Atmospheric Rivers and Bomb Cyclones Saturate California in the New Year

CoreLogic® is monitoring ongoing flooding and river flooding in California

The New Year brought unseasonably wet conditions to California. A series of atmospheric rivers and frontal systems, including a bomb cyclone, slammed into California beginning on Friday, December 31, 2022 and continuing through the week of January 9, 2023. The storm systems included multiple atmospheric rivers that provided excess moisture , resulting in heavy rainfall that saturated much of California and caused localized river flooding and mudslides.

Back-to-back heavy rainfall events kick off the New Year

Repeated atmospheric rivers dropped significant precipitation across the state of California. Since January 1, areas of California received over 30 inches of rain, according to the National Weather Service (NWS). The greatest rainfall depths were recorded south of Eureka, along the coast northwest of San Luis Obispo, and in Santa Barbara (Figure 1).

Figure 1: January 2023 precipitation totals in California and Nevada

Source: NOAA NWS 2023

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The NWS reported two to four inches of rain during a 24-hour period ending Jan. 1 across the Sacramento region and more than five inches of rain in San Francisco by mid-afternoon on Dec. 31. Heavy rain continued in central and southern California throughout the following day. On January 4, a second storm system brought heavy rain and hurricane-force winds to homes across California. A frontal system combined with a second atmospheric river brought persistent and at times heavy rain over a two-day period ending Thursday, January 5. One to two inches of rain fell over a 24-hour period in Sacramento, San Francisco and San Jose. Outside of these cities, maximum 24-hour rainfall totals exceeded five inches. Heavy rain continued over California through the weekend of January 7th. Two-day rainfall totals exceeded six inches along the coast near Santa Rosa, San Jose and San Luis Obispo. Additional rain fell over California throughout the week of January 9, as another storm surge brought moisture to the waterlogged state.

Normal monthly rainfall depths in the cities of Sacramento, San Francisco, San Luis Obispo, Santa Barbara, and Los Angeles range from one to three inches. According to the NWS, precipitation totals far exceeded the monthly average across the state (Figure 2, left). Santa Barbara’s January 2023 precipitation totals are already more than 30 inches above normal (Figure 2, right).

Figure 2: January 2023 precipitation totals relative to the January monthly average as a percentage (left) and number of inches above average (right)

Source: NOAA NWS 2023

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A bomb cyclone brought hurricane-force winds to California

Localized cases of hurricane-force wind gusts associated with the January 4 bomb cyclone were recorded across Northern California (Figure 3). A bomb cyclone is a storm system characterized by a drop in atmospheric pressure of 24 millibars within 24 hours, a phenomenon that often causes extreme wind speeds due to significant pressure gradients. According to CoreLogic Wind Verification Technology, areas just west of Paradise, California and south of Bakersfield, California experienced wind gusts exceeding 90 mph, a speed fast enough to cause some damage to buildings. Maximum wind speeds of 65 mph were observed in downtown San Francisco.

Figure 3: Maximum wind speed in northern (left) and southern (right) California as seen at the CoreLogic reactor. Maximum wind speeds exceeded 90 mph near Paradise and Bakersfield, CA

Source: CoreLogic, Inc.

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Waterlogged vehicles, broken tops, power outages and mudslides

The heavy rains associated with the December 31 storm quickly saturated the soils, preventing water from penetrating the soil surface. The increased runoff led to flash flooding in low-lying urban areas around Sacramento, San Francisco, and San Jose. Flash flooding was reported as far south as Los Angeles County. Multiple furrow breaches along the Cosumnes River led to major river flooding outside of Sacramento. Authorities were forced to close State Highway 99 due to flooding and numerous cars were stranded along the highway.

Ground conditions did not improve between the December 31 and January 4 storms. Soils throughout California remained saturated, which prevented additional infiltration. There is a high risk of landslides in the more mountainous regions of California, especially in areas of recent wildfire activity. Recently burned areas lack plant life, which helps increase soil cohesion. People living downslope of recent burn scars should monitor landslide activity. According to initial damage reports, mudslides are responsible for road closures and damage on State Highway 1. Heavy rainfall is responsible for road closures due to mudslides in Matilijas Canyon near the 2017 Thomas Fire site.

Stream gauges in Marin, Sonoma and Napa counties are currently in moderate to minor flood stages. Napa Creek on State Highway 29 nearly passed the record breaking stage on Jan. 9 at 5:30 a.m. local time. The gauges along Colgan Creek, Copeland Creek and Willow Brook surrounding Rohnert Park, California are in near major flood stages. South of San Francisco, flow gauges outside of San Jose along the Berryessa Creek and Santa Cruz on the San Lorenzo River are at moderate flood stages.

Hurricane-force wind gusts are responsible for widespread power outages. According to Poweroutage.us, over 180,000 customers were without power in Northern California as of January 5 at 8:16 a.m. local time. Power was restored to thousands over the weekend, but upcoming storms are expected to affect more customers.

Wave action and coastal flooding are responsible for damage in Santa Cruz County. According to Surfline.com, 20- to 30-foot waves crashed on the shoreline in and around Santa Cruz. Wave action is responsible for damaging piers and ships in the area.

Initial damage assessments are underway and the full extent of property loss due to water and wind is unknown at this time. Almost the entire state of California remained under flood, high winds or winter storm conditions as of January 9 at 6 a.m. local time.

More rain expected in California

The five-day forecast for California includes additional storms due to another atmospheric river moving over the state. Heavy rain is expected in most of the country. Many waterlogged areas could see an additional four to five inches of rain (Figure 4).

Figure 4: 120-hour rainfall forecast, valid from January 12 to January 17

Source: Pivotal Weather 2023

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The CoreLogic Incident Response Team is monitoring the situation in California. Updated impact summaries will be posted on http://hazardhq.com if more information becomes available.

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