The first major atmospheric river of the season is parked over Northern California, and it’s being held in place by two bomb cyclones — one of them among the strongest on record — in an unusual meteorological event that is expected to feed a continuous surge of moisture into the West Coast for the next several days.
As more rain falls, the risk for dangerous flash flooding, debris flows and landslides will increase as the ground becomes too saturated to hold and streams and rivers continue to rise and overflow.
What can make atmospheric rivers difficult to forecast is that they are narrow bands of moisture, and they hold dramatically less moisture around their edges than they do in the center. So a specific location could see a huge variety in the potential forecast in the days and even hours leading up to an event. For this storm, that means a place south of the Golden Gate Bridge, like San Francisco, could end up seeing only an inch of rainfall or more than six inches by Friday.
Here’s a look at what’s happened so far, what the region may face next and why it’s so hard to be sure.
This week so far
The storm began to move ashore Tuesday evening, first bringing pouring rain and howling winds to the northern regions of Washington and then working its way south. More than 600,000 customers lost power at some point overnight into Wednesday, and on Thursday morning more than 300,000 in Washington remained without power, according to PowerOutage.us, which tracks utilities.
By Wednesday morning, rain had arrived in the San Francisco Bay Area, where residents faced uncertain forecasts that ranged from less than an inch to more than 10 in accumulation, depending on the storm’s movements. From Seattle to San Francisco, the storm flooded roads, brought down trees and forced Amtrak to cancel trains.
Santa Rosa in the North Bay, which typically receives just over three inches of rain in November, reached over seven inches on Wednesday, its second wettest day in the last 120 years. Just south, across the bay, San Francisco received less than a half-inch. The storm also brought blizzard conditions to places at higher elevations, and forced the authorities to close Interstate 5 just south of the Oregon-California border for a stretch of more than 50 miles.
What comes next: Thursday
The Bay Area may see a brief lull on Thursday as a second storm system rapidly strengthens in the Pacific Ocean. As it intensifies, the narrow band of rain will shift north, even back into Oregon, before quickly shifting south again. This storm is not expected to be nearly as intense as the one that arrived on Tuesday, but it will keep the atmospheric river going into Friday.
A rare high risk alert for excessive rain was issued for portions of coastal Northern California on Thursday, from Ukiah to just south of Fortuna, and forecasters warned that the compounding days of rainfall over the same area could lead to life-threatening flooding. Another three to six inches of rain is possible with some areas likely to receive closer to 10 inches, possibly pushing storm totals over 16 inches in a few locations. There will be significant flood risks through Thursday night and early Friday morning as the ground becomes saturated.
As the day goes on, the risk for landslides and debris flows also increases, especially over burn scars where fires have left soil less absorbent. Forecasters are especially concerned about a burn scar left from the Park fire, which scorched hundreds of thousands of acres this summer between Redding and Sacramento. That wildfire was the fourth largest in California’s history.
Friday: More rain for San Francisco
The atmospheric river across California is expected to begin to weaken and drift south, bringing an end to the extreme wet weather over coastal Northern California. But that movement will center the storm over the Bay Area, and San Francisco is likely to see its heaviest rain of the week on Friday.
Late in the day and into the night, as colder air pushes through the Bay Area, this three-day atmospheric river should finally come to an end — but the chance of rain doesn’t go away.
Saturday: The atmospheric river is over, but the rain is not
An unsettled weather pattern and waves of nuisance-level rainfall are likely to continue to surge through the coastal ranges of central and Northern California on Saturday and beyond.
Forecasters with the San Francisco National Weather Service said that moderate rain is also expected on Sunday and Monday, but uncertainty is much higher for how much and precisely when. It is likely that rain will remain in the forecast, without a dry day for at least another week, these forecasters warned.
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