The Pacific Northwest and Northern California, especially Washington and Oregon, have had an onslaught of storms and the run of them is expected to continue over the next week.
These systems, known as atmospheric rivers, are predicted to be quick-hitting and weak to moderate in severity and unlikely to cause extreme weather on an individual basis.
Together, though, they will have an impact, heightening the risk of holiday travel disruptions.
“Because they’re piled up over time, we’re looking at a significant amount of rainfall over the next week,” Frank Pereira, a meteorologist with the Weather Prediction Center, said. “River flooding will become a growing concern as these systems continue to impact the area.”
Areas that could get some of the highest rainfall totals include the Olympic Peninsula in Washington and parts of far Northern California where six to 14 inches of rain is possible over the next seven days, according to the Center for Western Weather and Water Extremes.
Rivers are expected to swell and numerous ones in Washington and Oregon are at risk of flooding.
In the most populated areas, the projected totals are less impressive. Seattle is estimated to record about 3.4 inches of rain; Portland, Ore., up to 3.5 inches; and San Francisco around 1.5 inches, from Saturday to Friday, according to the National Weather Service.
While much of the West Coast gets soaked, Southern California is not expected to get any significant rainfall in the next seven days. There’s a chance of light rain in Los Angeles on Christmas Eve.
“Anything would be pretty light, probably under a tenth of an inch or so,” said Bryan Lewis, a meteorologist with the Los Angeles office of the Weather Service.
Atmospheric rivers are streams of moisture in the sky
Atmospheric rivers are narrow and elongated streams of water vapor transported by powerful winds in the sky.
These systems are known for releasing rain and snow when they hit land, especially in mountain ranges that squeeze out the precipitation.
Over the next week, the western side of several mountainous areas could get more than 10 inches of rain, including the Olympic Mountains in Washington, the Cascades and the coastal mountains of southwestern Oregon and northwestern California.
Common in winter, atmospheric rivers tend to be warmer and generally bring higher snow levels at 8,000 feet and above. This will be the case for most of the upcoming storms, but snow is more likely to occur even lower, at 6,000 feet, when temperatures drop at some point next week, affecting passes and potentially road travel in the Cascade Range and the Sierra Nevada.
Anyone who will be getting on the road on Christmas Eve or Day should check conditions before leaving.
These storms are known for stalling and unleashing dangerous amounts of rain over a focused area. That happened in Northern California in November. Some locations in the region just north of San Francisco recorded 20 inches of rain.
As of Saturday morning, the forecast indicated that none of the upcoming storms were predicted to slow down, said Julie Kalansky, deputy director at the Center for Western Weather and Water Extremes at Scripps Institution of Oceanography at University of California, San Diego.
The storms are not expected to sit in one place and release torrents of rain but these quick-hitting storms are coming in one after the next, with some brief breaks.
“The rapid sequence of them, the combined effect, could cause flooding-related hazards for some areas,” Dr. Kalansky said.
More storms are on the way
Atmospheric rivers brought rain to the West Coast on Friday and Saturday. Another system is expected on Sunday and another on Monday, with the Center for Western Weather and Water Extremes forecasting continued storm activity through late next week, especially across the Pacific Northwest.
“We’re getting storm after storm, but all told, we’re getting a break in between them,” said Jeff Michalski, a meteorologist with the Weather Service in Seattle.
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