BELLINGHAM Morning fog is forecast to blanket much of Whatcom County through the weekend, continuing the mostly gray days and occasional sunshine.
The recent fog is the result of a high-pressure system that has been sitting over the Northwest for a couple of weeks, working with an inversion to reduce visibility in low-lying regions, said National Weather Service meteorologist Josh Smith.
That's the basic explanation. Here's the more complex one: Clear skies allow the ground at night to cool more than usual, with the heat radiating up into space, Smith said. As the air cools, it shrinks, wringing moisture out like a washrag, creating the fog.
An inversion in which temperatures increase with height instead of decrease as usual makes sure the fog stays put, he said, because the warmer air is less dense than the cooler air below, and the two don't want to mix.
Light winds coming with the high-pressure system aren't enough to mix things up, either.
"When we have these extended fog events, usually there's an inversion," Smith said. "Normally it's more common a little bit later in the year, November or December."
Last week, fog delayed or canceled several flights at Bellingham International Airport and Sea-Tac.
But it's more than an annoyance - it can be deadly. On Thursday, a Washington state ferry nearly collided with a fishing boat in the fog near Clinton. On Tuesday, two people died in a head-on collision in heavy early-morning fog on Grandview Road northwest of Ferndale.
The high pressure system has created another rarity - conditions where the coast and mountains are sometimes more likely to see sunshine than lowlands and valleys.
The several weeks of high pressure, lack of rain, and dense low clouds and fog are strange for this time of year, University of Washington meteorologist Cliff Mass said in a Sunday blog post titled "Fogmageddon."
"I have forecast here for many decades, and I can not remember a situation that is even close," he wrote. "The underlying cause? An extraordinary persistent ridge of high pressure over the eastern Pacific and West Coast."
Forecasters say a change is possible by Monday to colder weather and clearing skies, but still dry.
Alexis Krell of The News Tribune in Tacoma contributed to this report.