Tonight through Tuesday, we’ll be mainly dry for most of the time but a few sprinkles are possible, especially early in the day and nearer the coast. Also patchy fog is likely in the morning along the water and in some river valleys. Morning lows will be in the 50s with highs generally in the 60s. By the way, fall begins at 6:30 Tuesday morning.
Out in the Pacific, a bonafide atmospheric river will be developing, aimed at Washington for Wednesday into Thursday. These atmospheric rivers are sometimes called a “Pineapple Express” but as the streams of copious atmospheric moisture don’t always originate near Hawaii (this one isn’t), the preferred term for any one of these long, soggy fetches is “atmospheric river.”
The leading edge of the atmospheric river looks to arrive early 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.
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 for a multi-day total of 2-3 inches, on average.
On Wednesday and Wednesday night, we’ll also likely see some breezy times with gales on some of our northern waters and around the San Juans as well as at the coast. Elsewhere, some wind gusts of 20-25 miles per hour (maybe a bit stronger near the water) 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 – neither is mountain snow.
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