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When will it rain once more within the San Francisco Bay Space?

After a soggy December, the San Francisco Bay Area has seen little rain in January, with the city recording a little over a half-inch since the start of the year.

Weather conditions have been completely dry for a week, with a high-pressure ridge hunkering down along the West Coast, blocking the jet stream and putting an end to the steady stream of storms that swept California last month.

So when is the rain coming back?

National Weather Service meteorologist Ryan Walbrun said a weak storm is forecast to bring light rain to Central California over the weekend, with locations such as San Luis Obispo and Santa Barbara likely being a bit wet. But he doesn’t expect the system to deliver even sprinkles to the Bay Area. Long-term weather models — which are only right half the time at 10 days and beyond — show the next chance for rain in the region in the last week of January.

“I would say with high confidence through Jan. 23, there’s no chance of rain,” Walbrun said. “When we get to Jan. 24, 25, 26, there’s some signal for a chance of rain. We might start to see a pattern change. But that’s two weeks out, so confidence in the forecast is low.”

Prolonged dry spells aren’t unusual during a Bay Area winter.

Jan Null, a forecaster who runs the private forecasting service Golden Gate Weather Services, combed through San Francisco rainfall data and found that over the past 71 consecutive rainy seasons going back to 1950, the city has experienced a dry period in December or January, averaging 19 days.

Null defines a “dry period” as consecutive dry days with no rain, or consecutive days that were broken by no more than two nonconsecutive intervening days of very light rain with less than 0.08 inch recorded.

“All of these dry periods began in December or January,” he wrote in an email, “with the exception of the 1964-65 period, which was 19 days but did not begin until Feb. 6.”

Null also noted that even the very wet El Niño seasons of 1982-83 and 1997-98 had intervening dry spells of 22 and 17 days, respectively.

“We get these mid-winter dry spells,” Walbrun said. “Not unusual, but the shorter we can keep it the better, so hopefully we do end it here by the end of the month. We’ve only got so many months of winter.”

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