(Bloomberg) -- A powerful bomb cyclone is ripping the Pacific Northwest with hurricane-strength winds as the season’s first atmospheric river comes ashore in Northern California, promising torrential rains, floods and mudslides across the region.
“The thing about this one is the persistence,” said Brian Hurley, a senior branch forecaster at the US Weather Prediction Center. “It is going to have some staying power over a three-day period.”
The harshest impacts are expected to spread across Northern California through Thursday with some areas getting as much as 18 inches of rain, said Hurley. While that is playing out, a Pacific storm that is “bombing out” will linger off the coast of Washington and British Columbia, spinning damaging winds into the coast.
A storm becomes a bomb cyclone when its central pressure, a way of measuring its strength, drops by 24 millibars in 24 hours. The storm off the Pacific Northwest has met that criteria, Hurley said. The US National Weather Service has issued hurricane force warnings as gusts are forecast to reach 80 miles per hour and waves will top out as high as 26 feet. Canada also has issued wind warnings for coastal British Columbia.
Hurley said the storm likely will remain offshore and wind down in coming days.
As that happens, the atmospheric river is expected to bring flooding rains from San Francisco to Vancouver through Thursday while hurricane-force winds will last at least through Wednesday, Hurley said.
Atmospheric rivers are ranked on a 1 to 5 scale, with storms reaching category 3 or higher considered to be dangerous and destructive. Areas along the California coast will range in the category 3 and 4 range, while farther north the impacts will be less severe.
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