Atmospheric rivers will hit the West Coast this week

Not one, but two atmospheric rivers — narrow plumes of strong tropical moisture — will flood the West Coast over the next seven days, bringing dangerous impacts and disrupting travel. Heavy flooding rains, landslides, significant mountain snow and strong winds all come in waves over the work week and weekend.

Wind advisories from Oregon's Willamette Valley near Monterey, California indicate wind gusts of 45 mph or greater. There are several high wind watches and warnings for wind gusts up to 60 mph within that zone.

Much of the same area is covered by flood watches. They cover the northern two-thirds of California's San Joaquin Valley and coast. A typical 3 to 6 inches of rain is expected — less in low-lying areas and more in higher elevations — which could cause flooding and shallow landslides in some pockets.

“There will be an increased risk of rockslides and landslides on roads,” the National Weather Service warned. “Creeks and streams may rise from their banks.”

All that moisture will be reduced Snow In the Sierra Nevada, snow levels drop from 7,500 feet to 5,000 feet or below as cold air filters.

The weather service said winds of up to 100 mph were possible in the Sierra Ridge: “Travel will be extremely difficult and impossible”.

The worst weather will arrive Wednesday and Thursday, but a second atmospheric river will hit Sunday through Monday. That can double rainfall totals and make the land even more saturated.

Unrelenting rains last winter and this year helped eliminate California's drought completely. A year ago, 99.36 percent of the state was experiencing at least some level of drought, according to the U.S. Drought Monitor. Right now, only 3.45 percent of California is “abnormally dry” — the lowest category of drought. The rest are dry.

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The first of two atmospheric rivers will begin to make landfall in Oregon and Washington tonight, then work their way up the coast Wednesday through Thursday. At any given location, the main effects of an atmospheric river last from 12 to 18 hours.

  • Time: Southern Oregon will see impacts Tuesday night into Wednesday. For Northern California, it's a Wednesday event; A storm surge will focus over the Bay Area Wednesday evening through early Thursday morning. Then it brushes through central and southern California for the rest of Thursday.
  • Rainfall: Totals of 3 to 6 inches in southwestern Oregon and 4 to 8 inches in coastal northern California. A general 3 to 5 inches for the north bay, and 3 inches to give or take around the bay area. Two to 3 inches for the rest of the coast up to the Mexican border. Below the snow line in the Sierra Nevada, 2 to 5 inches of rain will fall. The northern San Joaquin Valley should get 2 to 3 inches, with 1 to 2 inches throughout the central valley south.
  • Amount of snowfall: For the Sierra Nevada, 12 to 24 inches above 4,000 feet, 3 feet or more above 7,000 feet. Snow totals could be several inches over Nevada's Great Basin.
  • Air: Along the coast, central and southern Oregon is 60 mph to northern California, 55 mph south to the Bay Area, then 45 mph south to San Diego. Inland, gusts as high as 45 mph are possible for much of the Central Valley.
  • Thunderstorms: A few thunderstorms are possible during the second half of Thursday as cooler air moves in. Thunderstorms have the potential to produce pea-sized hail and an isolated funnel cloud.
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Another low pressure system in the northeast Pacific will pull a second patch of tropical moisture toward the coast with most of the impacts Sunday through Monday. Meteorologists refer to it as the “Pineapple Express” because the moisture mostly originates from the central Pacific region near Hawaii.

The details of the next Atmospheric River event are still hazy (after all, we're six days away), but there are a few things we do know:

  • Initially, this appears to be a California-centric event. The acquisition or purpose of the atmospheric river may have greater impacts in California versus the Pacific Northwest.
  • Rainfall may be high. This is especially true in the southern regions.
  • Winds of 45 to 55 mph are possible again along the coast.
  • Three to five feet of snow is possible in the Sierra Nevada.

We will refine the forecasts as more information becomes available in the coming days.

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