A weather system from the Pacific that typically carries dampness south from the Pacific Northwest in October has not developed this season, leading to record-low rainfall levels, according to forecaster Bob Benjamin.
Rainfall for December should be about several inches in Bay Area cities but "we're just getting piddling amounts," Benjamin said. "Most places are in the quarter-of-an-inch range."
Although the weather service does not keep rainfall numbers for the entire Bay Area, "for all the sites we maintain climate records, this is the lowest calendar year for rainfall amounts" for the area, Benjamin said.
"Everyone is going to be reporting very, very low, record low amounts," Benjamin said.
With only a day to go in 2013, the city of San Francisco, which started recording rainfall 164 years ago, has had only 5.57 inches of rain since January, far below the city's record low 9 inches in 1917, Benjamin said.
San Jose has had only 3.8 inches this year — the lowest since the city started recording rainfall 119 years ago — compared to its record low 6.04 inches in 1929, Benjamin said.
"We're not talking short-term records," Benjamin said.
Perhaps the biggest drop in rainfall this year has been in Kentfield, an unincorporated area of Marin County that is frequently the wettest part of the Bay Area, Benjamin said.
But Kentfield has seen a mere 7.8 inches of raindrops this year, much lower than its previous record low of 20.30 inches in 1989, according to Benjamin.
A high pressure "ridge" in the eastern Pacific Ocean that usually fluctuates and permits the passage of moist air into the Bay Area is for some reason more intense and blocking the air from moving south, Benjamin said.
"It's a unique weather pattern," Benjamin said.
The lack of moisture-filled air is also producing warmer than normal temperatures during the day and cooler than normal temperatures at night in the Bay Area, Benjamin said.
Normally this month Bay Area temperatures in the daytime are in the mid to lower 50s to lower 60s but highs have been in the 60s to lower 70s, while overnight lows that should be in the mid to low 40s have fallen below that, such as 35 degrees last night in San Jose, Benjamin said.
But weather patterns contributing to the high-pressure ridge may be changing and some rainfall is possible by next Tuesday or Wednesday, based on the weather service's models for rainfall, he said.