At times in May, many hiking trails in Denver were underwater. In one storm, 2 feet of hail landed on Colorado Springs, and in another, 4 inches of rain fell in an hour. Overall, Colorado — and the U.S. as a whole — had the wettest May on record.

Here in Arizona, these storms bailed out a water supply that’s on the edge.

They delayed a water shortage for the Central Arizona Project that many officials had feared was imminent. The Colorado rainfall landed in the Upper Colorado River and its tributaries, and from there headed south and west, first to Lake Powell, then to Lake Mead. Much of that water will ultimately go into the CAP, a 336-mile-long system of canals and aqueducts that brings river water to Tucson and Phoenix.

The event shows how lucky Arizona has been when it comes to weather compared to California, which has been hit by severe water shortages due to drought over the past few years.

On May 1, the official federal forecast was for 42 percent of normal flows into Lake Powell for the crucial April to July runoff season. Lake Powell at the Utah border stores water for use in both the Upper and Lower Basins of the Colorado River, including Arizona.

The odds of the CAP’s first shortage were over 50 percent for 2016 and 75 percent for 2017, the U.S. Bureau of Reclamation said at the time. Lake Mead at the Nevada border dropped to its lowest level on record last spring for the third time in five years. Mead stores water for the CAP, although the canal itself actually begins at Parker Dam farther south near Lake Havasu City.

But after the storms hit, the ultimate April-July runoff total of 6.71 million acre-feet was more than double the May forecast and 94 percent of normal, the federal Colorado River Basin Forecast Center said. The bureau now says odds of a CAP shortage are negligible for 2016, 47 percent for 2017 and in the 60-65 percent range from 2018 through 2020.

A shortage would slash water deliveries to Central Arizona farmers and to recharge projects used to set aside CAP water for future years.

How did we avoid that? A partial cause could have been El Niño, the cyclical weather event triggered by periodic warming in sea surface temperatures across the central and east-central equatorial Pacific. An errant jet stream storm track that bypassed California in favor of the Great Basin and Colorado also helped, experts say. But weather forecasters and analysts at the National Oceanic and Atmospheric Administration aren’t ready to credit El Niño.

The CAP may have helped itself by making a deal last winter to pay five Central and Southern Arizona irrigation districts $5 million to leave some of their CAP supplies in Lake Mead for three years. In addition, the city of Phoenix left 15,000 acre-feet in Mead, and the CAP left 70,000 acre-feet there, that it would normally bring to Central Arizona for recharge, a project official said.

“We’re very fortunate,” said Chuck Cullom, CAP’s Colorado River programs manager. The river might have had enough water to avoid shortages due to the supplies left in Mead by CAP users even if the Upper Basin’s water deliveries had been reduced, he said — “but it would have been nip and tuck.”

Now, many experts believe there’s a 90 percent chance El Niño will stick around through next winter before petering out in spring 2016.

But there’s no way to say whether it will prevent a shortage once again.

Our second escape

This is the second time this decade that unexpected wet weather helped Arizona escape shortages.

In 2010, many water officials were fairly certain that the CAP would have its first shortage in 2012 after Lake Mead dropped to its lowest level on record in October 2010. Instead, the Upper Basin got an extraordinary amount of snowpack and rainfall the next year.

Lake Mead rose 46 feet between January and December 2011, compared to an average drop of 12 feet per year in most years since 2000 — putting off all shortage talk for three more years. But unlike 2015, 2011 wasn’t an El Niño year.

The La Ni
a effect

For the Colorado River, El Niño has a checkered history.

El Niño and its cold-water companion, La Niña, are the polar opposites jn what’s known as the El Niño-Southern Oscillation Cycle. That’s a scientific term describing temperature fluctuations between the ocean and atmosphere in the east central Equatorial Pacific Ocean.

In much of the western U.S., El Niño’s behavior is fairly predictable. In Arizona and the Southwest, numerous studies have found that El Niño years usually bring more rain or snow — and stream runoff, particularly in the wintertime. In the Pacific Northwest, El Niño is typically linked with diminished snowpack and streamflow, studies have found. In La Niña years, these trends are flipped.

But in the Colorado River Basin, El Niño’s behavior is much more complex and less consistent, wrote scientists from NOAA and the U.S. Interior Department in a 2001 study. The northern part of the Colorado River Basin is typically drier than average and the southern part is wetter than average, the study said. In the most recent nine El Niño years in the basin, three were unusually wet, three unusually dry and three about normal.

Once in awhile, though, a monstrous El Niño breaks the rules. The most celebrated example came in 1983, when monthly forecasts through the winter and early spring consistently predicted below-normal runoff. But in May and early June, the weather turned cold and snowy, then very hot. The river’s nearly full reservoirs couldn’t handle the runoff, resulting in massive flooding.

In 1997 and 1998, another big El Niño hit the river basin, but this time federal officials released more water than normal well in advance to prevent overflow and flooding.

No flood worries now

This year, Lake Powell has been at barely half full or a little more all year, whereas Lake Mead is now 37 percent full, so nobody’s worried about flooding.

“The 1997 event came in the middle of plenty. The 1990s had been very generous to much of the western U.S.,” said Klaus Wolter, a federal research scientist and climatologist. “We have had historical low flows since about 2000, so now anything helps.”

In May and early June, there was a storm every few days — six in all, said Nolan Doesken, Colorado’s state climatologist. Along the Colorado River area, unlike Denver, the storms weren’t exceptional — typically a half-inch to 1 inch of rain, sometimes 2 inches at higher elevations.

“But they came every week with no chance to dry off between them,” he said.

He credited a combination of El Niño bringing tropical storms up from the Gulf and Pacific coasts of Mexico, and a jet stream that inexplicably caused its storm tracks to bypass California and head into the Great Basin over Nevada and Utah and then into Colorado. Essentially, the jet stream transformed into spinning vortexes of energy that were fed by the El Niño-driven moisture, he said.

Wolter, of the National Oceanic and Atmospheric Administration, said the behavior of May’s storms is consistent with El Niño’s development and strength, but he’s not convinced El Niño was a cause until he does a detailed analysis.

The wet weather is circumstantial evidence, he said, not proof.

Mild optimism for 2016

For next winter and spring, the federal Climate Prediction Center is mildly optimistic for Colorado, with a map showing bands of light green indicating wet weather stretching across the state.

For the fall, the weather forecast bodes well, since it’s unprecedented for tropical storms from the Pacific to bring rain to this region as early as they did, Wolter said. He expects at least one or two tropical storms this September and October benefiting the San Juan Mountains and central Colorado.

In the winter, Colorado’s high mountains often miss out on precipitation. But if they get 90 percent of normal snowpack or greater by the end of February, there’s a good chance of precipitation and runoff increasing into the spring, Wolter said. If El Niño precipitation continues into spring, odds of runoff increase in April.

But it would be foolish to count on that, the Central Arizona Project’s Cullom said.

“Expecting to hit the hydrology lottery every three years,” he said, “is not a sustainable strategy.”