Northern California and the Pacific Northwest are bracing for what is expected to be a powerful storm, with heavy rain and winds set to pummel the region and potentially cause power outages and flash floods. The Weather Prediction Center issued excessive rainfall risks beginning Tuesday and lasting through Friday as the strongest atmospheric river—long plumes of moisture stretching far over the Pacific Ocean—that California and the Pacific Northwest has seen this season bears down on the region, the AP reports. The storm system has intensified so quickly that it is considered a "bomb cyclone," says Richard Bann, a meteorologist with the National Weather Service Weather Prediction Center.
The areas that could see particularly severe rainfall as the large plume of moisture heads toward land will likely stretch from the south of Portland, Oregon to the north of the San Francisco area, Barr says. "Be aware of the risk of flash flooding at lower elevations and winter storms at higher elevations. This is going to be an impactful event."
- In northern California, flood and high wind watches go into effect Tuesday, with up to 8 inches of rain predicted for parts of the San Francisco Bay Area, North Coast, and Sacramento Valley.
- A winter storm watch was issued for the northern Sierra Nevada above 3,500 feet, where 15 inches of snow was possible over two days. Wind gusts could top 75 mph in mountain areas, forecasters said. "Numerous flash floods, hazardous travel, power outages, and tree damage can be expected as the storm reaches max intensity" on Wednesday, the Weather Prediction Center warned.
- A bomb cyclone occurs during the rapid intensification of a cyclone located between the tropics and the polar regions, according to the National Oceanic and Atmospheric Administration. It can happen when a cold air mass collides with a warm air mass, which is something that can occur over ocean waters, the agency says.
- The measurement needed to determine whether a cyclone can be classified a bomb cyclone can be tricky, but it largely concerns a swift drop in pressure, the AP reports. Atmospheric pressure is measured in millibars by the National Weather Service. If a storm decreases 24 millibars or more in 24 hours or less, it can be considered a bomb cyclone, said Stephen Baron, a forecaster with the weather service in Gray, Maine. "I would say rapid intensification of hurricanes is one of the more common times we see it," Baron says. "We do see it with Nor'easters occasionally."
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