The annual monsoon that drenches the U.S. Southwest is late this year, raising the risk of wildfires sparked by another of the region's meteorological quirks: dry lightning.
While heavy rain isn't expected in the normally arid area until mid-July, thunderstorms continue to form high above the Earth. But the air below is so parched that raindrops can evaporate on their way down. The only things that strikes the ground are lightning bolts -- dry lighting.
The phenomenon, which also occurs in Australia, is blamed for many of the wildfires that charred more than 3 million acres in the U.S. West last year. The threat is particularly severe in Arizona, Nevada and parts of Southern California, where utilities plan to cut power during high-risk conditions. And the strikes are notoriously difficult to forecast because they just need a storm to pass over a sliver of dry air.
"The difference between a wet strike and a dry strike can be 500 feet," said Heath Hockenberry, national fire weather program manager for the National Weather Service.
Dry lightning can occur in any desert or semi-arid region that draws in moisture from nearby oceans. The dry air that makes rain drops evaporate also dries out plants, making it easier for lightning to start fires. One storm in 2008 sparked hundreds of fires across California in a single night that took weeks to tame.
"As global warming causes temperatures to rise, evaporation rates are increasing, causing increased drying of vegetation," said Jeff Masters, a meteorologist with Yale Climate Connections. "The risk of lightning-sparked wildfires will increase."
Dry strikes are hard to forecast because they require finding an arid pocket in the atmosphere that can be just 500 feet thick, Hockenberry said. A typical thunderstorm can rise to 50,000 feet above the Earth. So a dry layer that can cause all the trouble is just 1% of a raindrop's journey to the ground.
The monsoon that typically starts soaking Arizona and New Mexico this time of year may not arrive until mid-July. Across 11 Western states almost 67% of the land is abnormally dry and 46% is locked in some form of drought as of June 23, according to the U.S. Drought Monitor. Almost half of California is in drought.
While the monsoon may ease threats in the Southwest, "July is the entry point into the core of the western fire season," according to the National Significant Wildland Fire Potential Outlook issued last week. This means the threats will rise throughout California, the Pacific Northwest and the central and northern Rocky Mountains.
The fire threat eases for much of the West in mid-September, but for California the severity creeps up with the start of the fall gusts, marked by the Diablo and Santa Ana winds.
The fire threat comes as PG&E Corp., which serves Northern California, has emerged from bankruptcy after its equipment sparked devastating blazes in 2017 and 2018 and left it facing $30 billion in liabilities.
There are 44 large fires burning now in nine states, and the threat of dry lightning is increasing the risk. The National Weather Service issued warnings this week across the West.
There have been more than 24,000 wildfires so far this year, burning 1.4 million acres, compared with about 19,700 blazes for the first half of 2019 that consumed about 1,10,000 acres, according to the National Interagency Fire Center in Boise, Idaho.
There's hope that the risk of fires from dry lightning will diminish in the next few weeks. Some forecasts show a large high-pressure system emerging across the Four Corners region where Utah, Colorado, Arizona and New Mexico meet, Hockenberry said. That may draw moisture off the ocean and trigger the monsoon that will soak the ground and bring rain to accompany the lightning.
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