Calif. Dept. of Water Resources
SACRAMENTO — Much of California could be in for a drier winter if the building consensus calling for a weak La Nina pattern turns out to be accurate, a National Weather Service meteorologist warns.
The federal Climate Prediction Center issued its winter outlook on Oct. 19, noting that oceanic and atmospheric conditions appear to favor wetter-than-average conditions across the northern U.S. and drier weather across the South.
For California, similar conditions early last fall led to one of the wettest seasons on record. But since 1950, only 10 percent of weak La Nina winters have been wet, noted Cindy Matthews, an NWS forecaster in Sacramento.
Sixty percent of such winters turned out dry, including 2011-12, which was the first of five years of drought.
“The main point here is, just like last year, past events do not guarantee a future outcome,” Matthews said in an email.
In a La Nina, a mixture of atmospheric and ocean surface temperatures tends to steer storms toward the Pacific Northwest.
In such cases, the prospects for a wet winter tend to be better in far Northern California, where Shasta Lake and Lake Oroville anchor the federal Central Valley Project and State Water Project, respectively, than in the rest of the state. And they improve if the ocean surface temperatures that drive the direction of storms revert to neutral, as they did last year.
As it is, the CPC’s long-range precipitation outlook through April shows a better-than-average chance of a wet winter from Salem, Ore., and Boise, Idaho, northward, and a likely drier-than-normal winter from Bakersfield, Calif., and Las Vegas south.
Elsewhere, Matthews said people can “get the dart board out” as winter outlooks show equal chances of above- near- or below-normal precipitation throughout much of the West.
Much of California received its first significant rainfall Oct. 19-20, but most areas are off to a slower than normal start to the water year and much slower than in 2016, when Northern California had one of its wettest Octobers ever.
But there’s more water in reservoirs now than a year ago, as Shasta Lake is at 72 percent of capacity compared to 59 percent at this point in 2016, according to the state Department of Water Resources.
That abundance comes after most areas finished the 2016-17 water year on Sept. 30 well above their normal precipitation. For instance, Redding sopped up 47.7 inches of rain for the season, easily eclipsing its annual average of 34.6 inches, according to the National Weather Service.
Sacramento’s 33 inches of rain in 2016-17 was nearly double its annual average of 18.5 inches, according to the weather service.