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Powerful atmospheric river pummels Northwest with over 8 inches of rain as rivers begin to rise | CNN

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Over 10 million people are under flood alerts across the Pacific Northwest Tuesday as a strong atmospheric river pelts the region already soaked by previous atmospheric rivers with more heavy rainfall.

More than 8 inches of rain has already fallen in the region over the last 24 hours and rivers are starting to rise into flood stage, with multiple rivers forecast to reach major flood levels over the next 24 hours.

Additional heavy rainfall Tuesday will only make matters worse in parts of Oregon’s Cascade mountains, southern Washington and far northwest California. A slight risk, or Level 2 of 4, of excessive rainfall is in place Tuesday, according to the Weather Prediction Center.

Coastal stretches of southern Washington and Oregon are expected to bear the brunt of this heavy rain threat, the National Weather Service said in an advisory.

The flood watches stretch from coastal Oregon and Washington into northeastern Washington and northern Idaho. While most are set to expire Wednesday, a few will last into Thursday.

Washington’s Olympic mountains could receive over a foot of rain, and the Cascades could see between 5 to 9 inches, the weather service said. Along the coast, residents can expect 3 to 5 inches and 1 to 3 inches in the interior lowlands.

Parts of the region had received ample rainfall from Saturday’s atmospheric river, including 9.07 inches in Red Mound, Oregon, and 8.5 inches in Gasquet, California.

Monday and Tuesday’s atmospheric river is also registering impressive totals: Olympic National Park in Washington recorded 8.67 inches in the last 24 hours with more rain to come.

cnnweather

Additional forecast rainfall as of Tuesday morning.

The sequence of back-to-back atmospheric rivers, called an AR family, began Saturday and did not leave much of a break period before Monday night’s system began. This lack of recovery time will lead to an increased risk for flooding.

“In scenarios where we see multiple atmospheric rivers move onshore in the form of a family, hydrologic impacts tend to be exacerbated due to the lack of time for rivers and soils to recede back to baseline,” said Chad Hecht, research and operations meteorologist at the Center for Western Weather and Water Extremes at UC San Diego.

For residents caught in flood conditions, the weather service recommends staying indoors or seeking higher ground if shelter isn’t available.

“If you’re stuck outside when a flash flood occurs, do not attempt to cross flood waters by vehicle or on foot,” the agency advised in a social media post.

By Thursday and Friday, storm activity is forecast to ramp down across much of the Northwest, but showers and some high-elevation snow remain possible.

But the break from soggy weather may be short-lived. Forecast computer models are showing some early signs of yet another atmospheric river hitting the region by the early weekend.

The exact strength and overall impact of this potential event should become clearer once this week’s atmospheric river comes to an end on Wednesday.

A Level 4 out of 5 atmospheric river event is forecast to hit roughly the entire coastline of Oregon this week but the event will be more severe in the far northwestern part of the state with a Level 5 out of 5 expected.

Not all atmospheric river events are bad. In fact, AR event levels 1 and 2 are considered mostly beneficial rains and are much needed across the western US to build water supply levels. But AR event levels 4 and 5 are more hazardous than they are beneficial as the risk for flooding and travel dangers outweigh the benefits.

“Given enhanced rainfall during the weekend, the soils will be more vulnerable and thus the potential for greater run-off and flooding,” the Weather Prediction Center said earlier this week. “Snow levels are initially expected to be rather high, and therefore western facing slopes of the Cascades may be subject to an additional risk of flooding due to snow melt.”

Parts of Utah and Colorado received snowfall on Monday, bringing total snowfall throughout the atmospheric river events to several feet in some places, including 50 inches in Collins, Utah, and 49 inches in Colorado’s Rabbit Ears Pass.

But this latest round of moisture also ushered in warmer temperatures, including a high temperature of 65 degrees in Portland, which ties with the city’s all-time high for the month of December last set in 1993.

These warm conditions have encouraged snowmelt across the Pacific Northwest and may lead to excessive runoff and rising creeks and streams.

Some rivers in western Washington are forecast to reach moderate and major flood stage this week. The Skagit and Snoqualmie rivers are of particular concern, with both forecast to flood roads, farms and even some residential areas. Water levels on portions of the Skokomish River in western Washington climbed to moderate flood stage Tuesday morning.

Heavy downpours also increase the risk of landslides or debris flows in areas burned by wildfires because the ground is less able to absorb the moisture effectively.

CNN’s Allison Chinchar and Sara Tonks contributed to this report.





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