PORTLAND, Ore. (KOIN) — Summer ending rains return to Oregon and Washington Saturday as an atmospheric river continues to develop offshore.

The first full day of autumn will bring the return of cooler and wetter weather to much of the Pacific Northwest. Temperatures will fall nearly 10 degrees from Friday’s high of nearly 80 degrees in Portland.

A cool and rainy start to the fall season in Portland, Saturday, September 23, 2023

Rain will start to fall along the Willamette Valley by the afternoon hours. The heaviest of the showers aren’t expected until the evening. Those heavy rain showers will remain until about 10:00 p.m. A mostly dry night is expected for the area Saturday night into Sunday morning.

Rain accumulation will near a couple of hundredths of an inch by the end of the day Saturday. Heavier and more widespread rain will fall Saturday. That’s when the effects of the atmospheric river will intensify during the late afternoon hours.

Oregon and Washington’s first atmospheric river of the season begins to push more moisture into the area Sunday. That’s where nearly a quarter of an inch of rain is expected to accumulate by Sunday night. Even more rain is expected to fall during the first half of the week. That’s where some rainfall totals will near three inches for some communities.

KOIN 6 Meteorologist Josh Cozart shares Portland’s cooler and wetter weather pattern for the first full week of autumn

Portland’s thunderstorm potential starts to increase by the start of next week. That comes as temperatures remain below average through Friday.

Heavy rain over the next week is increasing the flooding potential for those along Oregon’s south coast. This is where widespread burn scars are located from recent wildfires.

Oregon’s south coast is under a ‘Flood Watch’ as heavy rain moves over large burn scar areas this weekend and next week

Flooding and landslides are a possibility for for the southwest corner of the state has heavy rain moves over the recently burned area.