For anyone who didn’t check the forecast, the sound of rain pounding on San Francisco rooftops on Thursday morning may have come as a surprise.
“We do sometimes get rain in May, but I would call it uncommon,” said Brayden Murdock, a meteorologist with the National Weather Service office serving the Bay Area. “Usually, April is when we start to see the rain get cut off.”
A late-season storm has brought rain across much of California, particularly the northern half, and snow to the higher elevations of the Sierra Nevada Range. Many locations have recorded a quarter inch to an inch of rain this week.
The precipitation is welcome at a time of year when vegetation is drying out and the risk of wildfires is increasing.
“The rain has wetted the landscape and made it less flammable, and should prevent large, destructive wildfires in coming days,” especially in Northern California, said Brent Wachter, a fire meteorologist with the U.S. Forest Service in Redding, Calif.
But the low threat of fires is not expected to last long with a return to warm, dry weather forecast for next week. Mr. Wachter said the vegetation would quickly dry out again.
The storm pulled in a mass of chilly air as it started moving into California on Monday night. On Tuesday, thunderstorms rumbled through the Central Valley, and short bursts of rain and hail were reported in the greater Sacramento area. On Wednesday, snow dusted the Sierra Nevada.
The storm pivoted toward the San Francisco Bay Area on Thursday morning, and at around 6 a.m., rain fell heavily for less than an hour before easing into a light shower.
The average rainfall total for May in downtown San Francisco is 0.64 inch. By late morning the city had nearly matched that amount with 0.60 inch. It is the most rainfall on May 28 in records going back to 1849, but it is not the most to fall on a single day in May, Mr. Murdock said. Downtown San Francisco measured 1.42 inches on May 27, 1990.
“These are beneficial rains, nothing that causes flooding,” said Brian Hurley, a meteorologist with the Weather Prediction Center.
By late morning, the storm was heading into Central California and moving toward Los Angeles, where a few sprinkles were expected on Thursday.
Snow continued to fall in the Sierra Nevada, where the snow pack had been steadily melting in past weeks from warm spring weather. All but one ski resort has closed. Mammoth Mountain, a ski resort, announced on Wednesday that the fresh snow would allow it to remain open through at least June 7.
Mammoth received a dusting of snow on Wednesday and picked up three to five inches as of early Thursday morning.
The resort has a long history of trying to continue operations as long as possible and often stays open through at least Memorial Day, occasionally until July 4 and very rarely into August.
The storm was expected to exit California by Thursday night and push east toward Nevada and Utah and then the Rocky Mountains.
Amy Graff is a Times reporter covering weather, wildfires and earthquakes.
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