A powerful storm system with the strength of a hurricane is likely to whip damaging winds through the Northwest, create large ocean waves and usher in a drenching atmospheric river over Northern California this week.
Heavy rain and strong winds will begin moving through the Bay Area Tuesday night as the first major atmospheric river — a band of moisture that flows from the Pacific Ocean — begins a deluge extending into the weekend. The bulk of this week’s rainfall is expected to fall north of San Francisco, where more than a month’s worth of precipitation could fall over the next three to four days.
Marty Ralph, the director of the Center for Western Weather and Water Extremes, said this storm was predicted to produce as much as 20 percent of the rainfall the region would expect in a year. He said this one storm will help get the wet season in Northern California off to a strong start.
For the Bay Area, Wednesday will be the most intense day, forecasters said, with a brief reprieve on Thursday, but only in comparison with Wednesday. The heaviest rain will fall in the North Bay, with lighter showers across the city and to the south. Rain will continue on Friday.
Forecasters have issued a rare “high risk” of excessive rainfall on Thursday for parts of northwest California, where more than 16 inches of rain could fall. Over the past decade, some of the deadliest and most destructive floods have occurred in areas that forecasters said were at this level of risk.
For areas south of San Francisco, Friday will bring the heaviest rainfall. By the end of the week, nearly everyone in the Bay Area can expect at least an inch or two of rain.
It’s early enough in the wet season that the risk of flooding is mitigated by the fact that the ground is still dry and the rivers aren’t high, Dr. Ralph said. Typically, concerns about more considerable flooding rise when atmospheric rivers come back to back. This storm could set the stage for events coming down the line, but could still cause some flooding along smaller creeks and streams throughout the North Bay.
A possible ‘triple-bomb cyclone’ in the Pacific
This atmospheric river is connected to a storm system swirling over the northeastern Pacific Ocean that may reach one of the lowest barometric pressure readings ever recorded in that part of the ocean, forecasters said on Tuesday.
A storm’s pressure is an indication of its intensity, and this storm’s reading is expected to drop so quickly that meteorologists would call it a “bomb cyclone” — a storm that rapidly intensifies or dramatically drops in pressure in 24 hours. (The threshold measurement in millibars varies depending on the storm’s latitude.)
These “bomb cyclones” are more likely when there is an atmospheric river already present, like in this case, Dr. Ralph said. The fact there was already an atmospheric river where the cyclone began forming in the Pacific Ocean has given the storm more fuel.
While the worst of the storm will remain out at sea, the stronger this storm becomes, the more intense the winds from the Bay Area to Seattle will be. Oregon forecasters have issued a very rare hurricane-force wind warning for the state’s coastal waters, which means winds could surpass 74 miles per hour. They said that “very steep and mountainous seas of 29 to 34 feet” are possible.
Inland, the strongest damaging winds, with gusts up to 70 m.p.h., are possible along the coast and the high terrain of Northern California and parts of Oregon and Washington. This could lead to blizzard conditions where snow falls Tuesday night, including in locations such as the Cascades, where Seattle forecasters have issued a rare blizzard warning.
Where to expect atmospheric rivers this season
In the winter outlook from the National Oceanic and Atmospheric Administration’s Climate Prediction Center, forecasters warned that the season’s storm paths would favor abundant rainfall across the Northwest, a pattern often associated with La Niña — a weather pattern generated by cooler-than-average equatorial sea-surface temperatures in the Pacific and seen as the opposite of El Niño.
During La Niña, there are more frequent atmospheric rivers over the Pacific Northwest and a slight decrease over the southern tier of the United States, including in Southern California, Nat Johnson, a meteorologist with NOAA’s Geophysical Fluid Dynamics Lab, said.
During the El Niño years, storms take a more southern track, like in the winters of 2022 and 2023, when stronger and more frequent atmospheric rivers mostly lashed California.
As of now, the Pacific Ocean is still in a neutral phase and not quite meeting La Niña criteria. During a neutral phase, less predictable weather patterns can dominate, something Dr. Johnson called “weather wild cards.”
Right now, he is forecasting that this winter a marginal La Niña will take shape and warns that the impacts of this year’s La Niña could “punch above its weight” because the rest of the oceans are so warm. That could mean even more storms for the Northwest.
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