How big a problem is January’s dearth of rainfall?
Sunny skies. Balmy temperatures. Walks on the beach. Umbrellas back in the closet.
After a soaking wet December that ended fire season, delivered more 15 feet of snow to the Sierra Nevada and boosted hopes that California’s severe drought might be coming to an end, dry weather is back, in a big way.
Like a baseball player stuck in a hitting slump, it hasn’t rained significantly in the Bay Area since Jan. 4.
Although reservoirs received a nice boost from big storms in December and late October, they still remain well below normal levels in most parts of the state.
Time to start sweating that the state’s two-year drought might be turning into a three-year drought? Not yet, say experts. As it turns out, dry spells in the middle of winter are actually quite common. The key is how long they last.
There’s been a dry stretch in nearly every winter season in December and January back to 1950 in the Bay Area. The average duration is 19 days.
“It’s not unusual for us to have dry periods in December and January,” said meteorologist Jan Null with Golden Gate Weather Services in Half Moon Bay. “It’s the rule rather than the exception.”
What happens is that a ridge of high pressure builds up off the coast, diverting storms to the north or south, and bringing summer-like weather to wintertime Northern California. That’s what’s afoot now.
The shortest such winter dry spell was 8 days long, in 1957-58 and in 1994-95, said Null, who compiled the data.
The longest was a parching 56 days in 2014-2015, during the depths of California’s last drought. Only one winter season avoided the December-January trend — in 1964-65, when the dry period started in early February and continued for 19 days.
But there’s some reason to be concerned. The forecast calls for lots more dry, sunny weather across the Bay Area for at least the next two weeks.
And as Null noted, another two weeks of that would put the Bay Area’s dry streak at 28 days. That would rank it as the 11th driest dry spell in the past 71 winters, with no way to know how long the streak will continue.
And the longer the Bay Area goes without rain, the more likely a dry winter becomes. Overall, eight of the 10 winters since 1950 with the longest dry spells ended up with below-average rainfall.
On average, half of Northern California’s precipitation falls in December, January and February. For the state to continue to fill reservoirs and emerge from the drought, February and March will need to be wet.
“We simply don’t know what the rest of the water year is going to be like,” said Jeanine Jones, drought manager for the California Department of Water Resources in Sacramento. “We can’t count our chickens until they are hatched. We’ve had a very good start to the water year and we hope that continues.”
The lack of rainfall is returning some of California’s eye-popping December numbers back closer to historical averages.
On Jan. 1, the statewide Sierra Nevada snowpack was 168% of normal for that date. A week ago, it was down to 117%.
Rainfall totals around California cities show a similar trend.
On New Year’s Day, San Francisco had received 191% of its historical average rainfall for the winter season.
But a week ago, that was down to 157%. Oakland fell from 219% to 174%. And San Jose, which didn’t get as much rain during the big December storms, fell from a Jan. 1 total of 157% of normal to 116% a week ago.
How much more rain and snow will it take to end the drought?
“The answer depends on where you are,” Jones said. “If you are a local water agency with a full reservoir at the end of this winter season you might say from your perspective the drought is over.”
In Marin County, the seven reservoirs owned by the Marin Municipal Water District a week ago were 95% full — a stunning turnaround from three months ago, when they were just 32% full.
But many other areas are still struggling, she noted.
In Santa Clara County, the 10 reservoirs operated by the Santa Clara Valley Water District were just 26% full a week ago. That’s up from 11% a month ago.
But they remain very low because the area didn’t get as much rainfall as the North Bay. And because the district’s largest reservoir, Anderson, near Morgan Hill, is drained for federally required earthquake repairs.
The state’s biggest reservoirs have seen improvement. Shasta, the state’s largest, was 34% full a week ago. Oroville, in Butte County, was 44% full, having risen nearly 100 feet since this summer. Folsom, north of Sacramento, was 56% full.
“We’re better now than where we were in mid-December,” Jones said, “but we aren’t out of the woods yet.”
Brayden Murdoch, a meteorologist with the National Weather Service, said sunny skies with high temperatures in the mid-60s are likely for at least the next 10 days.
“Unfortunately, it doesn’t look like much is going to change,” he said. “We are expecting dry, warm conditions through the end of the month.”
That means water managers will be putting a lot of hope on February and March. The winter rainy season typically ends April 1.
“We had a very wet October, then a fairly dry November, a wet December, and a dry January,” Null said. “We can hope for the pattern to repeat and bring us a wet February.”