1 of 2 | Flooding during winter months is nothing new in the Pacific Northwest, and forecasters warn more could be in store this week, affecting travel (such as that experienced by motorists in flooded areas of Washington in 2010). An atmospheric river of rain will continue to affect western Washington and Oregon into Wednesday, forecasters say. File Photo by Kevin Dietsch/UPI | License Photo
As a firehose of rain, known as an atmospheric river, continues in western Washington and Oregon into Wednesday, small streams and rivers will become raging torrents and pose significant threats to lives and property. The atmospheric river has already turned deadly as a man was swept away along Johnson Creek near Portland, Oregon, Monday.
AccuWeather meteorologists reserve the term atmospheric river for the most extreme rain and snow events when a long plume of moisture is directed from the ocean to the land, and there is a risk of major travel disruptions or much more serious conditions.
|This image taken on Tuesday, Dec. 5, 2023, shows a plume of moisture, known as an atmospheric river, extending from hundreds of miles at sea over the Pacific Ocean to the northwestern United States. (AccuWeather Enhaced RealView&trade Satellite)
"While atmospheric rivers can vary greatly in size and strength, the average atmospheric river carries an amount of moisture roughly equivalent to the average flow of water of the Mississippi River in southeastern Louisiana," according to the National Weather Service (NWS). Exceptionally strong atmospheric rivers can transport up to 15 times the average flow of the Mississippi River and extend for thousands of miles.
In the case of the atmospheric river event into Wednesday, the rain may pour down at the rate of 1-2 inches per hour. An average rainstorm may only bring that amount of rain throughout its duration. Along with the copious amounts of rain, much warmer air is accompanying the atmosphere river, which will lead to quick-melting snow at high elevations in the Olympics and Cascades.
Storms from late last week deposited from 1 to 4 feet of snow on intermediate elevations, which is being wiped out in a matter of hours by the atmospheric river.
The combination of liquifying snow at high elevations with torrential rain will have small streams and rivers that flow out of the Cascades, Olympics and Coast Ranges in Washington and northern Oregon surging rapidly into Wednesday. Due to the rapid rise of these streams, dangerous and life-threatening conditions may evolve.
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During the 24- to 48-hour period ending Wednesday, more than a month's worth of rain will pour down. In some cases, that means that 4-8 inches of rain will fall, enough to lead to street and highway flooding in low-lying and poor drainage areas. But, when combined with rapidly melting snow from the intermediate and high elevations that have occurred and will continue, the release of water may be the equivalent of 12 inches or more of rain falling. There is an AccuWeather Local StormMax® of 18 inches of rain for this event which is most likely to occur along the west-facing slopes of the Cascades, Coastal Ranges and Olympics.
The sudden rainfall will cause the ground to become saturated despite prior drought conditions in the western portions of Washington and Oregon. The land may give way in some cases, leading to rockslides, debris flows and road washouts.
Travel along Interstate 5 in much of Washington and Oregon, including in the Seattle and Portland metro areas, will be a slow go, with torrential rain at times that will lead to poor visibility, excess water on the roads and the risk of vehicles hydroplaning.
Motorists dodging road closures due to rain and warm conditions will likely encounter poor visibility due to snow and fog with slippery and snow-covered highways in higher elevations as the week progresses in the Northwest.
Rain will fall over most of the passes into Wednesday evening. However, after that, enough moisture will continue to stream in from the Pacific as colder air arrives to bring several inches of snow from Wednesday night to Thursday night in the Cascades.
"From Wednesday to Thursday, the atmosphere river will break down into a more typical pattern with rounds of rain as it shifts southward into southern Oregon and Northern California, AccuWeather Storm Warning Meteorologist Joseph Bauer said.
The heaviest and steadiest rain will stay to the north of San Francisco, but the City by the Bay will get some showers for a time at midweek.
"It appears that the air will remain warm enough for just rain to fall on the high ground along Interstate 5 in southern Oregon and Northern California from Wednesday to Thursday," AccuWeather Senior Meteorologist Heather Zehr said, adding, "But it will turn cold enough for rain to change to snow and bring some snow and slippery conditions to Donner Pass, California, along I-80 from Wednesday night to Thursday night."
As the jet stream dips and pushes inland, it will create more of a storm across the interior West from Wednesday night to Friday, where it will become cold enough for mostly snow to fall from parts of southern Idaho and northern Nevada to portions of Utah, Wyoming, Colorado and northern New Mexico.
Building warmth over the Rockies into midweek will depart as the storm gets underway for the latter half of the week.
Salt Lake City will experience rain showers from late Wednesday night to Thursday. However, as the storm forms and creates colder air, periods of snow are likely from late Thursday night to Friday, with a slushy coating to an inch or two possible.
In the higher elevations of the Wasatch Range and the central Rockies, snow will ramp up from Friday to Friday night with several inches or more likely that should have skiers jumping for joy.
Motorists and airline passengers around Denver may experience delays due to slippery roads and deicing operations as temperatures crash from near 40 degrees Fahrenheit at midday through the 30s and 20s Friday night.
The system over the interior West will evolve into a major storm with severe weather, heavy rain, a band of heavy snow and severe thunderstorms as it swings out over the Central states this weekend.
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