SAN FRANCISCO (CBS SF) — Mother Nature’s Spring Surprise blanketed the Sierra with snow Thursday, triggering a winter storm warning and dumping more than a month’s worth of rain in some parts of the Bay Area.

Over the last 24 hours the storm front has dumped 1.96 inches of rain in Santa Rosa, 2.24 inches at St. Helena in Wine County, 1.59 inches in Kentfield, 1.42 fell in Napa, 2.37 inches in San Anselmo and 2.16 inches at Ben Lomond in the Santz Cruz Mountains.

Forecasters had initially predicted anywhere from 1/2 inch to 2 inches of rain to fall by next week. But those totals could be much higher before an extended period of dry weather begins next week.

Compare those totals with the recent average monthly rainfall in May — 0.83 inches in Santa Rosa, 0.70 inches in San Francisco, 0.44 inches in San Jose and 0.43 inches in Livermore.

“The past seven years have been extremely – or completely – dry in the Bay Area in the month of May,” said KPIX 5 Chief Meteorologist Paul Deanno. “That has given many of us the feeling that ‘it never rains in May.’ That’s true for July & August, but not true for May. We do typically get one storm or two moving through, but just not recently.”

Showers will return to the forecast on Saturday and Tuesday.

“The rainfall over the next 5-7 days is definitely above normal,” the national weather service said. “Parts of the region could see more than the monthly normal (rainfall) in just a few days.”

In the Sierra, more than 7 inches of snow has fallen near the Squaw Summit.

Meanwhile, Mammoth Mountain reported a foot of new snow on its peak, boosting the summit season total to nearly 58 feet at the resort, which had already announced it had enough to allow skiing and boarding through the Fourth of July.

“Looks like a Winter Wonderland in mid-May!” the Sacramento National Weather Service office said in a tweet showing traffic camera images of snowy Interstate 80, a major route that crosses over the Sierra north of Lake Tahoe.

Caltrans warned drivers using the high mountain passes in the Sierra on both Highway 50 and I-80 to be ready for delays and also make sure to have their tire chains.