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'Bomb Cyclone,' Meet 'Atmospheric River'

2 big weather events set to converge Saturday into Sunday on West Coast, and it likely won't be pretty
By Jenn Gidman,  Newser Staff
Posted Oct 21, 2021 9:10 AM CDT
Updated Oct 23, 2021 6:30 AM CDT
Rare 'River in the Sky' Could Slam Parched California
A van drives through floodwaters in Graton, Calif., on Jan. 7, 2017, right before an expected atmospheric river event there.   (AP Photo/Jeff Chiu)

(Newser) Update: As if California didn't have enough to not look forward to this weekend, warnings of a potentially rare "atmospheric river" are now being joined by ones for a hurricane-strength "bomb cyclone," which takes place when a low-pressure system's central pressure plummets 24 millibars or more in less than 24 hours. The Weather Prediction Center says these two weather events converging will dump "a fire hose of rich subtropical moisture into California," per CNN. The north and central parts of the state will see "flooding rains, wet snow, strong winds, and coastal surf" on Saturday night into Sunday, the outlet notes. Parts of Oregon and Washington may also be affected. Our original story from Thursday follows:

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California is thirsty for some rain—but maybe not this much. CNN reports that multiple storms sweeping in from the Pacific are headed toward the northern part of the state, and the rising moisture in the air could lead to a rare Category 5 "atmospheric river" that could spur flash floods and debris flows. The weather event—which the National Oceanic and Atmospheric Administration describes as being like a "[river] in the sky"—consists of a long, narrow band of concentrated moisture moving miles above the sea, with the water vapor equivalent to the amount of water you'd see flowing at the mouth of the Mississippi River.

Once an atmospheric river makes landfall, it can dump an unusual amount of precipitation. "An atmospheric river marked as a category 4 or a 5 is capable of producing remarkable rainfall totals over three or more days, likely to exceed 10-15% of a typical year's precipitation in some locations," Marty Ralph, who heads up UC San Diego's Center for Western Weather and Water Extremes, tells CNN. Northern California could see up to a foot of rain, and perhaps 3 feet of snow in the Sierra Nevada.

Because California has been suffering from a longtime drought—one of the driest on record, per CBS San Francisco—so much rain so quickly will likely produce runoff, flooding, and fast-moving masses of rocks, soil, and vegetation in "burn scar" areas, where the landscape is especially charred and repels water. "It's a big one," Ralph tells the Washington Post of the approaching weather pattern. There is some potential good news amid all this: The dramatic rainfall could signal the end of Northern California's wildfire season. (Read more California stories.)

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