Not one, but two atmospheric rivers — narrow plumes of robust tropical moisture — will deluge the West Coast over the next seven days, bringing a slew of hazardous impacts and disrupting travel. Heavy flooding rain, landslides, significant mountain snow and strong winds are all in the offing, coming in multiple waves during both the workweek and the weekend.
Wind advisories span from the Willamette Valley of Oregon to near Monterey, Calif., representing where gusts of 45 mph or greater are expected. Within that zone are several high wind watches and warnings for winds closer to 60 mph.
Much of the same region is also blanketed by flood watches. They cover the San Joaquin Valley and coastline for the northern two-thirds of California. A general 3 to 6 inches of rain is expected — lesser in the lowlands, and more in the higher elevations — which will cause some pockets of flooding, as well as shallow landslides.
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“There will be an increased risk of rock and land slides along roadways,” warned the National Weather Service. “Creeks and streams may rise out of their banks.”
All that moisture will come down as snow in the Sierra Nevada, with snow levels dropping from 7,500 feet to 5,000 feet or below as cold air filters into the region.
“Travel could be very difficult to impossible,” the Weather Service said, noting winds could gust up to 100 mph atop the Sierra Ridge.
The worst weather comes Wednesday and Thursday, but a second atmospheric river will hit Sunday into Monday. That could double precipitation totals and leave the ground even more saturated.
On the plus side, relentless rains last winter and this year have helped to entirely eradicate California’s drought. A year ago, 99.36 percent of the state was facing at least some tier of drought, according to the U.S. Drought Monitor. Now, only 3.45 percent of California is “abnormally dry” — the lowest category of drought. The remainder is drought-free.
Atmospheric river #1
The first of two atmospheric rivers will begin lapping at Oregon and Washington tonight, and then work down the coast Wednesday into Thursday. In any given place, the main effects of the atmospheric river will last 12 to 18 hours.
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- Timing: Southern Oregon will see its impacts late Tuesday night into Wednesday. For Northern California, it’s a Wednesday event; the atmospheric river will focus on the Bay Area from Wednesday evening through dawn Thursday. Then it brushes through central and Southern California during the remainder of Thursday.
- Rainfall totals: Totals 3 to 6 inches in southwest Oregon, and 4 to 8 inches in the Coastal Range of Northern California. A general 3 to 5 inches for the North Bay, and 3 inches give or take around the Bay Area. Two to 3 inches for the remainder of the coastline 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 see 2 to 3 inches, with 1 to 2 inches farther south across the Central Valley.
- Snowfall amounts: For the Sierra Nevada, 12 to 24 inches above 4,000 feet, with 3 feet or greater above 7,000 feet. Snow into the Great Basin of Nevada, too, likely to total several inches.
- Winds: At the coastline, 60 mph for central and southern Oregon into Northern California, up to 55 mph south toward the Bay Area and then 45 mph south to San Diego. Inland, gusts of 45 mph possible for much of the Central Valley.
- Thunderstorms: A few thunderstorms are possible as cold air moves overhead the second half of Thursday. Thunderstorms would have the potential to produce torrential downpours, pea-sized hail and an isolated funnel cloud.
Atmospheric river #2
Another low pressure system over the northeast Pacific will drag a secondary strip of tropical moisture toward the coastline with most impacts Sunday into Monday. Meteorologists refer to the overarching setup as the “Pineapple Express,” since the moisture often originates from the central Pacific near Hawaii.
Details on the next atmospheric river event are still hazy (after all, we’re still six days out), but there are a few things we know:
- Preliminarily, this looks like a California-focused event. The fetch, or aim, of the atmospheric river may favor greater impacts in California versus the Pacific Northwest.
- Rainfall amounts may be greater. This is especially true in southern zones.
- Wind gusts of 45 to 55 mph will once again be possible at the coastline.
- Three to 5 feet of snow will be possible in the Sierra Nevada.
We’ll continue to refine forecasts as more information becomes available in the days ahead.
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