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Originally published August 1, 2012 at 7:31 PM | Page modified August 1, 2012 at 7:40 PM

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Global warming may bring us more gloomy morning weather

Tired of the cloudy summer weather? Blame our proximity to the sea. We'll get a break this weekend, but then the clouds are expected to return.

Seattle Times science reporter

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The pattern is all too familiar in Western Washington: While the rest of the country basks — or swelters — under summer sun, our mornings bring a blanket of gloom. Throughout the day, people peer skyward, wondering when — or if — the clouds will burn off.

Some days we get lucky. But sometimes the skies never clear, as is likely on Thursday.

Frustration is understandable, but futile, says University of Washington meteorologist Cliff Mass. Our summer stratus sheets are a byproduct of living close to the sea — and could become more pronounced as the globe warms.

"Our simulations show we could have more of these low clouds in the spring and summer," he said.

Mass scheduled his annual celebration of the driest day of year for last Monday. But the revelers weren't treated to a speck of sun, because the burnoff never achieved ignition. "It stayed cloudy." Mass said.

The typical scenario starts when what's called the East Pacific High migrates north and parks itself off the coast of Washington and Oregon. Higher pressure offshore drives cool marine air inland, and clouds tag along.

At the end of a hot spell, there's nothing more refreshing than to feel that breeze chase the stickiness away. But on days when you long for a little solar radiation, it's demoralizing to wake to yet another expanse of gray. The marine air and clouds tend to push in overnight, Mass explained.

"So we get this cool, relatively shallow layer of moist air that's often full of clouds," he said.

Satellite maps clearly reveal the way the cottony covering clings to the lowlands and comes to a halt at the Cascade mountains. Climb a peak, and you can look down on it yourself.

"Burnoff" isn't technically the right term for what happens on days when the sun breaks through, Mass said, but it does paint a vivid picture. If the marine air is moving gently and the layer isn't too thick, rays will percolate through and start to warm both the air and the ground by midmorning. As the air warms, the clouds dissipate. But equally important is the atmospheric mixing that gets going once the air is toasty enough to rise.

Thursday is forecast to be a repeat of Monday, when a small storm front supercharged the marine flow. The result was a thicker layer that thumbed its nose at the sun's efforts to pick it apart.

On Thursday, the troublemaking system will be over British Columbia, said National Weather Service meteorologist Johnny Burg. The result will be a stronger onshore push of moisture and cool air. There's a possibility of rain by late morning — and hint of possible clearing by late afternoon, Burg said.

But signs are more favorable for Seafair weekend. By Friday, air flows will start to reverse as high pressure builds inland. That will cut off the marine flow and bring warmer air and sunny skies, with highs in the low 80s for Friday and Saturday. Sunday will be a tad cooler, then the weather gods hit the reset button.

"The clouds make a comeback Sunday night and Monday," Burg said.

Mass' climate models predict that global warming will heat up the continents faster than the oceans. As the warmer air rises, that will create even more of a pressure gradient to draw cool, cloudy marine air over Western Washington throughout the spring and summer.

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com

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