Weather

‘Atmospheric river’ pummels California with heavy rain and snow


A major storm known as an “atmospheric river” is pummeling California with heavy rain and high winds, continuing a streak of weather whiplash that has jolted the state from unseasonal heat to downpours in a matter of days.

The storm, spawned by a low pressure system off the Pacific north-west, delivered deluges across the San Francisco Bay Area as it made landfall on Monday night, prompting the National Weather Service to issue flood advisories and watches through large parts of central and northern California. The storm is forecast to soak the southern part of the state by Tuesday evening, although it will soften as it moves down the coast. Forecasters said California will experience unsettled weather through the week.

“A strong Pacific storm with a stream of high moisture will continue gusty to high winds over most of the western US, heavy to excessive rain along the coast, and mountain snow in the Sierras and Rockies today into Wednesday,” the National Weather Service reported, noting that the weather system could dump up to 5in of rain across parts of California and roughly 3ft of snow on mountain ranges. The agency also warned of the possibility of flash flooding, particularly in areas where wildfires have left “burn scars” that are vulnerable to floods due to a lack of vegetation.

By Tuesday morning, heavy rainfall flooded roadways and strong winds tore down trees and branches in the affected region, adding new travel hazards for commuters and those departing from holiday celebrations. Downtown San Francisco had received more than an inch (2.5cm) of rain and Mount Tamalpais in the northern Bay Area had more than 4in (10cm) of rainfall before dawn. Low-lying locations including rivers, creeks and streams could flood, according to forecasts, including in urban areas.

Winter storm warnings were issued for the Sierra Nevada, where motorists were advised that the combination of winds and snow could make travel hazardous. The greater Lake Tahoe area and Mono county were warned to expect heavy snow with wind gusts around 50 mph (80 km/h), and up to 100 mph (161 km/h) along Sierra ridgetops. Lake Tahoe was expected to have waves that could capsize small vessels.

As the storm surges southward, temperatures are expected to drop by 15F across the central coast and into southern California. “There will likely be 12 hours of steady rain,” the National Weather Service in Los Angeles/Oxnard said in a forecast discussion issued on Tuesday, adding that the downpour was expected to reach Los Angeles county by Tuesday evening.

California’s stint of wintery weather comes as other areas of the country emerge from a deadly arctic blast that caused temperatures to plummet over the holiday weekend. White-out conditions and frigid snow squalls wreaked havoc on several states and parts of Canada, as the enormous system stretched across the Great Lakes to the Rio Grande.

In the west, the wet weather is expected to do more good than harm. The rain and snow will provide a welcome reprieve for drought-stricken areas and soften the blow from drier days expected ahead.

“There will be quite a lot of water in the Sierra snowpack and soon in [northern California] soil/rivers/lakes streams from the upcoming storm sequence,” climate scientist Daniel Swain said in a weather discussion posted online, noting that even if the coming months are drier than usual, December’s storms provided a “substantial buffer”.

The Associated Press contributed reporting



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