Heightened Fire Danger in Forecast for Desert Southwest

(July 27) -- It happens every year: The long, dry summer in the western U.S. results in a heightened fire danger. But two weather factors -- last winter's rainfall and a developing La Nina -- could result in a worse fire danger than normal in the Desert Southwest through the fall.

As is the case every summer, it's been dry in terms of a lack of rainfall and lack of humidity in the West. This naturally arid climate strips the ground and existing vegetation of their moisture more effectively than in more humid areas of the country to the east.

The inevitable heat of the summer weather also strips moisture from the ground and vegetation. That's even been the case this summer when temperatures in the Pacific Northwest, much of the Rockies and parts of the Desert Southwest have been cooler than average. If it had been hotter, current fire conditions might be worse.

Regardless, a large area of the West already has fire danger conditions that range from moderate to very high, and as it does every summer, the fire danger will continue to increase in August and September. Storm systems from the Pacific will not bring beneficial rainfall until later in the fall, while the scattered thunderstorms more likely during the late summer often bring a greater risk of fires caused by lightning.

(Image courtesy of the Wildland Fire Assessment System)

For Southern California and the Desert Southwest, the risk might become higher than normal this fire season.

The rain during last winter greatly reduced drought conditions, especially across California, but it also promoted the growth of natural vegetation. This increased vegetation has been drying in place during the summer, and it will provide potential tinder in the coming months -- more tinder than there would have been had less rain fallen in the winter and spring.

In addition, the developing La Nina is likely to result in drier-than-normal conditions in the Desert Southwest and Southern California in the upcoming winter. In other words, the storms needed to end the fire season are less likely to arrive, potentially extending the fire danger.

In the Pacific Northwest, though, where a La Nina brings the likelihood of more storms than average, an early arrival of winter storms could shorten the fire season.