A trio of storms this week gave Northern California communities an inch or more of rain. The weather also brought welcome relief from the state’s long dry spell, but – no surprise – the drought is by no means over.
Despite a week of weepy skies and gray clouds, precipitation so far this season has been below average across California. In the northern Sierra Nevada, where the snowpack is a crucial water supply, precipitation as of Friday morning was about two-thirds of average for the period since Oct. 1. In Sacramento, rainfall is about 50 percent of average since July 1.
Even so, those numbers are an improvement over the same period last year, which says a lot about the severity of this drought, which is in its third year.
“In fact, we have right now more (rainfall) in the bag than we had until February of last year,” said Maury Roos, chief hydrologist at the California Department of Water Resources. “I wouldn’t hang my hat on it, except that it’s better to have it looking wetter than drier.”
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After years of drought, it is easy to forget what winter is supposed to feel like in California. Warm weather and light rain, when it comes, seem to have become the new normal.
In reality, said Holly Osborne, a meteorologist with the National Weather Service in Sacramento, we should be seeing wetter storms more often, based on historical averages. The wet season typically begins in October as storms cross the Pacific Ocean and soak the state with some regularity.
“Typically, we start gearing up and getting storms every week or so in October,” Osborne said. “As we get more into winter time, we see storms usually more frequently.”
Roos said five or six storms each winter are typically “atmospheric river” events, in which a narrow band of wind drives across the Pacific Ocean, bringing heavy tropical precipitation on an express train from the tropics to California. These are the storms that, throughout history, have caused the worst flooding in California. Just one can deliver 10 percent or more of California’s annual precipitation.
These heavy storms refill reservoirs and make slopes busy for Lake Tahoe ski resorts. But only two reached California last winter, Roos said, and none so far this season.
“The last two years have been real tough on the economy up here,” said Jo Abbott, a board member of the Lake Tahoe Ski Club Foundation, which provides financial assistance to young ski racers. “We have ski races for high-level athletes, but they had to go and chase the snow – they had to leave California and go to other places and race.
“I think it’s a hardship on the economy and on the families up here if your child is involved in ski racing. It’s a little more expensive year if you want your child to continue in the sport.”
Determining how much rainfall is needed to end the drought is difficult. That’s because drought is a subjective term based on many factors, including water storage in reservoirs, groundwater depletion, soil moisture, condition of vegetation and economic effects. But it will clearly take an above-average winter to reverse the drought, because all these categories have been trending downward for three years.
“If we just have one year around normal, or slightly above, it’s not going to end the drought,” Osborne said. “It’s going to take a lot, because we have a deficit of several years.”
California reservoirs have shrunk to record-low levels. Lake Shasta holds just 39 percent of its average water supply for this time of year. Lake Oroville is at 42 percent and Folsom Lake is at 59 percent. All three are below the water levels at this time in 1976, the driest year on record.
Some reservoirs could be refilled by a single average winter. For example, Folsom Lake holds about 1 million acre-feet of water, and the average annual runoff from its watershed is about 2.5 million acre-feet. Others, like Lake Shasta – the state’s largest reservoir – will require more than average runoff.
Groundwater aquifers also take a long time to refill and may not recover from even a single wet winter. The land is so dry that much of the rain from the first few storms will simply soak in, without running off into reservoirs or refilling aquifers.
“There’s a whole host of impacts you could be measuring,” said Paul Iniguez, science and operations officer at the National Weather Service office in Hanford. “When it does start raining, how do you start coming out of that really deep drought? It’s a very difficult question, and something that people struggle with across the country, basically.”
The wettest months in California are usually December, January and February. Sacramento averages 3 to 4 inches of rain in each of those months, which works out to about one good storm every week. If even one of these months turns out dry for the state, drought becomes likely.
Last winter, December and January were both dry in California, producing the longest continuous dry spell in state history.
On the bright side, winter has not yet officially begun. And on Thursday, the Climate Prediction Center at the National Weather Service released long-range predictions that suggest December is likely to be wet in California. But its three-month prediction, running through February, has no strong indication of either wet or dry conditions.
Abbott, who lives in Truckee, said the recent storms make her feel as if winter is returning to normal, even though they did not produce much snow.
“I have a very good feeling it’s going to be a great year,” she said. “We’re one of the top ski areas in the country. We have some of the top ski racers in the country. We definitely need a good year.”
Call The Bee’s Matt Weiser at (916) 321-1264. Follow him on Twitter @matt_weiser.