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High Pressure System Locking In Clear Blue Skies This Weekend

Tim Durkan Photography
November sunset over Puget Sound, with the Olympic Mountains in the background.

Enjoy the weather this Thanksgiving weekend. High pressure above us has set up in a stable pattern that will stick around until Monday, keeping rain away until sometime Tuesday. 

"No precipitation (guaranteed), sunny, with little clouds ... a tonic against Seasonal Affective Disorder," says KPLU weather expert Cliff Mass.

Expect clear skies and cooler temps, with highs in the mid-40s and lows down into the 20s or even the teens in western Washington. There might be some fog in a few places and there is a possibility of some air quality issues, as smog builds up.

All of this, Mass says, comes courtesy of “the ridge.”  

Ridgeology 101

“When meteorologists use the term 'ridge,' we really mean high pressure – an area of high pressure in the atmosphere and these high pressure areas are quite big, they’re a few thousand miles in size,” explains Mass, who teaches atmospheric sciences at the University of Washington.

Low pressure is what brings in storm systems, he says.

High pressure can create strong winds or it can lock in calm conditions, depending on its location.

“When high pressure is right above us, the pressure gradient, the difference in pressure over the region is very weak and winds tend to be light,” he explains. Winds are produced by differences in pressure.

"On the other hand, when high pressure is inland from us, for example in eastern Washington or the interior of British Columbia, that can produce very strong winds in certain locations," Mass says.

“Particularly in gaps and downstream of gaps in the Cascades,” he says. For example, British Columbia's Frasier River gap can deliver gusts as fast as 30-50 miles per hour to places like Bellingham or over the San Juan Islands.

That was a source of the strong winds last Tuesday and Wednesday in those areas — as well as all the way down the western side of Puget Sound, knocking out power in places such as Poulsbo.

Air Quality Issues

Mass says high pressure above us tends to create poor air quality and stagnation, which are the triggers of burn bans. It does so in a few ways.

First: weak winds – when the high is right over us, there’s less turbulence. “So there’s less of that mixing of pollutions from the surface, up into the atmosphere,” Mass says.

Also, when there’s high pressure, we tend to have clear skies and cooler air gets locked in below, with warmer temperatures above. This is called an inversion. “And inversions are very, very stable features,” Mass says.   “It tends to act as a cap on the atmosphere above and you tend not to get mixing when you have an inversion.”

And on top of all that, he says the colder air temperatures near the surface when there’s a ridge means that more people are using their wood fireplaces and other heat that produces air pollution and gets locked in.

Rain Again Wednesday

Mass says the ridge that’s over us now will last through at least through Monday, but on Tuesday a series of storm systems connected to the jet stream off the west coast will start pushing in. Wednesday and Thursday, he says expect rain.

“We’re back into the precipitation again – so we’ve had a nice break over Thanksgiving, but it won’t last.”

The weekly KPLU feature "Weather with Cliff Mass" airs every Friday at 9 a.m. immediately following BirdNote, and twice on Friday afternoons during All Things Considered. The feature is hosted by KPLU 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 a podcast of “Weather with Cliff Mass” shows.

Bellamy Pailthorp covers the environment for KNKX with an emphasis on climate justice, human health and food sovereignty. She enjoys reporting about how we will power our future while maintaining healthy cultures and livable cities. Story tips can be sent to bpailthorp@knkx.org.