California’s Water Supply for 2016: Context is Everything

The pivotal end-of-March snow survey results are in from California’s Sierra Nevada--and whether the results are good or bad depends largely on your point of view. The current snowpack is about average when viewed from long-term climatology, bountiful against the backdrop of four years of punishing drought, and paltry next to what other “super” El Niño events have produced. Manual measurement on March 30 at Phillips Station, near Echo Summit (elevation 7382 feet), found that the snow water equivalent--the amount of water held in the soggy 58.4-inch snowpack--was 26 inches, or 97% of the long-term average for the date. Manual readings are collected each month at Phillips Station, largely as a ceremonial exercise and media event, with the end-of-March numbers closely watched because the snowpack typically peaks in late winter or early spring. A much more comprehensive picture of snowpack emerges from daily monitoring by more than 100 automated snow gauges across the entire Sierra Nevada. As of March 30, the statewide snow water equivalent was 24.4 inches, or 87% of average for the date. The percentage was down to 82% as of Monday.


Figure 1. What a difference a year makes: the Sierra snow survey being conducted at the Phillips course on April 1, 2015 (left), with California governor Jerry Brown, and on March 30, 2016, with Frank Gehrke, chief of the California Cooperative Snow Surveys Program for the California Department of Water Resources. Image credit: California Department of Water Resources (left); AP Photo/Rich Pedroncelli (right).

The two states of California, drought edition
There is no question that California’s water supplies are in far better shape now than a year ago. The April 1, 2015, snow survey took place on bare ground (Figure 1), with no snow at all present. Statewide, the snow content was only 5% of average, the lowest value since records began in 1950. Water supply agencies across the state faced their first-ever mandatory water restrictions in 2015. This February, the state water board extended those restrictions through October, although they will be re-evaluated later this month. Some users aren’t waiting. Citing the above-average water levels of Folsom Lake, the San Juan Water District has removed water restrictions for its 160,000 customers in central California as of March 23. The district is now calling only for a 10% voluntary reduction rather than the state-mandated 33% reduction.

Meanwhile, as of March 17, agricultural users in California are projected to receive 30% of water requested from the State Water Project supply this year. This compares to 20% in 2015 and 0% in 2014. Despite the improvement, we can expect many farmers and ranchers to continue drawing from California’s largely unregulated groundwater, a practice that has led to subsidence so dramatic--up to 2 inches per month in some areas--that it is detectable by NASA satellites.


Figure 2. Percent of average precipitation for the 180-day period from 12Z October 6, 2015, to April 3, 2016. For the Pacific states, the water year is commonly measured starting on October 1. Image credit: NOAA Advanced Hydrologic Prediction Service.


Where it’s been wet
The 2015-16 El Niño has produced buckets of rain and snow; it’s just that they’ve fallen a few hundred miles north of where one might have expected. Precipitation this winter has broken all-time records in Seattle (43.33” from October through March, already just a half-inch away from setting an October-April record). As seen in Figure 2, it’s been considerably wetter than average in far northern California and slightly above average through much of the San Francisco-to-Reno corridor. An unusually large fraction of northern California’s rain and snow either fell as rain or melted during midwinter warm spells, which has led to healthy replenishment of the state’s large northern reservoirs.

Toward Los Angeles and San Diego, the winter’s moisture has been much more disappointing. Los Angeles is currently sitting just below half of its typical total rainfall for the water year to date, with a mere 6.59” recorded from October 1 through April 3. The four climatologically wettest months of the year--December through March--actually came in drier for downtown L.A. in 2015-16 than in 2014-15 (6.13” vs. 6.67”). Late-summer rains helped give a boost to the full water-year totals if we extend them back to July 1, as noted by Chris Burt in his recent round-up of March and water-year precipitation across California. (The U.S. Geological Survey defines October 1 as the water-year start nationwide, but there are variations across California.) From this point into summer, a typical water year produces another 1.5” or so in Los Angeles and another 1” or so in San Diego. With models suggesting that an active subtropical jet stream will move into Southern California next week, it’s quite possible April will end up wetter than average--although L.A. would need close to its wettest April and/or May on record just to have a shot at an average water year for 2015-16.

