For the first time in nearly a month, a storm system carrying rain for the entire Bay Area region descended into the area from the Pacific Northwest early Wednesday, spreading precipitation beyond just the North Bay.
Still, it’s the system that’s trailing it by a few days that’s really going to deliver the haymaker.
“That system is supposed to move in by Sunday,” National Weather Service meteorologist Roger Gass said. “That one will continue on and off during the week. So Christmas travel and any plans will be 100 percent affected by it.”
That rain from the last of three systems making their way across the Pacific Northwest is expected to be generated by an atmospheric river that is expected to measure at least an inch for the entire region and perhaps even 1½ inches minimum, according to the weather service. Forecasters believe areas of the North Bay could get between 2 and 5 inches.
That atmospheric river will be receiving it’s moisture from a plume that extends all the way to the Hawaiian Islands — a term that weather forecasters once called a “Pineapple Express,” a term that is largely out of favor among meteorologists now, according to the weather service.
The first of the two rain systems preceding the atmospheric river brought light drizzle only to the North Bay on Monday, and some light showers in parts of the region late Tuesday into early Wednesday.
By 2 p.m., most of the region had been touched by at least some measurable rain. The weather service measured about one-third of an inch in downtown San Francisco and a quarter-inch fell at Point Reyes Station in Marin County. Castro Valley received three-tenths of an inch, Livermore four-hundredths of an inch and Concord .02 inches.
It marked the first time since Nov. 20 that the Bay Area’s rain gauges in those counties showed anything.
Some of the heavier overnight rain happened in San Francisco, where the weather service measured between three-hundredths of an inch and .07 inches in the 12 hours before 6 a.m., Gass said. The North Bay received a bit more; the weather service said nearly a half-inch fell near Guerneville and .13 inches fell in that same period.
“This rain is still kind of considered the first system,” Gass said. “This is the tail end of it. As it progresses south, it kind of falls apart. Then we get that’s a bit more substantial by Friday. Then that third system comes in Sunday.”
The arrival of the rain already has changed one part of the recent pattern significantly. Tule fog that had blanketed the region for much of the dry spell was effectively obliterated by the ocean influence from the storm systems, Gass said. The dense fog that brought more than one advisory — including one on Monday morning — is expected now to stick close to its natural home in the Central Valley.
“Anything offshore that gives us influence from the Pacific Ocean is going to keep the fog away,” Gass said. “We’re finally getting that influence back.”
Another change that has come with the rain is the lifting of what have been chilly temperatures. The thermometer was expected to get as high as 64 degrees in areas of the East Bay and South Bay on Friday, an increase of nearly 10 degrees from Monday.
“Those cold temperatures were largely due to the tule fog spreading in the Bay Area,” Gass said, adding that the conditions created the fog trapped the cold temperatures. “We’ve kind of washed that pattern out.”
