Timeline: Atmospheric river bringing weekend downpours for Portland area

PORTLAND, Ore. (KOIN) – Another round of heavy rainfall brought in by a strong atmospheric river is moving into the Pacific Northwest Friday evening -- and the rain will last through the weekend.

Saturday will be the wettest and wildest weather day, offering a range of disturbances and plenty of inconveniences for travelers this weekend.

Here's what you need to know for the storm's timeline.

Friday afternoon and evening: Atmospheric river arrives

Rain will begin to spread around Northwest Oregon and Southwest Washington around and after 3 p.m. on Friday thanks to a warm front with the atmospheric river. Rainfall will get heavy starting overnight into Saturday, which is when the stormiest day starts.

Saturday morning is the soggiest & windiest

A high wind warning is in effect starting from midnight until 3 p.m. on Saturday for Oregon and Washington beaches and headlands, with damaging winds expected, and projected rainfall totals put Portland at 1+ inches of rain in just an eight-hour time period. That means when you wake up Saturday morning you may find localized ponding on your neighborhood streets and freeways. It's during this time you can expect gusty winds of 20 to 30 mph in the Willamette Valley.

Afternoon offers the best chance for thunderstorms

As a cold front arrives Saturday midday, a couple of things will happen. The main plume of moisture for Portland will advance to the south/southeast towards the Cascades and Eastern Oregon. But, it's in this postfrontal environment that all the action kicks off. There will be additional instability with colder air aloft. With enough moisture remaining we have a chance to encounter thunderstorms anywhere from the coast to the valley Saturday afternoon.

Meanwhile, keep your eyes to the sky as you may spot a high-based funnel cloud. Fall and spring are the seasons in which we have the best chance to see cold core funnels.

Calms down by Saturday evening, Sunday sees more rain

Rain tapers off Saturday night but then get ready for the third surge of moisture. Another atmospheric river is lined up for the Pacific Northwest Sunday, but rainfall amounts will be considerably less. For Portland, we'll have hit-or-miss rain showers, likely less than 0.50" from Saturday night to Sunday afternoon. There will be some dry breaks Sunday, but more rain is expected Sunday night through Monday.

Moral of the story is - it's rainy season and you best have that waterproof jacket ready to go.