Californians are watching anxiously to see if a “Miracle March” or “Awesome April” salvages the worst snowpack season on record thus far in parts of the Sierra Nevada. Snow that piles up across the mountain range from autumn through spring furnishes more than 60% of the state’s water for consumer and agricultural use. Winter precipitation across California hasn’t been too far from average, but even more than usual, most of it has fallen in a relatively small number of wet storms, mainly in the first half of December and early February. The San Francisco Bay area saw its first bone-dry January in more than a century of weather records. Outside of December, which brought 17 wet days, San Francisco (downtown) has seen just 14 days of measurable rain since October 1.
Figure 1. Departures from normal for the height of the 500-millibar surface (in meters), averaged for the period Nov. 1 - Feb. 14 in 2013-14 (top) and 2014-15 (bottom). The positive departures (red) correspond to the location of the “Ridiculously Resilient Ridge”. Image credit: NOAA/ESRL, via California Weather Blog.
In many ways this winter resembles 2013-14, when the “Ridiculously Resilient Ridge” just offshore steered wet systems well north of California. This year’s upper-level features (Figure 1, right) have tended to park a little bit east of last year’s, enough to extent record warmth to the Great Basin and allow the occasional big storm to push its way onshore while smaller, weaker storms spin across Southern California. Upper-level ridging has strengthened over the past month, leading Daniel Swain (California Weather Blog) to proclaim the arrival of the Ridiculously Resilient Ridge, Redux.
A cardinal feature of this winter’s storms from California to Washington is their unusual warmth, which means that much of the water ran off from the Sierra as rain rather than accumulating as snow. Reservoirs in northern California got a healthy boost, now running from 70% - 100% of average for this time of year, but central California reservoirs are still hurting, most of them holding half at best of the seasonal average. The “reservoir” of water within snowpack is in far worse shape from the Sierra Nevada north through the Cascades, with most sites reporting 25% or less of the amount of water typically stored in snowpack at this time of year. Figure 2 illustrates the depleted state of snowfall in Yosemite National Park at elevations of more than 8,000 feet.
Figure 2. On February 19, the NWS office in Hanford, CA, tweeted this photo of a nearly snowless landscape, taken in Yosemite National Park at an elevation of 8,100 feet. Image credit: Elizabeth Christie.
California is entering its fourth consecutive year of widespread drought, as measured by the U.S. Drought Monitor, which takes into account soil conditions and streamflow as well as precipitation. This is the third multiyear drought in California since 2000, and as Figure 3 shows, it’s the worst of the three in terms of the geographic extent of the most dire drought categories. Each year of consecutive drought magnifies the impact, as progressively tougher adaptations must be called on. Last month, California’s State Water Project anticipated being able to meet only 15% of the contracted water needs of its customers (which include 25 million Californians) in 2015. Most of the burden will fall on agriculture, the main user of water in California, but cities will be affected as well. David Behar (San Francisco Public Utilities Commission) told me that his agency has asked customers to cut 10% from their total water use. “That’s happened across our service area in a very solid way. I think we’re seeing people take this drought very seriously,” he said. "Even a 20% cut would be tough, but we could weather it.” The bigger question is what might happen in a drought lasting a decade or longer: ”Every drought we live through this century will be a dry run, no pun intended, for what we might see in the future.”
Figure 3. The percent of California’s land area at various stages of drought over the last 15 years, as defined by the U.S. Drought Monitor. Image credit: NWS/Hanford and drought.gov.
Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
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