January is a rainfall bust
How big of a problem is that?
Sunny skies. Balmy temperatures. Walks on the beach. Umbrellas back in the closet.
After a soaking wet December that ended fire season, delivered more than 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 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 mid
dle 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 eight days long, in 1957-58 and in 199495, said Null, who compiled the data. The longest was a parching 56 days in 20142015, 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 the longer the region 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.