SF has record-low rainfall in 2013

An unusual weather pattern brought mild temperatures to the Bay Area at the end of 2013, but it also blocked moisture from the Pacific Ocean, leading to record-low rainfall in many of the region’s cities.

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, National Weather Service forecaster Bob Benjamin said.

“It’s a unique weather pattern,” Benjamin said.

The lack of moisture-filled air produced warmer-than-normal temperatures during the day and cooler-than-normal temperatures at night in the Bay Area, Benjamin said.

Normally in December, Bay Area temperatures in the daytime are in the mid- to lower 50s to lower 60s, but highs were in the 60s to lower 70s, while overnight lows that should have been in the mid- to low 40s fell below that, Benjamin said.

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, has had only 5.59 inches of rain since January, far below the city’s record-low 9 inches in 1917, according to National Weather Service data.

But weather patterns contributing to the high-pressure ridge may be changing and some rainfall is possible in the next few weeks, according to long-range models, the National Weather Service said Tuesday in an email.

— Staff, wire report

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