PORTLAND, Ore. (KOIN) -- A weak atmospheric river will likely impact the end of September for much of western Oregon and southwest Washington.

Fall is officially here and the first major autumn storm of the season is starting to take shape. A weak atmospheric river will likely bring cooler temperatures and more frequent rain to the Portland area by the end of the weekend.

An atmospheric river is a stream of moisture extending from Hawaii to the West Coast. These soaking rains bring the majority of the year's precipitation to the West Coast. The greatest chance of seeing an atmospheric river take shape over the Pacific Ocean is from fall to spring. This first round of rain for the autumn season is expected to impact the greater Portland area by Sunday.

Clouds will increase by the end of

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