Fall begins Tuesday, heavy rain close behind for Puget Sound region

The smoke has moved out of Western Washington and some showers have returned but summer 2020 is going to officially be over at 6:30 a.m. Tuesday. Close on the heels of the autumnal equinox will be some very fall-like weather, with gusty winds at times, heavy rain, and rising rivers.

Beginning the week mainly dry, we’ll have clouds at times along with some sun for Monday and Tuesday. Some patchy early-morning fog is possible both days. Smoke and poor air quality shouldn’t be an issue for us, though some spots in southern Washington and areas east of the Cascades will still deal with smoke.

Out in the Pacific, a bonafide atmospheric river will be developing, aimed at Washington and British Columbia for the end of the week. These atmospheric rivers are sometimes called a “Pineapple Express” but as the streams of copious atmospheric moisture don’t always originate near Hawaii, the preferred term for any one of these long, soggy fetches is “atmospheric river.”

You can really think of it as a “river of water” in the atmosphere. These atmospheric rivers are fast moving streams of very rich water vapor in the lowest few miles of the atmosphere. They’re sometimes thousands of miles long but often just a few hundred miles wide — long, but narrow like a river.

The “firehose” of moisture will bring heavy rain and some breezes

The leading edge of the atmospheric river looks to arrive around Wednesday for Western Washington. As the moisture slams into the Olympic and Cascade mountain ranges from the southwest, the mountains will force the moisture to rise, where the water vapor will condense into clouds and cause heavy rainfall.

In the mountains on Wednesday, two to four inches of rain is expected with even more possibly falling into Thursday and Friday, depending on the exact trajectory of the atmospheric river. This will cause rivers to rise, though I doubt we see any river flooding because all our rivers are low at the end of summer, especially so this September as it has been quite dry. The only exception: The Skokomish River in Mason County could approach flood stage during this event.

There will be the localized possibility of flooding near where large areas of land has burned, particularly in northern Oregon if the atmospheric river dumps rain that far south. The soil in these “burn scars” cannot absorb rainfall easily so runoff is quick and can cause flash flooding.

In the lowlands around Western Washington, including Puget Sound, we can expect a half-inch to more than an inch of rain Wednesday through Wednesday night with even more rain falling in showers through Friday. We could wind up the work week with more than two inches of rain in some of the wettest spots around the Sound. Great news for the trees and shrubs, some of which have been looking rather sad and droopy after our dry times.

More wet weather is headed our way this week…just in time for the autumnal equinox. First day of fall is Tuesday, Sept 22 at 6:30 AM PDT. 🍂 #wawx pic.twitter.com/vEMm9cRzG9

— NWS Seattle (@NWSSeattle) September 21, 2020

On Wednesday and Thursday, we’ll also likely see some breezy times with gales on some of our northern waters and around the San Juans. Elsewhere, some wind gusts of 20-30 miles per hour (maybe a bit stronger) are possible, but major wind impacts aren’t likely.

We should see the wet weather settling down next weekend with some periods of sunshine but still some showers around, particularly Saturday.

Temperatures around the Sound will still top out in the low-mid 60s during this mild, rainy event this week, so serious chilly weather isn’t upon us.

Nor is any mountain pass snow. We’re not there yet!

Written by KIRO 7 TV Chief Meteorologist Morgan Palmer.