Scientists say a vast swath of warmer-than-normal water known as El Niño circulating in the eastern Pacific Ocean is poised to be one of the strongest of the past seven decades. It has the potential to play a key role in the formation of storms that could bring much needed rain and snow to California and at least ease the effects of the state’s severe four-year drought.
SPECIAL REPORT: FROM DROUGHT TO EL NIÑO
But there’s that one word – “potential” – that commands attention for the state’s residents, farmers and water regulators, according to weather experts. They add that it will take more than one good winter for the region to escape the grasp of a sustained drought.
A strong El Niño shifts the odds in our favor. But even if you go to Las Vegas and play craps with your own loaded dice, you’re not going to roll a seven every time.
Mark Halpert, NOAA Climate Prediction Center deputy director
“A strong El Niño refers to how much warmer than average the equatorial Pacific Ocean is,” said Mike Halpert, deputy director of the National Oceanic and Atmospheric Administration’s Climate Prediction Center in Maryland. “The one we’re seeing now is currently strong. … It’s among the top two or three events we’ve seen going back to 1950.”
The warmest versions of El Niño to develop in the past seven decades occurred in 1997-98 and in 1982-83.
“In both of those events, California saw a lot of rain and snow,” Halpert said. “The hope is that this one delivers a similar type of pattern.”
Even with an El Niño that last month had sea surface temperatures warmer than normal and on the rise, there’s no such thing as “a sure thing” that the Valley and neighboring mountains will get above-normal rain and snow this winter and spring.
“The conditions are what we’d call ‘favored,’ but you can’t guarantee anything about climate. There are an awful lot of things that go on in climate,” Halpert said. “A strong El Niño shifts the odds in our favor. But even if you go to Las Vegas and play craps with your own loaded dice, you’re not going to roll a seven every time.”
23.57Inches of rainfall in Fresno in 1982-83, a very strong El Niño year
20.16Inches of rainfall in Fresno in 1997-98, a very strong El Niño year
How much?Inches of rainfall in Fresno in 2015-16, expected to be a very strong El Niño year
Forecasters anticipate that this El Niño will bring a wet year for California, “especially in Southern California, but that’s not necessarily where we want it to be,” Halpert said, noting that most of the state’s water storage falls as mountain snow in Northern California.
“You can get too much, too fast sometimes, and that creates flooding and mudslides. And you got a taste of that recently,” he added, referring to October mudslides that trapped trucks and cars on Interstate 5 over Tejon Pass south of Bakersfield and on Highway 58 over the Tehachapi Pass between Bakersfield and Mojave.
History shows that El Niño tends to be about an even-money proposition when it comes to dropping substantially more rain and snow on the Valley and the central Sierra Nevada. Both of the previous “very strong” El Niño events, in 1982-83 and 1997-98, packed a wet punch across the entire state. But other El Niño events have been duds.
“There have been prior strong El Niño events that did not bring the Valley above-average precipitation,” said Brian Ochs, a meteorologist at the National Weather Service office in Hanford. “Back in around 1965-66 (considered a strong El Niño), Fresno got below-average rainfall and there was snowpack in the Sierra that was average to below average.”
Since 1950, El Niño patterns have occurred 24 times. In 12 of those years, annual rainfall in Fresno has been below the 30-year norm of 11.5 inches. In the other 12, Fresno saw more rainfall than the long-term average. And in the Sierra, at Kaiser Pass in eastern Fresno County, the precious water content of the benchmark April 1 snowpack was actually below normal in 13 of the 24 El Niño years, and above normal 11 times.
That historical context is important for people to remember this year. But forecasters are getting more comfortable with the odds of more rain and snow this winter as the equatorial Pacific keeps warming up.
“A 50 percent chance of above-average rainfall around here is usually pretty confident,” Ochs said. “We’re pretty confident that we’ll see more precipitation than recent seasons, especially the past four years.”
The Valley has been locked in a sustained drought, and the current conditions are considered “extreme,” measured by the Palmer drought index. A database maintained by NOAA’s National Centers for Environmental Information indicates that it would take between 125 percent and 150 percent of normal rainfall – between 15 and 18 inches – in the Fresno region over the next six months just to ease the current drought conditions to half the current severity on the Palmer scale. Even with a potent El Niño warming up in the Pacific, however, the NCEI suggests that the chances of getting that much rain are less than 20 percent.
