'It's off the charts': 3,500-foot-deep marine layer blankets Bay Area

FILE -- Fog returned to San Francisco on Sept. 27, 2019. FILE -- Fog returned to San Francisco on Sept. 27, 2019. Photo: Adam Treadwell/Getty Images/500px Prime Photo: Adam Treadwell/Getty Images/500px Prime Image 1 of / 27 Caption Close 'It's off the charts': 3,500-foot-deep marine layer blankets Bay Area 1 / 27 Back to Gallery

A blanket of fog, measuring up to 3,500 feet deep in some spots, covered most of the San Francisco Bay Area on Friday morning, delivering drizzle and wetting sidewalks.

"It's off the charts," says Suzanne Sims, a forecaster with the National Weather Service. "This is about as deep as the fog gets. You can see it on satellite going well into the Sacramento Valley. There's a clearing patch over San Francisco Bay and San Jose."

The soupy weather arrives after the region saw a heat wave earlier in the week and temperatures Friday will be up to 30 degrees cooler than they were on Wednesday.

Coastal areas are forecast to be in the 60s, bay front locations in the high-60s to low-70s and inland valleys in the 70s.

San Francisco saw a high of 94 degrees on Wednesday. Today, the after high is projected to be 66.

Sims says there will be some clearing of the clouds in the afternoon but the fog will return in the evening when the sea breeze kicks up.

Similar weather is forecast for the weekend with temperatures below normal for this time of year. Next week, temperatures will rise slightly and are expected to be closer to normal with inland areas approaching the low-80s by mid-week.

The shift in weather arrives with a trough of low pressure digging into the West Coast, kicking up the ocean sea breeze that's blowing cold air inland. What's more, a cold front from the Northwest is pushing chilly air into Northern California.

"Compared to where we were two days ago, it's welcome relief," says Sims.

Amy Graff is a digital editor for SFGATE. Email her at agraff@sfgate.com.