It’s been so dry this spring in California that, as they say in Texas, the trees are whistling for the dogs.

State water officials on Thursday announced that the Sierra Nevada snowpack — a key source of drinking water for millions of people — is at an anemic 52 percent of its historic average.

That’s the lowest reading for the beginning of April of any time since 2007, which was the start of a three-year drought. Only two years during the past 20 have had less Sierra snow at this time of year, 1994 and 2007.

“We’re seeing a lot of melted-out spots, a lot of grass and rock where we normally see snow,” said Dave Rizzardo, chief of snow surveys for the California Department of Water Resources in Sacramento.

The amount of snow and rain that fell in the northern Sierra between January and the end of March is the least since records were first kept in 1920. In San Francisco, the rainfall over the same three months (1.72 inches) is the lowest since records were first kept in 1850, and the second lowest in San Jose, whose records go back to 1874. It’s a similar story in Oakland, with the driest spring so far since the 1930s, when consistent records there were first kept.

A forecast for mild weekend showers across the Bay Area, with a 70 percent chance of rain Sunday, won’t be enough to change the overall picture, water experts said.

Nevertheless, in an odd mix of events, so much rain and snow fell in Northern California in November and December that reservoirs and ground water banks remain healthy, and no large Bay Area water agencies say they expect water restrictions or mandatory rationing this summer.

“We have planned for the possibility that the winter would be as dry as it is. We are prepared with other water supplies,” said Marty Grimes, a spokesman for the Santa Clara Valley Water District, in San Jose. “We are fine through this year. But obviously we’ll be hoping for a wetter winter next year.”

In the East Bay, water officials held back more of the runoff from heavy storms late last year in their reservoirs than normal, leaving plenty of water “in the bank” when the spring rains failed to materialize.

The on-again-off-again rainfall this year has made for a wild hydrologic ride.

“We got more rain in a few hours during one storm on Dec. 23 than we’ve had in the whole three-month period from January until now. It’s been a strange year,” said Charles Hardy, a spokesman for the East Bay Municipal Utility District, which serves 1.3 million people in Alameda and Contra Costa counties.

The district’s main storage area, Pardee Reservoir in Calaveras County, was 88 percent full Thursday. At worst, the district’s customers will get a voluntary conservation request this summer, Hardy said.

Same thing in Silicon Valley, where the Santa Clara Valley Water District is comfortably sitting on a year’s supply of water in its vast underground aquifers, roughly 326,000 acre feet, and has another year’s supply, about 308,000 acre feet, stored underground at the Semitropic Water Storage District near Bakersfield.

The bone dry last three months haven’t even particularly affected the state’s ski resorts. At Heavenly at Lake Tahoe, there is a 4-foot base, some of which is left over from the November and December storms, and the U.S. National Freestyle Skiing Championships are set for this weekend.

“The snowpack was generated early. We are just riding it out through the season,” said Heavenly spokesman Russ Pecoraro. “People have been coming up and raving about the conditions.”

Some Californians, particularly some San Joaquin Valley farmers, however, will face a more parched summer than their Bay Area neighbors. That’s because the low snowpack means state and federal officials won’t be able to pump as much from the Sacramento-San Joaquin River Delta, a key source of water for the farms that use 80 percent of the state’s developed water supply.

Officials from Gov. Jerry Brown’s administration on Thursday used the snowpack news to try to boost the difficult political fortunes of their $23 billion plan to build two massive tunnels under the Delta to make it easier to move water from the north to the south.

“With today’s snow survey, the table has been set for yet another very dry year,” said state Natural Resources Secretary John Laird. “Add to that pumping restrictions imposed this winter because of vulnerable smelt and salmon populations, and it is clear that the security of California’s water supply is threatened.”

State hydrologists and private meteorologists, however, aren’t ready to say the state is in a drought.

That’s because it typically takes two dry years in a row for a drought to materialize in California, and although last year was drier than normal, the wet November and December have left plenty of water in reservoirs.

California would have received lots of rain and snow this winter, but a persistent zone of high pressure off the coast has forced storms into Canada and down into the Midwest, which has resulted in large amounts of snow there. In other words, the Rust Belt and Bible Belt are shoveling up what should have been California’s rain.

Jan Null, a metrologist with Golden Gate Weather Services in Saratoga, offered one explanation:

“It just falls into the category of ‘stuff happens.'”

Paul Rogers covers resources and environmental issues. Contact him at 408-920-5045. Follow him at Twitter.com/paulrogerssjmn