From Sunday through Wednesday, heavy rainfall and snow are expected across the Sierra Nevada and Cascade mountain ranges, affecting much of the Western U.S.
Two separate storm systems-a prolonged "atmospheric river", followed by a cold, weaker storm-coming from the Pacific Northwest is expanding southward into California and eastward into the Northern Rockies, prompting the National Weather Service (NWS) to issue Winter Storm Warnings across areas of California, Nevada and Oregon.
SFGATE reported that the first system-a moisture-rich, slow-moving atmospheric river-began sweeping across the Cascades and the northern Sierra Nevada on Sunday morning, and is expected to bring heavy snowfall through Tuesday.
The NWS predicts that the heaviest snowfall will arrive Monday, causing major travel impacts and delays, and continue into Tuesday morning. "Lengthy travel delays are certain," the NWS said on Sunday morning. "There may be a brief break in snow Wednesday, but difficult travel can be expected for the next five days."
"Travel could be very difficult to impossible due to heavy snow and whiteout conditions," the service said. "Very strong winds could cause tree damage."
The NWS is discouraging travel, especially through mountain passes, due to the danger. It said whiteout conditions, road closures, downed trees and power outages are all likely. Even avalanches are a possibility across the Sierra.
While the effects of these two systems are spread over a large area, the NWS generally predicts that the storm will drop between one and three feet of snow in the Olympics, Cascades and Northern Rockies; three to five feet in the California ranges, including the Sierra Nevada; as much as six feet of snow at elevations of 5,000 feet in the Shasta area.
San Francisco's NBC Bay Area news reported that, not only are several feet of snow possible around Lake Tahoe and regions to the north, but winds could get up to 100 miles per hour over the mountain ridges.
California's valley areas will receive several inches of rainfall, creating the conditions for flashing flooding and scattered debris, especially near areas that have been recently burnt by fires.
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