Remember umbrellas and windshield wipers?
Bay Area residents woke Monday morning to something they hadn’t seen in six weeks: Rain.
A fast-moving cold front from the Gulf of Alaska brought several hours of showers across Northern California on Monday, providing countless free car washes, clearing the air and wetting down vegetation after a hot dry weekend. It was the first significant rain in the Bay Area since April 1.
How rare is rain in May? Not unheard of, experts said Monday.
“We don’t get a lot. But we usually do get some,” said Jan Null, a meteorologist with Golden Gate Weather Services in Half Moon Bay.
More precisely, 84% of the Bay Area’s annual average rainfall — 19.28 inches out of a yearly total of 22.89 inches, measured by the gauge in downtown San Francisco — falls in five months: November, December, January, February and March, a feature of the state’s Mediterranean climate. And 59% falls in just three months, December, January, and February.
May? Normally sunny. On average, the Bay Area receives .7 inches of rain each May, or 3% of the annual total. That puts May as the 8th wettest month of the year, well behind December, January, February, March, November, April and October, and ahead of June, September, August and July.
“We’re in the shoulder months between spring and summer,” said Karleisa Rogacheski, a meteorologist with the National Weather Service. “We get weak low pressure systems that sometimes come across Northern California. This was beneficial rain.”
The rain cleared the air, reduced fire danger for a short time, and helped boost seasonal totals, she noted.
Some years there isn’t any. On average, it rains 4 days a year in May.
Monday’s showers came in before dawn, causing flight delays of about 47 minutes on average at San Francisco International Airport, and 15 minutes at Oakland and San Jose airports.
It was gone by mid-morning. The wettest areas were across Bay Area hills and peaks.
Tilden Park in the East Bay Hills had nearly half an inch, with .47 inches; Mount Diablo near Walnut Creek had .31 inches, the same as Ben Lomond in the Santa Cruz Mountains. Marin County’s Mount Tamalpais saw .30 inches, while Hayward had .25, Concord .20, Oakland .16, San Francisco .15, Mt. Hamilton .12, San Jose .09, Palo Alto .08, Richmond .06 and Watsonville .04.
It was the most rain in the Bay Area since Most rain in the Bay Area since April 1, when San Jose receive .25 inches.
The forecast? Dry weather for the rest of the week.
“I’m looking at nice blue skies,” Null said.
Since Oct. 1, rainfall totals have been higher in Northern California than Southern. Santa Rosa on Monday was at 126% of normal, San Francisco at 89%, San Jose at 71%, Los Angeles at 59%, and San Diego at 49%. But after three above-average or near-average winters in a row, reservoirs across the state are full, and no summer water shortages are expected.
“It’s been a real gradient from the Oregon border to Mexico this year, with higher numbers in the north and less and less as you go south,” Null said. “A lot of the storms we saw in the winter got cut off around Interstate 80.”
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Average Bay Area rainfall amounts by month
(Downtown San Francisco: 1991-2020, with % of annual total)
January: 4.40
February: 4.37
March: 3.15
April: 1.60
May: .70
June: .20
July: .01
August: .06
September: .10
October: .94
November: 2.6
December: 4.76
Total average: 22.89
