An atmospheric river is bombarding the Pacific Northwest, bringing heavy rain to the valleys, snow to the mountains and strong winds.

Wind advisories for gusts up to 45 mph and flood watches are up in Seattle and Tacoma into Friday night and Saturday, and Portland is expecting strong winds too.

The Center for Western Weather and Water Extremes is classifying the episode as a level 4 out of 5 on its 1 to 5 atmospheric river scale.

Atmospheric rivers are streams of moisture that are typically thickest at the mid-levels of the atmosphere, often stretching thousands of miles from the deep tropics to the mid-latitudes.

In this case, some of the humidity can be traced all the way back some 8,000 miles to near Guam and the Northern Mariana Islands.

The corridor of moisture is being swirled in between a high pressure system between Hawaii and the West Coast and a low in the Gulf of Alaska.

Like interlocking gears, the pair of weather systems are able to entrain a filament of moisture that will aim directly at the Pacific Northwest.

Seattle was already in the waterlogged westerly flow as of early Friday, and will be until dry air to the north abruptly arrives overnight into early Saturday.