An atmospheric river called the Pineapple Express is bringing heavy rain and warm air to the Pacific Northwest. The combination will melt snow and the NWS cautions lead to river flooding. How long will the rain continue?

SEATTLE – fears in the Pacific are growing as rivers and streams in and start to rise thanks to a series of storms that are delivering heavy rain to the region, which is now melting mountain and sending water into low-lying areas.

It was a wet and dangerous weekend across the region with snow falling in the Cascades, including more than a foot at Mr. Rainier’s Paradise Ranger Station. 

A car spins out on Interstate 90 in central Washington on Dec. 1, 2023.

(Washington State Patrol / FOX Weather)

The snow also led to several crashes that forced officials to close a stretch of Interstate 90 through Snoqualmie Pass near on Friday.

One person was killed in a crash in Stevens County in northeastern Washington after the driver lost control on a snow-covered road and hit a tree, according to the Washington State Patrol.

All aboard the Pineapple Express

A three-hour radar loop showing where rain (green) and snow (blue) are ongoing. Severe Thunderstorm Warnings are indicated in yellow. Tornado Warnings are indicated in red, while Tornado Warnings with a confirmed tornado are indicated in purple. Flash Flood Warnings are indicated in green, while Flash Flood Emergencies are indicated in pink.
(FOX Weather)

 

An atmospheric river called the slammed into the Pacific Northwest Monday on the heels of the atmospheric river that impacted the region over the weekend.

This is a warm flow of moisture that connects tropical moisture from Hawaii to the Pacific Coast. The river of moisture can hold up to three Mississippi Rivers worth of water vapor. Coming from the tropics, the air is mild and the heavy rain and warmer temperatures will start to melt all that new snow. Tuesday, a handful of cities are even expecting record highs.

Seattle drivers had to deal with inches of water on I-5 Monday as heavy rain fell.

(@marushki_ via Storyful / FOX Weather)

That stream of moisture will weaken slightly on Tuesday, but the most impactful part of the storm – river flooding – will just be getting started.

Moderate to major flooding expected

Rain forecast.
(FOX Weather)

 

The FOX Forecast Center said the warm air surging off the Pacific will raise snow levels above 9,000 feet, which is higher than they were over the weekend.

“It started around 3,000 feet, and then the snow elevation went up to 6,000 feet,” FOX Weather Meteorologist said. “The problem was that between 3,000 and 6,000 feet, we still have a conveyor belt of rain. So, what we had basically was rain on top of all that snow. All of that snow starts melting. It leads to runoff. And that’s what led to a big flood concern, especially in the western Cascade Mountains.”

The FOX Forecast Center said the combination of an additional 3-12 inches of rain and the melting of the 2-4 feet of snow on the ground in the Cascade and Olympic mountain ranges will send numerous rivers into flood stage, with some major flooding possible.

Flood alerts in effect through Wednesday, Dec. 6, 2023.
(FOX Weather)

 

Because of the flooding threat, Flood Watches have been posted in the Pacific Northwest from western Washington through western Oregon. This includes major cities in the region like Seattle and in Washington and , , and in Oregon.

said excessive rainfall is likely Monday and Tuesday, with 7-10 inches possible over the Olympics, 5-9 inches over the Cascades, 3-5 inches along the coast and up to 3 inches in the interior lowlands.

Because of this, excessive runoff may result in the flooding of rivers, streams and other low-lying and flood-prone areas.

There’s also a risk of urban and poor-drainage flooding with low-water crossings that could be flooded, as well as extensive street flooding and the flooding of rivers and creeks.

The Snoqualmie River hydrograph at Carnation, Washington.
(FOX Weather)

 

The FOX Forecast Center said water levels will begin rising significantly on Tuesday and could crest sometime between Tuesday night and Wednesday night.

Moderate to major flooding is expected along the Skagit, Snohomish, Snoqualmie and Skokomish rivers. Communities along these rivers, like , Washington, will need to be on alert as numerous roads and structures could be flooded.

Flooding impacts from the Snoqualmie River at Carnation, Washington.
(FOX Weather)

 

The is expected to crest on Wednesday at 58.2 feet. That is below the record crest of 62.21 feet and is just over major flood stage.

At 51 feet, the river spills over its banks, and shallow flooding occurs. At 56 feet, widespread flooding from through occurs.

At 60 feet, swift floodwaters causes widespread inundation.

Flooding likely.
(FOX Weather)

 

The flood alerts in effect in the Northwest will continue through at least Wednesday night.

However, the flash-flood risk will continue through at least Thursday morning, though it will be focused farther south in Oregon and parts of Northern .