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Wet weather systems becoming increasingly rare in Southwest

RENO, Nev. – The southwestern United States has already begun a long-predicted shift into a decidedly drier climate, a new study looking at the last 35 years of weather pattern concluded.

What’s now considered a normal year of rain and snow in the Southwest is one-quarter drier than it was before the 1970s, according to the federally funded study posted online in the journal Geophysical Research Letters. The study said atmospheric conditions that bring the region most of its rain and snow will become more rare.

“This is something we expect from global warming,” study lead author Andreas Prein of the National Center for Atmospheric Research. “We see that in observations. It’s happening already.”

“Droughts are occurring there more easily,” Prein said last week.

Prein said the new data doesn’t prove climate change is responsible for increasing frequency and duration of drought. That’s because the researchers didn’t look for the link in this study, but will do that next.

The findings are consistent with earlier climate models that predict human-made global warming will move north a belt of drier high pressure that sits closer to the equator. The drying area includes San Francisco, Los Angeles, Las Vegas, Phoenix, Albuquerque, Salt Lake City and Cheyenne, Wyoming.

They drew their conclusions after identifying a dozen weather patterns from 1979-2014 that are typical for the weather activity in the contiguous United States and then looking to see whether those patterns are becoming more or less frequent.

They found that the three patterns that tend to bring the most wet weather to the Southwest all involve low pressure centered in the North Pacific just off the coast of Washington state, typically during the winter.

“This study is important as it connects the dots between long-term trends and changes in specific weather patterns that appear to be driving those trends,” said Katharine Hayhoe, a climate scientist at Texas Tech, who wasn’t part of the research.

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