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Atmospheric river unloads record rainfall on California, spurring flooding and mudslides

Floods and mudslides across California triggered evacuations Feb. 14 after an atmospheric river unloaded record rainfall hit the state. (Video: The Washington Post)

Californians knew for a while that an atmospheric river was coming in the latter half of this week. But that was about all that was certain, at least until just before it arrived.

A two-day north-south combo punch turned out to be a soaker — and it hit almost everywhere.

From Wednesday to Thursday, the river in the sky packed an astoundingly waterlogged wallop.

Combined with a pair of low-pressure systems sliding down from the Gulf of Alaska, it dumped more than a foot of snow on the north Central Valley city of Redding, the most there in more than 50 years.

The next day, Palomar Observatory, located on a mountaintop 60-plus miles north-northeast of San Diego, took in 10.1 inches of rain — its wettest day ever recorded. Palm Springs posted 3.68 inches, its wettest February day on record and the third heaviest 24-hour rainfall in any month.

In between came report after report of washed-out and flooded roads and highways.

Thankfully for California, it did not suffer intensive flooding caused by rain melting away the deep snowpack accumulated over this wet winter. At Lake Oroville, the site of that scary spillway failure two years ago, the reservoir was taking in 66,000 cubic feet per second, yet there was plenty of capacity for this inflow.

Meanwhile, social media was awash in videos and footage of flood-control infrastructure — from neighborhood retention ponds to the concrete channel cutting past Angel Stadium of Anaheim.

That’s not to say California escaped unscathed.

Late Thursday, possible flooding on the Sacramento River prompted the Butte County Sheriff’s Office to issue an evacuation order near Chico. Also, there were several water rescues due to motorists getting caught in rising floodwaters.

But the Californian with the worst experience of the AR might have been the 70-year-old woman in Sausalito, on San Francisco Bay just north of the Golden Gate Bridge, who had a mudslide crash into her house and a neighbor’s. More than 2.5 inches of rain fell in the San Francisco area.

According to the days-old Ralph Scale, which measures the strength of AR events, this was a Category 3 — “strong.” Notably, the scale — which factors intensity and duration — knocked down the rating from a Cat 4 (“extreme”) in Northern California because of a lower-intensity IVT and in Southern California because the storm passed through in less than 24 hours.

The forecasting models picked up on this event early, focusing on a Kona low swinging into place, ready to turn on the tap for the West Coast. If that wasn’t convincing, there were impressive satellite photos (We see you, newest GOES West! Better late than never!) and reports from a storm-battered Hawaii.

But until just hours before that tropical moisture began streaming across the state, those same forecasting models couldn’t reach consensus on how the AR would interface — “phase,” in the technical term — with the two storm systems swooping down from the north. The tone of meteorologists’ forecasts ranged from frustration to bewilderment.

The state’s wet winter doesn’t appear to be coming to an end anytime soon. Late Thursday, another storm was dragging a squall line across Northern California, while two more low-pressure systems from the north were expected to arrive Friday and Sunday. But the latter pair don’t have the subtropical moisture to tap into, and so they aren’t expected to make much noise.

Freelance writer Mike Branom has covered weather/climate stories for newspapers and the Associated Press. He rafts atmospheric rivers in Pasadena, Calif., where he works at a cannabis consultancy.