Bay Area cities are setting the lowest levels of rainfall in recorded history this year as unusual conditions are blocking moisture from the Pacific Ocean, a National Weather Service forecaster said.
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 was 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.
San Francisco, which started recording rainfall 164 years ago, had only 5.57 inches of rain since last January, far below the city's record low 9 inches in 1917, Benjamin said.
San Jose had only 3.8 inches in 2013, 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 in 2013 was in Kentfield, an unincorporated area of Marin County that is frequently the wettest part of the Bay Area, Benjamin said.
But Kentfield saw a mere 7.8 inches of raindrops in 2013, 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 at this time of year, 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 below 32 degrees in Pleasanton.
But weather patterns contributing to the high-pressure ridge may be changing and some rainfall is possible by late this week or next week, based on the weather service's models for rainfall, Benjamin said.