Drought across the Western U.S. has forced California to ration water to farms. Hydroelectric dams barely work. The smallest spark — from a lawnmower or even a flat tire — can explode into a wildfire. While this region has always had dry summers, they’re supposed to follow a pattern that leads to relief with the arrival of the annual rainy season in November. But a break is no longer guaranteed.

Here are three forces desiccating the region.

A Second Consecutive La Nina Looms

Parched Western Winter

Last winter’s precipitation, as a percent of the 20th century average

Source: National Oceanic and Atmospheric Administration

Note: Precipitation data from December 2020-February 2021

The Climate Prediction Center just issued a forecast water managers in the Western U.S. didn’t want to hear. The latest report, released Thursday, puts the odds in favor of a second straight year of La Nina conditions in the Pacific Ocean.

La Nina tends to steer the storm track north of California, leaving most of the state and the Southwest parched. Last year’s La Nina is one of the reasons for the current drought. If the forecast had instead called for El Nino, the odds would have favored a wetter than average winter for California and the Southwest—something the region badly needs.

“If we want to see improvement of the drought across the West, the last thing you want to see is a back-to-back La Nina,’’ said Tom Di Liberto, a meteorologist with the National Oceanic and Atmospheric Administration. While it doesn’t always lead to a dry winter, it stacks the deck in favor of one.

La Nina is driven by a vast pool of unusually cool water near the equator in the eastern Pacific, just as El Nino is driven by warmer water in the same place. The consequences of La Nina aren’t all bad, since additional storms sent into the Pacific Northwest and Western Canada will help subdue devastating wildfires there.

The effects in Northern California are harder to predict. “California has the highest variability in precipitation anywhere in the U.S.’’ said Jeanine Jones, interstate resources manager for the California Department of Water Resources. “We cannot say what next year is going to be like.”

If the coming winter brings little rain and snow, the results will be troubling. California has already suffered through two dry years, leaving the soil so parched that what little snow fell in the Sierra Nevada Mountains last winter either evaporated into the air this spring or sunk straight into the dirt, leaving little runoff for rivers and reservoirs. Even with average winter rain and snowfall, runoff would remain low just because the land is so dry.