File photo of Lake Powell from Alstrom Point, Utah. Federal forecasters predict the spring-summer runoff into Lake Powell will be only 43 percent of normal this year. The lake is the reservoir that supports the Colorado River’s Upper Basin.

Colorado River runoff that flowed like wine last year is sputtering this year, boosting future shortage possibilities.

A cutback in Central Arizona Project deliveries in 2019 is considered highly unlikely at best. But shortage risks increase dramatically in the following years, the U.S. Bureau of Reclamation says.

Federal forecasters predicted last week that the spring-summer runoff into Lake Powell will be only 43 percent of normal this year. That’s due in part to a poor winter snowpack season and an expectation that the next two months’ weather will be about normal.

If the forecast pans out, it will be the sixth worst runoff into the lake from the river’s Upper Basin over 54 years of record-keeping.

The odds of a normal runoff season are only 3 percent today, said Greg Smith, a hydrologist for the federal Colorado Basin River Forecast Center.

The center’s annual Lake Powell prediction in April is generally considered the year’s most crucial forecast, coming at the start of the runoff season.

Lake Powell sends its runoff into Lake Mead. There, water is stored for future deliveries for the $4 billion CAP. It provides drinking water to Tucson and Phoenix and irrigation water to Pinal and Maricopa county farmers.

The new forecast marks another turn in the highly irregular pattern of Colorado River flows.

Last year, spring-summer runoff into Powell was 114 percent of normal, following three years of runoff exceeding 90 percent. The high runoff provided the river enough water to survive this year’s very low runoff with minimal chances of shortage for 2019.

But overall, river flows have generally declined steadily since 2000. Lake Mead has dropped from nearly 1,214 feet in elevation at the end of 1999 to 1,082 feet at the end of 2017.

That’s due partly to continuing drought, and partly to a structural deficit between the amount of water people take from the river and what nature provides, state and CAP officials have said. A shortage will happen if Mead drops below 1,075 feet at the end of a year.

The steep increases in shortage risks through 2022 were triggered by this year’s bad runoff forecast and the “annual imbalance” between water supply and demand from the lake, said Dan Bunk, a Bureau of Reclamation hydrologist.

The runoff forecast gives Arizona water officials “grave concern,” said Tom Buschatzke, director of the state Department of Water Resources.

While continued conservation by CAP could keep the lake at 1,075 and above through 2019, California’s future use of Mead water remains a big unknown, Buschatzke said.

Lake Mead could lose a total of 175,000 acre feet of water controlled by California next year due to various plans by Southern California’s Metropolitan Water District, he said.

But if a river shortage looms more likely, and there’s no agreement among the three Lower Colorado River Basin states to conserve more water, the Metropolitan could pull up to 400,000 more acre feet from the lake next year, he said. That by itself would drop the lake 5 more feet than expected, making a shortage more likely.

Bill Hasencamp, manager of the Metropolitan district’s Colorado River program, acknowledged that if next year is dry in California, “we may have to take a substantial amount” out of Lake Mead, but 200,000 acre-feet is probably the most it would have to take. But if it looks like the drought plan won’t happen, “we may have to re-evaluate,” he said.

CAP manager Chuck Cullom offered a more optimistic picture. Because of continued conservation by water project users, “we expect to avoid shortage in 2019 and likely in 2020” if the weather and river runoff are favorable, he said in a written statement. Project officials expect to conserve up to 180,000 acre feet of river water this year.

The low runoff forecast comes after CAP and Arizona Department of Water Resources officials have been at odds for well over a year over how to manage the lake and river. Their disagreements dimmed hope for a three-state Drought Contingency Plan agreement to force additional conservation of river water beyond what’s already planned. Gov. Doug Ducey’s efforts to secure more state authority to conserve more water in Lake Mead have so far gone nowhere in the Legislature.

One thing looking increasingly unlikely is for another “Miracle May” to bail out the river system as it did in May 2015. Heavy rains in Colorado that month prevented a shortage that had seemed almost certain.

That year, the spring-summer runoff forecast in mid-April was 300,000 acre feet higher than the current forecast, hydrologist Smith said.

A Miracle May this year is “not out of the range of possibility but it’s a very low probability.”

Contact reporter Tony Davis at tdavis@tucson.com or 806-7746. On Twitter@tonydavis987