Atmospheric river hits north of PNW, rain possible later this week

PORTLAND, Ore. (KOIN) -- Fall is here and so, too, is the potential for the West Coast to see atmospheric rivers return to the forecast.

The Pacific Northwest will miss out on the opportunity to see atmospheric river rains this week, but that doesn't mean an atmospheric river isn't present over the Pacific Ocean.

Parts of Alaska and British Columbia are getting hit hard with atmospheric river rains Monday. An atmospheric river is defined by the NOAA as, "relatively long, narrow regions in the atmosphere – like rivers in the sky – that transport most of the water vapor outside of the tropics. While atmospheric rivers can vary greatly in size and strength, the average atmospheric river carries an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River."

This relatively strong atmospheric river for the first week of fall will start moving south later this week. But as it moves into the Pacific Northwest Wednesday it will weaken significantly. Rain chances are expected to ramp up around western Oregon and southwest Washington Wednesday afternoon, but only about a tenth to a quarter-inch of rain is expected.

Despite no super soaking rains this week, the atmospheric river potential for the Pacific Northwest does begin to increase during the autumn months.

Stay with the KOIN 6 Weather Team as weather whiplash takes Portland through a nearly 20-degree temperature swing this week.