CoreLogic® is monitoring the ongoing flash and riverine 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 starting Friday, Dec. 31, 2022 and continued through the week of Jan. 9, 2023. The storm systems included multiple atmospheric rivers that provided excessive moisture, resulting in heavy rainfall that saturated much of California and caused localized riverine and flash flooding and mudslides.
Back-to-Back-to-Back Heavy Rainfall Events Start the New Year
Recurring atmospheric rivers dropped substantial rainfall across the entire state of California. Since Jan. 1, areas of California received over 30 inches of precipitation, according to the National Weather Service (NWS). The greatest precipitation 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
The NWS reported two to four inches of rain over 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 over the central and southern regions of California throughout the following day. On Jan. 4, a second storm system brought additional heavy rainfall and hurricane-force wind gusts to homes across California. A frontal system combined with a second atmospheric river brought constant, and at times heavy, rainfall over a two-day period ending Thursday, Jan. 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 rainfall continued over California throughout the Jan. 7 weekend. Two-day rainfall totals exceeded six inches along the coast near Santa Rosa, San Jose and San Luis Obispo. Additional rainfall fell over California throughout the week of Jan. 9, as another atmospheric riverine brought moisture to the waterlogged state.
Normal monthly precipitation 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, rainfall totals have greatly surpassed the monthly average across the state (Figure 2, left). The January 2023 precipitation total in Santa Barbara is already more than 30 inches higher than normal (Figure 2, right).
Figure 2: January 2023 Precipitation Totals Relative to the January Monthly Average as a Percent (Left) and Number of Inches Above Average (Right)
A Bomb Cyclone Brought Hurricane-Force Wind Gusts to California
Localized instances of hurricane-force wind gusts associated with the Jan. 4 bomb cyclone were recorded across Northern California (Figure 3). A bomb cyclone is a storm system characterized by a 24-millibar atmospheric pressure decrease within 24 hours, a phenomenon that frequently causes extreme wind speeds due to significant pressure gradients. According to CoreLogic Wind Verification Technology, the areas just west of Paradise, California and south of Bakersfield, California experienced wind gusts exceeding 90 mph, a speed fast enough to cause some building damage. Maximum wind speed gusts of 65 mph were observed in downtown San Francisco.
Figure 3: Maximum Wind Speed Gusts in Northern (Left) and Southern (Right) California as Seen in CoreLogic Reactor. Maximum Wind Speeds Exceeded 90 mph Near Paradise and Bakersfield, CA
Submerged Vehicles, Breached Levees, Power Outages and Mudslides
Heavy rainfall associated with the Dec. 31 storm quickly saturated soils, preventing water from infiltrating the ground 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 levee breaches along the Cosumnes River led to extensive riverine flooding outside of Sacramento. Authorities were forced to close state Highway 99 due to flooding and multiple cars were stranded along the highway.
Soil conditions did not improve between the Dec. 31 and Jan. 4 storms. The soil across California remained saturated, which prevented additional infiltration. There is an elevated risk of mudslides 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 residing downslope of recent burn scars should monitor mudslide activity. According to initial damage reports, mudslides are responsible for road closures and damage on state Highway 1. Excess rainfall is responsible for a road closure due to mudslides in Matilija Canyon, near the site of the 2017 Thomas Fire.
Stream gauges in Marin, Sonoma and Napa counties are currently at moderate to minor flood stages. Napa Creek at state Highway 29 nearly surpassed the record stream stage on Jan. 9 at 5:30 a.m. local time. Gauges along the Colgan Creek, Copeland Creek and Willow Brook surrounding Rohnert Park, California are at near-major flood stages. South of San Francisco, stream gauges outside San Jose along the Berryessa Creek and Santa Cruz on the San Lorenzo River are in 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 Jan. 5 at 8:16 a.m. local time. Power was restored to thousands over the weekend, but upcoming storms are expected to impact additional customers.
Wave action and coastal flooding are responsible for damage in Santa Cruz County. According to Surfline.com, 20- to 30-foot waves crashed into the coastline in and around Santa Cruz. The wave action is responsible for damage to piers and ships in the area.
Initial damage assessments are underway, and the full scope of property loss due to water and wind is currently unknown. Nearly the entire State of California remained under flood, high wind watch or winter storm watches, as of Jan 9 at 6 a.m. local time.
More Rainfall Expected in California
The five-day forecast for California includes additional storms due to another atmospheric river moving over the state. Heavy rainfall is expected across much of the state. Many waterlogged areas may see an additional four to five inches of rain (Figure 4).
Figure 4: 120-Hour Precipitation Forecast, Valid Jan. 12 Through Jan. 17
The CoreLogic Event Response Team is monitoring the situation in California. Updated impact summaries will be posted to http://hazardhq.com if more information becomes available.