California is poised to shatter an all-time weather record by notching the driest January-February period in recorded history across the northern Sierra Nevada.
The northern Sierra is crucial to statewide water supplies because it is where snowmelt accumulates to fill Shasta and Oroville reservoirs. These are the largest reservoirs in California and the primary storage points for state and federal water supply systems.
If February concludes without additional storms -- and none are expected -- the northern Sierra will have seen 2.2 inches of precipitation in January and February, the least since record-keeping began in the region in 1921.
That is well below the historical average of 17.1 inches.
The dry conditions have not been limited to the northern Sierra. January and February have produced 1.32 inches of rain in the city of Sacramento, ranking as the third driest since record-keeping began in 1850, said Johnnie Powell, a forecaster at the National Weather Service in Sacramento.
The only drier years in the state capital were 1889 and 1920. Had it not been for a leap year in 1920 -- which added a 29th day to February that year and 0.42 inches of rain on that day -- 2013 would rank as the second-driest, Powell said.
The average in downtown Sacramento is 7.9 inches of rainfall in January and February.
California began winter with a bang. November and December delivered about 200 percent of average precipitation in the Northern Sierra. It was enough to bring reservoir storage above average for that point in the season.
But the faucet has shut off almost entirely in the new year.
Maury Roos, chief hydrologist at the California Department of Water Resources, said a wet March or April could still turn the winter around. It has happened before, notably with a "Miracle March" in 1991 that reversed a similarly dry winter.
Absent additional big storms, however, it is likely the state's major reservoirs will end the summer below average, with potentially grim implications for boaters and water users across the state.
"The forecasts still don't look that juicy," said Roos. "If it stays totally dry, then the only water we have would be left over from the snow that's now on the hills."
January and February are normally the wettest months of the year in California. To essentially lose both from the state's water supply picture has prompted a lot of cold sweats across the state.
"We entered into this water year in really good shape; in fact, we were almost in flood-control conditions back in December," said Dan Nelson, executive director of the San Luis and Delta-Mendota Water Authority, which delivers water to a vast area of the San Joaquin Valley. "So to be in this situation now is pretty frustrating."
Nelson noted that farmers are beginning to make planting decisions for the year ahead. Many have scaled back their plans as a result of the poor water supply conditions.
To a certain extent, this is a response to reduced water diversions in the Sacramento-San Joaquin Delta that have been imposed over the past two months to protect the Delta smelt, a threatened fish.
But on Monday, the U.S. Bureau of Reclamation announced in an initial forecast that its agricultural water customers south of the Delta would receive only 25 percent of their contracted water amounts. This forecast is largely a result of the declining Sierra snowpack, which serves as a bank for summer water supplies.
Had the smelt protections not been necessary, Nelson said, the forecast probably would have been 40 percent.
The delivery forecast may increase if the final months of winter improve. But Nelson said many farmers can't wait.
"There will be a lot of land fallowed," Nelson said. "There's just not enough water."
The dry conditions have been caused by a persistent blocking ridge of high pressure off the coast that has diverted storms around the state. Powell called it "highly unusual" to see a blocking ridge stick around for two solid months.
"We just haven't had the storm tracks," he said.
Bill Patzert, a climatologist at the NASA Jet Propulsion Laboratory in Pasadena, said there is a larger cause: a phenomenon called the Pacific Decadal Oscillation, which is a periodic warming or cooling cycle in the North Pacific Ocean that can last for as long as two decades.
The oscillation differs from the El Nino-La Nina temperature cycle, which takes place in the equatorial Pacific over a shorter period, typically 18 months to two years.
The Pacific Decadal Oscillation has been in a negative or cool phase since 2000, Patzert said. When it began, he predicted 20 years of drought for California. That has largely been the case so far, he said, noting that rainfall in Los Angeles has been below average for seven of the last nine years.
In addition, another phenomenon called the Arctic Oscillation is also currently negative. This means cold Arctic winds have shifted south, causing the recent blizzards that have struck the Midwest. Sometimes these Arctic winds sweep through California, as they did in 2011, bringing massive snows to the Sierra.
"Sometimes we get lucky and they hit the West, but not this year," Patzert said. "They've mostly been east of us."
The prospects for more rain this winter are not good. The National Weather Service's Climate Prediction Center released a long-range forecast on Feb. 21, saying that the odds favor dry conditions across California and the Southwest through May.
Contact The Bee's Matt Weiser at (916) 321-1264. Follow him on Twitter @matt_weiser. ___