Atmospheric River Slams California with Coastal Flooding, Severe Thunderstorm Threat

Heavy rains worked their way through coastal Southern California on Thursday along a cold front, marking the final—and perhaps most dangerous—phase of a prolonged atmospheric-river storm. Several flash flood warnings from the National Weather Service were in place by late Thursday morning across parts of Ventura and Santa Barbara counties, where the catastrophic Thomas wildfire last December left huge areas vulnerable to fast-flowing water, mud, and debris.

Thousands of residents near the burn scars were under mandatory evacuations, reported weather.com. “Significant mud and debris flows are likely near the Thomas burn scar, including the area near Ladera Road. Flooded roads with large rocks are also likely,” warned the NWS in a flash flood warning Thursday morning.

Cold Spring Basin is doing its job above #montecito. #805strong #cawx pic.twitter.com/FY8ucLOhhX

— COSB Public Works (@COSBPublicWorks) March 22, 2018

Impacts were modest across the region through Wednesday night, although several locations set precipitation records for the date, including Santa Barbara with 1.85”. Much heftier amounts were observed further up the central California coast. In the 48 hours through 10 am PDT Thursday, Rocky Butte recorded 9.20” and Atascadero racked up 7.76”. Several major highways were closed across the region, including Highway 1 in San Luis Obispo County.
 

Figure 1. The WU WunderMap radar composite showed extensive heavy rain across the Los Angeles area at 11:34 am PDT Thursday, March 22, 2018.

At midday Thursday, flash flood watches extended along and near the coast from Paso Robles to San Clemente, including the Los Angeles area, and inland through Bakersfield to the foothills of the Sierra Nevada.

Heavy snows were expected in the higher Sierra, with snow levels dropping through the day Thursday. Up through 9 AM Thursday, 18” had already fallen at Diamond Peak. Another 1-2 feet is possible above 7000 feet. Even though much of the moisture will be falling as rain at lower elevations, the storm will still help contribute to enhancing the 2018 water supply for California, pushing the water-year precipitation total closer to average.

Figure 2. GOES visible satellite image of the California storm as of 11 am PDT Thursday, March 22, 2018. Image credit: NOAA/NESDIS.

Short-lived tornadoes possible Thursday across Central Valley

The broad Central Valley at the heart of California doesn’t rank with the Great Plains for tornado action, but it does get severe weather at times—and this week’s atmospheric river has led to a prime setup for Central Valley thunderstorms. Rich moisture at low levels has worked its way well into the valley, with dew points on Thursday morning as high as 59°F in Modesto. The mild, moist air beneath cold air aloft will lead to impressive instability for this part of the country. Add some vertical wind shear, and Thursday has the potential for plenty of small hail and the chance of one or more short-lived tornadoes.

The NOAA/NWS Storm Prediction Center placed a large swath of the Central Valley under a marginal risk for severe weather.

“It's been a while (perhaps, ever) that I've seen such a set of ingredients coming together for California's Central Valley,” said John Monteverdi (San Francisco State University), an expert on severe weather in California. Based on atmospheric profiles from the NAM model, Monteverdi noted the potential for supercell storm structures and tornadoes in parts of the Central Valley, including the area between Sacramento and Fresno.

On Wednesday, at least one highly visible tornado formed just east of Arboga, CA (about 35 miles north of Sacramento) around 6:30 pm PDT. Rated as a mere EF0 on the Enhanced Fujita Scale, the twister produced minor damage.

@kcraFinan @KCRAdverdoorn @NWSSacramento @KristinasCall #Tornado SE of Marysville pic.twitter.com/tP50QX31Yt

— Sac Weather Pics (@SacWeatherPix) March 22, 2018