Firmly in the grip of a historic statewide drought, Sonoma County needs a deluge to offset the lack of substantial rain for the past year and a half; but there is no such prospect on the meteorological horizon, with the once-promising forecast of a strong El Niño season fading away.
Over the past 18 months, from January 2013 to June 2014 — the driest such period in California’s records — the state has amassed a rainfall deficit that exceeds 19 inches, according to the National Climatic Data Center. Santa Rosa’s deficit — the difference between actual and average precipitation — is nearly twice as large, at 37.26 inches.
That means it would take more than double the city’s average rainfall, or about 70 inches, this coming winter and spring to balance the shortfall in local rain gauges and break the drought.
It’s been three years, however, since Santa Rosa even matched its 30-year average of 32.2 inches, and the likelihood of a wet season that significantly beats that mark has waned, leading weather and water officials to speculate that the drought is likely to persist through next year.
“Many parts of California have missed out on nearly a year’s worth of precipitation, and it will take a long time to gain back that deficit even in a best-case scenario,” Daniel Swain, a Stanford University environmental science graduate student, said recently in his California Weather Blog.
The combination of the long-term dry spell and record warmth has produced the “most severe drought conditions experienced in California in living memory,” he said. One set of data suggests that the drought is “more intense” than any other in the 20th century.
The U.S. Drought Monitor on Thursday added most of Northern California, including Sonoma, Mendocino, Lake and Napa counties, to the area classified as “exceptional drought,” the highest level of drought.
Overall, the portion of California experiencing “exceptional drought” jumped to 58 percent, up from 36 percent the previous week, when most of that area extended from San Francisco and the East Bay south to Orange County. Three months ago, just 25 percent of the state was in that category.
The Drought Monitor noted that California’s 154 reservoirs were short more than a year’s worth of water for this time of year and that soil moisture reserves are “nearly depleted” after three years of drought.
Meanwhile, the outlook for an El Niño that would make a difference has dimmed.
Earlier this year, the meteorological world was abuzz with forecasts for a strong El Niño developing in the fall and winter, said Jake Crouch, a climate scientist with the National Climatic Data Center, a National Oceanic and Atmospheric Administration agency.
But the Climate Prediction Center’s latest report, released July 10, said that computer models indicated a “weak- to moderate-strength” El Niño would peak during the late fall and early winter. The overall chance of an El Niño during that period is close to 80 percent.
“It sounds good,” said Jan Null, a San Francisco meteorologist. “We’re most likely going to have an El Niño.”
But the El Niño anticipated in 2012 did not materialize, he said, and the conventional wisdom that equates El Niño with abundant precipitation is shaky.
“Basically, anything can happen,” said Null, who has crunched El Niño data back to the 1950s. During the 22 El Niño events over the past six decades in Central California, including in the San Francisco Bay Area, rainfall has been above normal half the time and below normal the other half, he said.
In six strong El Niño seasons since the 1950s, Santa Rosa has twice received below average rainfall and four times had above average precipitation. The city’s 30-year rainfall average is 32.2 inches.
Rain season; Inches of rain
Santa Rosa's rainfall totals for the last four seasons, which run from Jul 1 to June 30, are: