A stream of tropical moisture, originating from near Hawaii, will direct a firehose of heavy rain with heavy mountain snow at the northwestern United States for a time later this week.
An atmospheric river, is expected to extend from the tropical Pacific to the coast of the northwestern U.S.
The plume of moist air will originate from near Hawaii, nearly 2,600 miles away. Meteorologists refer to this type of atmospheric river, which extends all the way to Hawaii, as a "Pineapple Express."
Forecasters caution that even though this system will be fairly short-lived, it could still be disruptive enough to produce episodes of flooding and mudslides and travel-snarling snow over the mountains.
"Even though below-average rainfall has occurred in the Northwest this autumn, enough rain can occur with this setup to lead to isolated flash flooding and mudslides, especially along the western slopes of the Coast Ranges and Cascades from later Thursday to Saturday," AccuWeather Senior Meteorologist Brett Anderson said.
Rainfall over these low-elevation ocean-facing slopes is likely to average 3-6 inches with an AccuWeather Local StormMax™ of 12 inches in parts of western Oregon.
Seattle, Washington, and Portland, Oregon, can expect a thorough soaking from the storm with travel delays associated with urban flooding, fog and a low cloud ceiling during Thursday and Friday.
"One limiting factor to the rain on this particular setup will cause the firehose of rain to gradually shift southward from Thursday to Saturday, rather than linger in the same spot along the coast for days," Anderson stated. "The steady movement of the atmospheric river may be enough to avoid major stream and river flooding."
However, minor flooding of small streams and short-run rivers flowing out of the Cascades are likely.
With cold air in place into Thursday, heavy snow is forecast for the Cascades. Slippery conditions with snow-covered roads are in store for Snoqualmie, Stevens and White passes in Washington. Milder air and less intense precipitation falling in the form of rain or a rain and snow mix may allow travel conditions to improve by Friday in at least Snoqualmie Pass and probably Stevens Pass. Conditions may remain wintry over White Pass, which is the highest of the three passes.
Snowfall over the Olympics and Cascades above 4,000 feet will top 1 foot and could reach between 3 and 5 feet over the high country by Friday afternoon.
By Saturday, the atmospheric river is likely to shift southward from Oregon to part of Northern California. During the weekend, as the plume of moisture continues to drift southward, it is likely to weaken.
Even with the moving and weakening scenario, enough rain can fall as far south as San Francisco and Sacramento, California, by Sunday to cause travel disruptions ranging from urban flooding to low cloud ceilings, just like that of Seattle late this week.
Rainfall from the Bay Area to the Sacramento Valley is likely to average 0.50 of an inch to 1 inch with locally higher amounts on the lower ocean-facing slopes of the Coast Ranges and the western-facing slopes of the Sierra Nevada.
The Siskiyou Summit along Interstate 5 may be spared snowfall this weekend. However, snow or a mixture of snow and rain may occur over Donner Summit late Sunday and Sunday night.
In this photo taken Tuesday, Nov. 26, 2019, provided by Caltrans, are cars and trucks in stopped traffic on Interstate 5 near Dunsmuir, Calif. Drivers on Interstate 5 near the Oregon-California border spent 17 hours or more in stopped traffic as blizzard conditions whirled outside. Some slept in their vehicles. (Caltrans via AP)
From Sunday to Monday, spotty showers are forecast to sweep into Southern California. Snow is not expected to reach pass level or hinder travel across the southern part of the Golden State.
Some rain showers are possible in Los Angeles and San Diego late this weekend to early next week.
During early next week, a weakened version of the tropical moisture and a storm system can combine to send rain and mountain snow inland over the interior West just prior to Christmas.
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