Two weeks after back-to-back atmospheric rivers delivered powerful rainstorms up and down California, another series is poised to bring yet more excessive rain to the state over the next few days, potentially leading to flash flooding and deep snow that could make driving treacherous.
The first of the storms, which is expected to begin Saturday morning and last into Sunday morning, should be relatively weak compared to the one coming immediately after it. That storm, which began taking shape Friday morning over the central Pacific Ocean, is expected to begin affecting the state Sunday afternoon and will last through the middle of next week.
In early February, an atmospheric river event caused severe wind and wave damage and mudslides, and it led to multiple rescues as rivers quickly overflowed their banks.
As of Friday morning, the details of how these storms will unfold were still relatively unknown. They could change as the storm system approaches land and meteorologists better understand how strong it will be.
“The Sunday system is complicated, and that makes it hard to predict accurately,” forecasters with the National Weather Service’s office in San Francisco said early Friday.
Here’s what to know going into the weekend.
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The forecasters expect Saturday’s system to be lighter but to act as a “primer” for the next storm. Any rain that falls will make it harder for the ground to absorb rain during the next one, increasing the risk of flooding.
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The second system could bring widespread rain from Northern California to Southern California from Sunday into Wednesday. It will most likely have enough strength to push rainfall farther inland into the valleys, but the heaviest rain will fall along the coastal ranges.
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Winds are expected to be strong with the second storm, but as of Friday, they were still not expected to be as destructive as the storm earlier in the month.
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High surf, including waves of up to 20 feet, is expected with the second storm, potentially causing coastal erosion and damage to some structures.
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By the end of the event, the Sierra Nevada above 6,000 feet are likely to have another two to three feet of snow, with even higher amounts near mountain peaks. Forecasters warned anyone going to the mountains for the long holiday weekend to be prepared for travel to be difficult to impossible.
Why this forecast is complicated.
At the cruising altitude of most airplanes, a strong area of winds known as the jet stream ebbs and flows like a river. The jet stream this weekend could have winds screaming at 200 to 250 miles per hour, which will especially have an impact on the second storm system. The jet stream is what will steer and drive the second system straight at the California coast Sunday into Monday. If the storm sits just right under a section of those strong winds, it could intensify rapidly. That could also not happen.
By the time the storm gets close to the coast, the jet stream is supposed to weaken, leaving the storm to just saunter toward land over several days. But where it stops and slowly moves toward will significantly affect which location gets the most rain.
The computer models forecasters use to make predictions don’t wholly agree on how these details will unfold. On Friday morning this left them with only moderate confidence in the level of impact this storm could have.
In the San Francisco Bay Area, forecasters said the most likely case would be three to six inches of rain in the coastal ranges. But there is a reasonable worst-case scenario in which this storm brings more like six to eight inches.
Forecasters in Los Angeles shared a similar sentiment, saying they expected less rain in Los Angeles County than the early February storm — with a caveat. The forecast could be off in either direction by an inch. That might not sound like a lot, but it could be the difference between a little rain event and a significant rain event in the city.