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Here’s How Big California’s Downpour Really Was
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The atmospheric river can be seen reaching from Hawai’i to California, a tributary of the moist band of air circling the tropics. 
NOAA
NOAA
SAN FRANCISCO — Californians are drying their socks, draining their basements, and de-flooding their freeways after a massive rainstorm drenched the state late this week. The brunt of the downpour came on Dec. 11, when an average of 2.5 inches of rain fell across the state.
The rain came via a tributary of atmospheric moisture that snaked its way up from the tropics near Hawaii. Historically, this transport of wet air isn’t uncommon—the phenomenon is nicknamed the “Pineapple Express”—and is typically responsible for about 30-50 percent of the annual precipitation in the dry western states. This storm was particularly dramatic, as it came in the midst of the state’s current three-year drought.
Infrared radar shows clouds hitting the west coast on December 11. NOA
NOA
Rainfall was most dramatic in coastal Northern California, with San Francisco getting 3.46 inches of rain. All three major Bay Area counties are coming close to doubling their average annual amount of rainfall.
Towns north of the bay were hit even harder, with places in the wine-producing Russian River valley getting as much as 10 inches of rain. Some areas were getting half an inch of rain per hour as early as December 10.
The Dec 11 storm caused a massive surge in the Russian River. USGS
USGS
The storm disrupted infrastructure as well. An estimated half a million homes lost power statewide, and flooding caused at least 18 highway or freeway closures across the state.
Two-hundred and thirty-six flights at San Francisco International Airport were cancelled on Dec. 11, accounting for 40 percent of the nation’s total canceled flights. Multiple roads closed due to flooding, notably a section of the busy Highway 280 in San Jose. Rain also caused a roof to collapse over the produce section at a San Jose grocery store.
PG&E reported that over 208,000 homes in the Bay Area lost power, with over 94,000 of those in San Francisco.
The rest of the state got off a little easier, with Los Angeles and San Diego averaging an inch of rain each. But, with dry soil unable to soak up the all the water, the downpour triggered flooding and mudslides. Sixty houses had to be evacuated because of mudslides in Southern California, and 18 inches of mud shut down a stretch of highway in Ventura County. Two people had to be rescued from flash flooding in the Los Angeles River.
Across Southern California, over 44,000 homes lost power, and nearly 20,000 in San Louis Obispo, Santa Barbara, and Kern Counties.
The storm didn’t just bring rain. Winds up to 50 miles per hour were reported in San Francisco, 49 mph in Lake Tahoe, and 80 mph in Sacramento. In Oregon, wind knocked over trees, killing two people. These were the storm’s only reported casualties.
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