Sun likely to break up shroud of clouds by midweek

From Manhattan to Moscow, summer has been a sweltering affair for much of the globe.

But from Seattle to San Diego, the West Coast has been trapped in a twilight zone of drizzle, fog and temperatures more conducive to fleece than bikinis.

The blame belongs to an unusually strong high-pressure system sprawled across more than a million square miles of the Pacific. The blob has been shoving cool marine air and clouds toward the coast, University of Washington meteorologist Cliff Mass said.

Add a conveyor belt of storm systems funneled from Alaska, and you have August days when you might be tempted to turn on the furnace.

Meteorologists say the pattern will begin to break up midweek, leading to a toasty weekend. But there's no guarantee the gloom won't return.

"It's been pretty persistent," said Art Gaebel, of the National Weather Service's Seattle office.

It's also been record breaking in terms of one measure of cloudiness.

UW postdoctoral researcher Jim Johnstone has been gathering current and historical data on the height of the cloud deck, estimated or measured with lasers several times a day at airports across the region. So far this summer, the amount of time the Seattle area has been blanketed in low clouds and fog is nearly triple the historic average.

Since July 10, the data paint an even glummer picture: nine hours a day of low cloud cover, compared to the seasonal average of 1.6 hours.

And pity the folks who live on the coast.

The town of La Push in Clallam County has been under a near-constant pall for the past 10 days.

This summer appears to be the most cloud-shrouded in Seattle since measurements were started in 1951, Johnstone said.

"Things are really quite unusual."

Johnstone also has studied Northern California's famous fog. He found a marked decrease in fogginess there since the early 1900s, a trend that could be harming coastal redwood forests dependent on the moisture.

This year notwithstanding, fogginess along the Oregon and Washington coasts also appears to be declining, although Johnstone's analysis isn't complete and historic records don't go as far back as in California.

If the trend proves true, it would be at odds with some climate models, which predict more low clouds and fog west of the Cascade Mountains as the planet warms due to greenhouse-gas emissions.

In those models, the cloudiness is driven in part by higher pressure over the Pacific, but it's impossible to draw a link to this summer's pattern, Mass said.

"The whole West Coast has been anomalous," he said.

San Diego experienced its coolest July in nearly 80 years. At Half Moon Bay near San Francisco, only eight hours out of the past 11 days have been cloud free.

"It's just brutal," said Johnstone, who visited the Bay Area last weekend.

Some days in Seattle, the layer of low-lying marine clouds has been thin enough to burn off by early afternoon. But other days the mass has been so thick it refuses to dissipate.

Expect more of the same Tuesday, with a few sprinkles thrown in.

There should be a glimmer of sun by Wednesday, which will expand as the high-pressure system over the ocean migrates northward and the storm-system pipeline breaks down.

The result will be sunshine and highs in the 80s or 90s by the weekend, Gaebel said.

"Hopefully, the good weather will stick around awhile this time."

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

Information in this story, published Aug. 9, was corrected Aug. 11. The high-pressure system sprawls across more than a million square miles. A previous version stated it sprawled across 1,000 square miles.