You might’ve noticed a conspicuous absence lately: rain.
In fact, with a scant few days remaining in the month, much of Northern California is on track for a record-dry January. The winter storms that had us scrambling in December have largely dried up, raising the prospect of a fourth year of drought. We had two big bursts that qualify as atmospheric river storms and then … crickets.
If this sounds somewhat familiar, flash back to the beginning of 2013, when, after a similarly soggy December, almost in sync with the New Year's ball dropping in Times Square, the tap suddenly shut off -- and stayed off.
It turned out that a big, bloated bubble of high pressure had parked itself over the West Coast and did not move. It caught the eye of Daniel Swain, then a 23-year-old doctoral student in climate science at Stanford University.
"It was going on and on, well beyond that maximum that we normally see and persisting over months," Swain recalls. "And not only over months but then recurring essentially over the course of two consecutive winter seasons."
Pockets of high and low pressure — meteorologists like to call them “ridges” and “troughs" — are a regular feature of California’s winter weather, moving through like waves.
"These pressure patterns in the upper atmosphere define wind flow," Swain explains, "and where the wind’s coming from defines the storm track and whether California receives its wintertime precipitation."
Or doesn’t. When we’re in a trough, the usual winter storm track in the Pacific is ushered right into California. But as Swain points out, "When it’s pushed farther north, as has been the case when we see these big ridges of high pressure occur, that storm track gets shifted well to the north, to British Columbia, to Alaska."
As weatherman Paul Deanno at San Francisco's CBS-5 put it, "It’s like a bouncer in the sky. Nothing makes it here with the ridge right on top of us."
Swain offers another image: "They’re like boulders in a small stream of water that deflect the flow of the water to either side of the boulder."
In this case, that stream is the jet stream, the high-level winds that are the conveyor belt for California’s rain and snow.
"And it’s kind of literally true," Swain says, "because these pressure patterns in the atmosphere are actually defining fluid flow, except in this case the fluid is air in the atmosphere rather than water in a stream."
So now, here we are again, with an obstinate ridge parked over us. Does this mean the Triple-R has returned?
"No," California State Climatologist Mike Anderson told me at a recent drought briefing. "This is very different from the pattern that set up last year, where we had a ridge that extended up into Canada, and was reinforced and lasted six weeks."
"The pattern we’re in now is more of a transient pattern where you may see a ridge develop but it may just as easily break down," Anderson continued. "In this case, the patterns that we see in the jet streams and the oceans are not all moving in the same direction to create such a ridge. The jet stream is in a position where it will push through and will have an easier time than it did last year."
"But this is a really critical time of year," he adds. And if this high-pressure stays parked into February? "Then we have to start thinking about the potential that we’re looking at another dry year, despite the fact that we saw so much rainfall in early December," Swain says.
Some other things we don’t know: whether this is some kind of “new normal” and if it is, whether human-induced climate change is to blame.
What we do know is that winter doesn’t last all year. January is the middle leg of a three-month stretch in which California typically gets half its precipitation (in the Bay Area, December, January and February are virtually tied for the wettest months on average). And while there have been “March miracles," at some point, time runs out.
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