SEATTLE -- Was this autumn's opening salvo, a month early?
The warm temperatures and tropical mugginess to the day Wednesday might suggest otherwise but at least for Seattle, Wednesday morning's rain (0.21" at Sea-Tac) will goes down as the wettest day in over two months, since 0.35" fell on June 25. Some spots in south King County got over 0.30-0.45", According to the CoCoRahs volunteer rain reporting network. There were even a few rumbles of thunder reported around the Kent Valley.
(Note, speaking of CoCoRahs, it suggests Wednesday was not the wettest day in two months north of Seattle -- a storm system on Aug. 2 brought about a quarter inch up there, but only 0.08" in Seattle.)
Wednesday will hold its distinction for wettest day of the past two months for... about 24 hours. A stronger system is due in Wednesday night and Thursday making for a very rainy day around the Puget Sound area. Forecast models now paint anywhere from 0.50" to 1.00" of rain in the greater Seattle area -- perhaps even enough to bring us above normal for August rainfall, the first time since July 2012 that we had a summer month with above normal rainfall. The exception to the rain will be an expected rain shadow around the northeastern corner of the Olympic Peninsula.
So is this really an early start to autumn? Probably not. Rain in late August and early September isn't too unheard of (the big Seattle Labor Day Festival is called "Bumbershoot" for a reason...) While just a few weeks ago we were at the pinnacle of the dry season, with rain expected about once every 9-10 years, by now, that frequency increases to about once every 3-4 years. (Don't ask about October and November.)
And while showers remain in the forecast at times through Labor Day Weekend, long range models do suggest some potential for another warm spell out there. Seattle Septembers typically do have what I call a "parting shot" of sunny and warm weather after a rainy spell that just when you think summer is done, we get one last burst. That may be coming sometime around Sept. 10-15.
It's raining: Yawn. But what's with this humidity again?
Seattle is not really known as a muggy place, no matter what the New York Times thinks but this August has been one of the muggiest months I can remember.
The dew point has reached 60 or greater on six dates this month -- including Wednesday. It's the most in summer since July 2009 and those high numbers were connected to the super heat wave of the time.
The predominant pattern this summer has been one of two things: A ridge of high pressure offshore and another one further inland that has keep the differences in pressure quite light across our region, and in turn kept our marine breezes that cool us off and keep dew points low very weak. That's also why it's been so sunny and warm.
The other pattern has been occasional upper areas of low pressure that spin offshore pulling in warm, moist air from the south and some rain and/or thunderstorms to add even more moisture to the mix for muggy conditions.
The latter pattern is expected to persist into early next week so muggy conditions will continue to be a factor for a while in addition to the scattered showers. But if we do get that "parting shot" of summer, that would be a much drier pattern.