‘Wet-tember’ arrives: expect a wet fall, followed by a ‘neutral’ winter | KNKX

‘Wet-tember’ arrives: expect a wet fall, followed by a ‘neutral’ winter

Sep 20, 2019

September is one of those months in the greater Puget Sound region that can be glorious, with sunny blue skies serving as a stunning backdrop for crisp autumn leaves and classic Northwest views of mountains and water.

Not so much this year. It’s been cloudy and cool. And very wet.

“Some people are calling it ‘wet-tember,’” says KNKX weather expert Cliff Mass. “We’re about an inch and a half above normal right now for the month – that’s more than we normally get for the whole month."

Mass says the wet weather is driven by the jet stream, a plume of moisture that comes from the tropics and fuels precipitation here. It normally stays north of Washington in September, but has moved south more quickly than normal, bringing us additional rain.


Mass says the extended forecast for the rest of the fall looks a little bit wetter than normal. He says that’s based on the predictions from models run at the European Center.

“It’s the best forecast in the world,” Mass said. “They go out 46 days. And those forecasts suggest a little wetter than normal over the next month and a half.”

But he says temperatures should be near normal over land – from Puget sound to Eastern Washington – and slightly warmer than normal near the coast and offshore.

“Because that blob hasn’t disappeared yet," Mass said, "we still have that warm water off shore.” 


Mass says the dice are weighted to fairly normal conditions – neither warmer nor cooler than usual and he expects precipitation to be near normal. That’s based on temperatures observed in the tropical Pacific, expressed as the El Niño-Southern Oscillation cycle. Last year was an El Niño, with slightly warmer and wetter conditions. This year is shaping up to be a neutral year, with nothing out of the ordinary.  

“That’s the best outlook we have,” Mass said of the extended outlook. “There’s no guarantee we’ll have a certain type of winter, but during neutral years, we tend to have near normal temperatures.”

However, neutral years are associated with extreme weather and, in the past, the most dramatic storms we've had often happened in years with the neutral outlook.

“So, as an example, the big Pacific wind storms, the inauguration day storms, the Columbus Day storms – they tend to occur during these kind of neutral years,” Mass said. “Also, strangely enough, the biggest snow storms also tend to occur during these neutral years.”

But, Mass says, that doesn’t mean any certainty of a snowy year in the passes around here. He says there’s a better chance of good conditions for show sports than in an El Niño year, but it’s still less likely than in a La Niña. And global warming has worsened the snow pack overall, in any year.

Still, he says if you’re wondering whether to buy a season pass for the ski areas, it’s not a terrible idea. 

“This is the kind of year, you’re betting, but I think it’s worth the bet,” he said.

Before then though, if you want to get out and enjoy the "wet-tember" weekend, here’s Cliff’s forecast:

Saturday: mixed clouds, temperatures around 70.

Sunday: a Pacific front comes in overnight, bringing rain in the morning and into the afternoon, temperatures in the mid-60s.

Monday- Tuesday: partly cloudy, with some showers. “Nothing serious, pretty typical for this time of the year."

Weather with Cliff Mass airs at 9:02 a.m. Friday, right after BirdNote, and twice on Friday afternoons during All Things Considered. The feature is hosted by KNKX environment reporter Bellamy Pailthorp. Cliff Mass is a University of Washington professor of atmospheric sciences, a renowned Seattle weather prognosticator, and a popular weather blogger. You can also subscribe to podcasts of Weather with Cliff Mass shows, via iTunes or Google Play.