Drought continues to put the squeeze on the Southwest’s water supplies, with Colorado River runoff forecasts declining for the second straight month.

The details:

What: The April-July forecast for Colorado River runoff into Lake Powell is 74 percent of average, down from 80 percent in early March. It’s the second straight decline in the monthly forecast from a February prediction of 94 percent, said Brenda Alcorn, a senior hydrologist for the federal Colorado River Basin Forecast Center.

Why runoff is low: In February, the culprit was mainly warm weather, which triggers evaporation of river water. In March, the problem was more attributable to very dry weather in the southern half of the Colorado’s Upper Basin, including the Dolores River in Colorado and the San Juan River in New Mexico and Utah.

Why it matters: Much of the runoff into Lake Powell at the Utah border eventually makes its way to Lake Mead at the Nevada border. Mead is where much of Tucson and Phoenix’s drinking water is stored; it is pumped uphill to the two cities via the Central Arizona Project canal system.

What it means: The current low flows aren’t bad enough to trigger a shortage in CAP deliveries for 2017. But they make it more likely that a shortage will occur in 2018, said Chuck Cullom, CAP’s Colorado River program manager, and Bill Hasencamp, Colorado River resources manager for Southern California’s Metropolitan Water District. The feds currently predict a 54 percent chance of a 2018 shortage.

Why a 2017 shortage is unlikely: The U.S. Bureau of Reclamation, which operates the reservoirs, is scheduled to release extra water from Powell to Mead this year because Powell is storing more water than Mead. It will release 9 million acre-feet, compared to 8.25 million on average.

How a 2017 shortage could still happen: If it stays dry and runoff is very low into the Colorado from its tributaries in the Lower Colorado Basin, such as the Little Colorado, Paria and Virgin rivers. But it’s unlikely that it will be that dry and runoff will be that low, Cullom and Hasencamp said.