Persistent High Pressure Will Bring Another Week Of Sun, Morning Fog and Ice

Dec 8, 2017

A strong ridge of high pressure has brought a string of sunny days to the Puget Sound, as well as patches of morning fog that is often accompanied by black ice. KNKX weather expert Cliff Mass says he expects that pattern to stick around for quite a while.  

“One way or the other, it’s going to continue at least the next week,” Mass said. “I expect no precipitation, no storms, no nothing for the next week.”

The afternoon sun can be dazzling in December. But it comes with the risk of slipping or sliding on black ice in the mornings. That phenomenon was pretty extreme in many parts of Seattle early Friday.  Mass says stagnating air was the reason.

“The offshore flow weakened a little bit. Fog became much more persistent overnight. And so between fog, which has water in it, and the surface cooling down under the clear skies, we have a lot of ice around the area,” he said.

Mass says some of the ice is from frost as well – and some of it is from fog going over cold roadways. Either way, it’s important to be aware of it, whether you’re walking or driving.

“It is pretty icy out there, so people have to be extremely careful,” he said.

Mass says he doesn’t expect the situation to change very much over the weekend.

“No rain, probably some low clouds and fog in the morning, burning out most days, temperatures getting up to around 50, so a pretty big range of temperature,” he said.

Mass says it could get as warm as 55 in parts of western Washington. And the ridge of high pressure that’s at the root of all this isn’t moving out anytime soon. He expects it to stick around for another week.

“This is going to be an extraordinarily persistent pattern. I don’t see any changes at least until next weekend,” he said.

To hear the full conversation, including an explanation of why the high pressure can cause black ice and bad air quality – as well as how this pattern is connected to the high winds that are fanning wild fires in California – you can click on the 'play' icon at the top of this post. 

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.