The same climate shift that is melting polar ice is depriving the Southwest of critical spring moisture — lengthening the time between the last winter rain and the first summer monsoon.

Two researchers at the University of Arizona have documented a decrease in late winter and spring storms in the Southwest, and ticd it to a previously documented northward shift in the storm track.

Stephanie A. McAfee and Joellen L. Russell will publish their findings in Geophysical Research Letters, a journal of the American Geophysical Union.

McAfee, a doctoral candidate in geosciences, said the wind shift and precipitation decrease are subtle, but capable of producing dramatic change in a region that receives so little rain.

Losing a storm or two each March and April lengthens the seasonal drought that puckers the cacti and dries out wildfire fuels, she said.

A widening gap between storms at that critical time would explain “at least in part, the increasing number of really catastrophic fires in the Southwest,” said noted fire researcher Tom Swetnam, director of the UA laboratory of tree-ring reseach.

“Those last winter storms, while not numerically important for our annal rainfalll total, are critical for the distance we have to go,” said Russell, an assistant prrofessor of geosciences at UA.