What the future holds
The odds are even longer for bringing the drought to an “end,” a standard defined as improving the region to a -0.5 on the Palmer scale, as opposed to the -4 it is at now. To do so would take between 150 percent and 175 percent of normal rainfall – or a minimum of 18 inches, and likely much more – in the area over the next six months. The NCEI analytics place the chances of that happening at less than 11 percent.
Kevin Werner, the Seattle-based Western region climate services director for the National Centers for Environmental Information, said history can be instructive for people’s expectations for winter weather, with or without an El Niño.
Historically, we characterize the climate in the Southwest and California as long periods of dry punctuated by much shorter periods of wet, and sometimes extreme wet. … That climate tends to be one that’s highly variable and tends to see a lot of dry days, dry months or even dry years. That’s the normal.
Kevin Werner, National Centers for Environmental Information western region climate services director
“Historically, we characterize the climate in the Southwest and California as long periods of dry punctuated by much shorter periods of wet, and sometimes extreme wet,” Werner said. “Regardless of El Niño or climate change or anything else, that climate tends to be one that’s highly variable and tends to see a lot of dry days, dry months or even dry years. That’s the normal.”
“There are many other four- or five-year droughts in the paleo-climate data that go back to 1000 AD,” he added.
A scientific evaluation of tree rings, produced last year for the state Department of Water Resources, reinforces that historic climatic perspective. The width of trees’ annual growth rings tends to correspond to wet and dry years as indicated by river flows.
In the San Joaquin River watershed, an analysis of tree rings indicates that over the past 1,100 years, the region has endured 35 sustained droughts of at least four years in length before the current drought began. Five of those periods have occurred since 1900. The average length of those extended droughts was more than six years, and the two longest drought periods reflected in tree rings were 12 years, from 1450 through 1461, and 13 years, from 1471 through 1483.
Ready for rain
The people in charge of keeping Fresno’s streets and neighborhoods from flooding say they’re confident of being able to handle the rainfall if the Valley gets a string of winter storms.
“The pipeline collection system, our underground pipes, are built for a two-year intensity storm,” said Alan Hofmann, general manager of the Fresno Metropolitan Flood Control District. “It’s built to handle about a half-inch of rain per hour. It’s quite a bit of rain.”
Hofmann recalled one May storm that dumped about three-quarters of an inch of rain in a 15-minute period, “and it flooded intersections (on Blackstone Avenue) like crazy. But an hour after it stopped, the water went away into the basins.”
Most of the storm-drain basins in Fresno’s system can handle about 6 inches of rainfall.
“Our criteria is for us to have enough capacity for the largest two-day storm on record,” Hofmann added. “We’ve got 100 years of records, so we went in and found the largest two-day storm, so we know we can handle that.”
If multiple days of heavy rain start to fill the basins to their brims, a system of pumps can move water from basin to basin and eventually into irrigation canals that carry water to the countryside.
In the years since the last big El Niño winter in 1997-98, the flood district has continued to develop and enlarge its system of basins.
“Even in the 1997 El Niño, I can’t think of more than a handful of basins that went into the streets because they exceeded their capacity,” Hofmann said. “We have considerably more capacity today than we did in the 1997 event.”
Weather experts add that even a gully washer of a winter for the Valley and the adjacent Sierra cannot fully heal the region’s drought wounds. That’s because breaking a drought of this magnitude requires more than just rain falling by the bucket-load and snow burying the mountains.
“It’s possible in one year do a pretty good job of filling your reservoirs,” said Halpert. “But when you look at drought, you’re looking at time scales of short-term drought and long-term drought. … A long-term drought goes more into the groundwater. Over the last few years, your area has survived by draining the groundwater – and that’s not going to recharge in one year.”
The underground water table takes years to recharge, “and it doesn’t take nearly as long to drain it,” Halpert added. “That’s a challenge that folks in California are going to be dealing with for a long time.”