Morning fog is forecast to blanket the South Sound on Thursday.
And Friday. And Saturday. And Sunday.
This month’s fog is related to a high-pressure system over the region working with an inversion, National Weather Service meteorologist Josh Smith said Wednesday.
To get fog, you need clear skies and winds that aren’t strong, both of which high-pressure systems usually cause, Smith said. An inversion helps keep the fog in place, he said.
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That’s the basic explanation.
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.
The light winds 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."
Wednesday, the fog faded in the afternoon, which it should continue to do until Monday, which is forecast to be mostly sunny.
Temperatures will be in the mid 40s and low 50s, and possibly upper 50s when the fog breaks each day, Smith said.
The several weeks of high pressure, lack of rain, and dense low clouds and fog are very 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."
Fog is not unusual for Sea-Tac Airport, spokesman Perry Cooper said.
"We deal with this stuff every year," he said. "Fog is common around here."
The hazy stretch has affected only a few flights, including five diverted Saturday, he said.
The airport has one of the best low-visibility landing systems in the country, he said.
"You don’t have to be able to see the runway to land at Sea-Tac with the technology that we’ve got," Cooper said.
But to see the sun during the foggy spell, it will require a trip, meteorologists said.
"Maybe take a drive toward the foothills, over toward Mount Rainier," Smith said. "If you get far east enough, you can enjoy the sunshine."