An atmospheric river is poised to funnel gigantic amounts of rain and snow to the Northwest over the next few days.

As much as 15 inches of rain is forecast in the mountains along with several inches in coastal areas, including Portland and Seattle. It could be Seattle's wettest weather since February, the National Weather Service said.

There is also a risk of flash flooding in western Washington and northwestern Oregon on Thursday as a result of the heavy rainfall, the weather service warned. 

Snowfall will expand throughout the Washington and Oregon Cascades and parts of the northern Rockies on Thursday — with some light snowfall in higher elevations of northern California. By Friday, snow will continue across the Cascades and throughout the northern Rockies. 

On the very tops of the Cascade mountains, a whopping 9 feet of snow could fall, the weather service said.

Some of the rain will make it down to fire-ravaged California in the next few days. However, while a few periods of rain are in store for northern California late Thursday through Friday, a widespread, fire-quenching rainfall is not expected, AccuWeather said.

Another one of these rivers in the sky could funnel rain into Oregon by the weekend.

Atmospheric rivers

Made visible by clouds, the ribbons of water vapor known as atmospheric rivers can extend thousands of miles from the tropics to the western USA. They provide the fuel for the massive rainstorms and subsequent floods along the U.S. West Coast.

The one that's fueling the storms this week stretches some 5,000 miles, all the way from Asia.

Though beneficial for water supplies in the western USA, these events can wreak havoc on travel, bring deadly mudslides and cause catastrophic damage to life and property, the National Oceanic and Atmospheric Administration said.

One well-known nickname for an atmospheric river is the "Pineapple Express," which occurs when the source of the moisture is near Hawaii. A single strong atmospheric river can transport up to 15 times the water vapor compared with the average flow of water at the mouth of the Mississippi River, according to NOAA's Earth System Research Laboratory.

Last winter, an onslaught of atmospheric rivers knocked out the five-year drought in Northern California. Much of the Sierra Nevada saw its rainiest and snowiest October-February period on record, the weather service said.

Warmer, drier next week

The cooler, damper weather will likely be short-lived. Another warm, dry pattern is forecast for next week in the West, especially in southern California. 

"A strong upper ridge building across the southwestern U.S. should bring some record warmth to the region early next week," the weather service said. "Current forecast highs above the 100 degree mark over Southern California may break some daily records." Hot, dry, breezy conditions could spur on a fire weather risk during the period.

High temperatures are expected to soar as much as 30 degrees above average in Southern California, with the worst heat arriving Tuesday.

Almost the entire state of California will see highs in the 80s and 90s by early next week. 

“Record heat is forecast for early next week,” the Los Angeles office of the weather service said.

https://www.usatoday.com/story/weather/2017/10/18/5-000-mile-long-river-sky-deliver-heavy-rain-feet-snow-northwest/776060001/