'Ridiculously Resilient' 400-Mile Blanket Of Fog To Keep Bay Area Cool And Dry, But For How Long?

SAN FRANCISCO, CA — Another round of stubborn morning fog is expected to settle across Bay Area valleys this week, keeping temperatures cooler as the region remains locked in a prolonged stretch of stable, dry weather, forecasters said.

This weather phenomenon has lingered in the Bay Area following a wet October and November, as the saturated lower atmosphere has been trapping moisture, resulting in "tulle" fog, NWS Bay Area meteorologist Roger Gass previously told SFGATE.

That fog has kept temperatures chilly for weeks. Consistent morning tule fog and slightly more low stratus coverage will be most likely across the North Bay Valleys, Interior East Bay Valleys, and the Santa Clara Valley, according to the National Weather Service.

“There are several sites in the North and East Bay that are definitely below normal because of the fog,” NWS meteorologist Matt Mehle told SFGATE. “A great example of that would be San Rafael. Last week, it barely even hit 47 at the max temperature.”

As winds calm through Monday morning, clouds could dip to the surface and generate patchy fog across several inland valleys.

Eastern San Mateo County has a lower chance of being socked in fog, about 30 percent, due to warmer Bay waters creating slight instability that may lift fog into a low cloud deck unless winds fully ease, forecasters said Monday.

Visibility at San Francisco International Airport sometimes drops under similar conditions. Historically, SFO reports visibility below 3 miles about 8 percent of December mornings, the NWS said. With a light north-northeast wind, the forecast for Monday, that probability rises to 17 percent.

Fog and low clouds are expected to gradually clear through the afternoon, though forecasters say temperatures may struggle to climb into the 60s in areas blanketed by morning stratus.

In addition to persistent fog, the weather service has recorded a 18-day dry spell in the San Francisco area. And that streak is likey to extend late into the month.

"How unusual is that? Well it turns out a long dry spell in the middle of the rainy season is actually pretty common," forecasters in the Bay Area explained Monday.

The longest recorded dry spell in San Francisco during this time of fall was 60 days from November 17, 1876 to January 15, 1877.

"We're not expected to get anywhere near that record as it looks like a pattern change is finally on the horizon," the NWS said, adding that there's a "hint" of rain at the end of this week.

Forecasters said Monday its a "ridiculously resilient ridge" that's keeping the storm track well to the north.

On Dec. 15, there will be a chance of rain as that ridge finally weakens.

“That's been a moving target,” Mehle told SFGATE. “Some days it looks like it's going to be wet, and other days the long range is showing it’s staying farther north. So not a lot of confidence on rain returning.”