Discovery Files

New method predicts summer rainfall in the U.S. Southwest months in advance

Seasonal forecasts of North American Monsoon can help drought management

As reservoir levels dwindle in the arid southwestern U.S., scientists have developed a method to estimate summer rainfall in the region months in advance. Such seasonal predictions can help state and local officials make key reservoir storage and water allocation decisions earlier and support more efficient water management.

Current seasonal forecasts are not able to accurately predict summer rains across Arizona and New Mexico. However, a team of scientists found that a variable in those same forecasts -- the amount of water vapor in the lower atmosphere -- could, starting in April, predict precipitation trends between the months of June and October across a large part of the region, performing especially well in Arizona. They detailed their findings in a study in Geophysical Research Letters.

"The method is surprisingly successful, enabling us to look at individual catchments and correctly predict months ahead of time whether they will get above or below average rainfall," said Andreas Prein, a scientist at the National Center for Atmospheric Research who led the study. "The desert Southwest is one of the most water-stressed regions in the world, and water management decisions have to be made way in advance before rainfall occurs."

The findings will be put to the test immediately. The U.S. Bureau of Reclamation, which provided funding for the study, will evaluate the prediction system in the Southwest this year in both the Colorado River and Rio Grande basins. The study also received funding from the U.S. National Science Foundation, which sponsors NCAR.

Water scarcity is a major challenge for the U.S. Southwest. The region is contending with one of its most severe droughts in decades, even as temperatures are becoming hotter and water demands are increasing from a fast-growing population. Mountain snowpack supplies much of the water in reservoirs, but snow levels are diminishing, a trend that is expected to continue with climate change.

To help offset the loss of snowpack, water managers would like to make better use of rainfall from the North American Monsoon. This annual climate phenomenon, which results from southerly winds bringing moisture from the Pacific Ocean, Gulf of California and Gulf of Mexico during much of the summer and early fall, delivers approximately 60%-80% of the annual precipitation in the desert Southwest. But it is highly variable from one year to another, and scientists have not been able to accurately predict whether an upcoming monsoon season will deliver an average amount of rainfall or be particularly wet or dry.

Prein and his co-authors turned to the long-range forecasts of several leading weather models to see whether they could develop months-ahead predictions of the monsoon.