The caveats of probability
Because there have been so few strong El Niños in modern records (1950 onward), and only two “super” events (1982-83 and 1997-98), experts stressed that a wet winter in California was highly probable but not guaranteed. The biggest surprise is that Southern California got the short end of the stick, as strong El Niños tend to be most reliably wet toward the south. In a study led by Andrew Hoell (NOAA Earth System Research Laboratory), published in Geophysical Research Letters in January, a multimodel ensemble of climate simulations keying off a century’s worth of data found that strong El Niño events significantly raise the odds of a wetter-than-average year statewide, but particularly across Southern California. The study found a more-than-90% chance of getting above-average precipitation in Southern California during a strong El Niño, which we’ve had in place this winter.

We should see this winter’s SoCal outcome juxtaposed against Hoell’s study as a reminder that even unlikely events do happen, a point emphasized by NOAA seasonal forecasters last month. “The results for 1982-83 and 1997-98 were probabilistic,” Hoell told me in an email this morning. Within the 130 simulations of the period 1979-2014 carried out for his study, Hoell added, “there were model simulations for 1982-83 and 1997-98 that resulted in exactly the same precipitation patterns that we saw this season. Assuming 1982-83 and 1997-98 are appropriate analogs (which is up for debate) then this event fell within the realm of possibility.” Preliminary model-based studies by Hoell and colleagues suggest this winter’s sea-surface temperature anomalies from El Niño were not as effective as those in 1982-83 and 1997-98 in generating wet conditions in California. It will be fascinating to see what insights emerge from the data collected by the El Niño Rapid Response Field Campaign, which spanned much of the February-March period that left Southern California largely high and dry.

The surprising aridity of the last few weeks has extended well into the Southwest U.S. Phoenix hasn’t seen a drop of measurable rain since January 31, although the city is a long way from its record-long dry stretch (160 days, set in 1972). Near Las Vegas, Lake Mead has recorded its lowest water levels for any January, February, and March since the lake was filled in the late 1930s. Further up the Colorado River, Lake Powell was at just 46% of full capacity on March 17. The snowpack upstream was at 94% of average, reflecting heavier winter snowfall in southwest Colorado than in neighboring areas.


Figure 3. The ruins of the Hannig Ice Cream Parlor are shown in the ghost town of St. Thomas on August 3, 2015, in the Lake Mead National Recreation Area, Nevada. The town was founded in 1865 by Mormon pioneers at the site where the Muddy River flowed into the Colorado River. At one point, it had about 500 settlers. The town was abandoned in 1938 after the construction of the Hoover Dam caused the Colorado River to rise. The area was once submerged in 60 feet of water but became entirely exposed as severe drought over the last 15 years has caused Lake Mead to drop to historic low levels. Image credit: Ethan Miller/Getty Images.


Figure 4. Departures from average in the height of the 500-mb surface for the three months from January through March 2016. The northeast Pacific is dominated by upper-level troughing (purple)--typical of an El Niño winter--while hints of a weak ridge (yellow) extend from the eastern tropical Pacific through the southwestern United States. Image credit: NOAA/ESL/PSD Map Room.

How California is going to extremes: a new analysis
A paper published last weekend in Science Advances shows that the upper-level features that drive California’s highly variable water-year precipitation have trended toward greater extremes in recent decades, with the patterns leading to dry years on the increase while wet patterns have held steady (or perhaps even increased as well). Led by Daniel Swain (Stanford University), the study analyzes October-to-May circulation patterns over the northeast Pacific for the years 1949 through 2015, examining how each water year compares to the five wettest, driest, coolest, and warmest. One ominous signal is a robust increase in the strength and persistence of upper-level ridging along the West Coast. The most dramatic recent example is the Ridiculously Resilient Ridge (RRR) that helped intensify California’s drought and record warmth. The RRR is gone, at least for the time being, but a lower-key, lower-latitude version may have played a role in this winter’s Southwestern dryness.

“It is definitely interesting that this winter featured some relatively subtle but persistent subtropical ridging between California and Hawaii,” Swain said in an email. “This is what kept Southern California so unexpectedly dry during this very strong El Nino winter, and prevented Northern California from being even wetter than it was.” Swain added: “Even though there were many more periods of active storminess in Northern California this winter than during recent drought winters, there were still some remarkably, unusually long stretches of dry and inactive weather caused by full-latitude West Coast ridging--even if it lacked the "resilience" of the RRR.”

The full paper is available through open access at Science Advances. There’s also a summary at the California Weather Blog. We’ll be back with a new post by Wednesday at the latest. Meanwhile, if you’re in the Midwest or Northeast, bundle up--it looks like a very chilly week ahead. Millions of people are under freeze warnings that extend across much of the Ohio Valley and a large chunk of the I-95 corridor from Washington, D.C., to New York.

Bob Henson