five hours.

YouTube video

By the end of the week, forecasters anticipate that between 125 to 200 mm (5 and 8 inches) of rain will have fallen in several areas, with some places potentially experiencing even higher amounts.

This continued precipitation keeps the risk of flash and urban flooding high.

The storm system is not only bringing heavy rains but also heavy snow over the Sierra Nevada Mountains and impacting other areas like the Washington and Oregon Cascades and parts of Idaho and Nevada.

noaa goes west 2110 utc on december 21 2023 bg
Image credit: NOAA/GOES-West, RAMMB/CIRA, The Watchers. Acquired at 21:10 UTC on December 21, 2023

“A relatively slow-moving low pressure system off the southern California coast will begin to gradually accelerate off to the east and move onshore into the Southwest U.S. by tonight,” NWS forecasters Kong and Orrison noted.

Deep southerly flow ahead of the low center will continue to feed moisture-laden air from the subtropical Pacific Ocean toward southern California today. Heavy rainfall will most likely be found once again along the wind-facing coastal ranges of Southern California where prolific rainfall amounts had already been reported in the past 24 hours, with additional significant flash flooding expected.

Urban flash flooding will also be possible including areas of the Los Angeles basin early this morning.

Debris flows and mudslides will remain likely near burn scar areas and areas of steep terrain.

The Weather Prediction Center has maintained a Moderate Risk of excessive rainfall for these areas early this morning, December 22.

As the low pressure system moves onshore later today, the heavy to excessive rainfall threat will begin to spread east into areas of central and southern Arizona as well by Friday night and into early Saturday. A Slight Risk of excessive rainfall is depicted across these areas where there may be some isolated to scattered areas of flash flooding. Some of the local slot canyons and burn scars closer to areas of terrain, and also the dry washes/arroyos over the deserts, will be vulnerable to seeing these runoff impacts. Generally, much of the rainfall across these areas should be beneficial.

Update:

A 28-year-old man died in a car accident in Carson. According to Los Angeles County Sheriff’s Lt. Pasquale Aiello, the vehicle went into the Dominguez Channel off Avalon Boulevard. The incident is under investigation, but a witness reported that the car was hydroplaning and speeding at approximately 110 – 120 km/h (70 to 75 mph). It was raining lightly at the time of the crash.

References:

1 Flash flooding wreaks havoc on Los Angeles area as powerful storm slams into Southern California

2 Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 346 AM EST Fri Dec 22 2023

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