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‘Dry Thunderstorms’ Are Forecast for Montana, Raising the Wildfire Risk

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What’s a ‘dry thunderstorm?’ They’re a growing concern during wildfire season.

July 19, 2021, 1:20 p.m. ET

A forecast for thunderstorms would seem to be a good thing in a region like the Northern Rockies that is suffering from high temperatures, which were set to peak on Monday.

But along with providing some relief from the heat, the storms predicted for Montana and other parts of the region early this week will also raise the risk of wildfires.

A dry thunderstorm, like those in the forecast, produces little or no precipitation at the surface, according to the National Weather Service. These storms can produce rain just below the clouds, but because the air is so dry below, it evaporates before reaching the ground.

The drier the storm — particularly when paired with dry vegetation — the more efficient it is at igniting fires. And the lack of rain means any blazes sparked by lightning strikes can spread more easily.

Dry thunderstorms are rare, said Cliff Mass, a meteorologist and professor of atmospheric sciences at the University of Washington. “Most thunderstorms have some precipitation reaching the ground,” he said. “They virtually all have rain, but how much rain reaches the surface can vary.”

Climate change is a major factor in the growing impact of lightning strikes, because large parts of the West are becoming more dried out. A lightning fire that might not have spread so quickly decades ago can now leap more readily across the landscape of dry vegetation.

Not all of the storms forecast for the West this week will be dry. Monsoonal rain was expected Monday in Salt Lake City and other parts of Utah, as well as in southwest Wyoming. Similar storms were predicted in Boise, Idaho, through Wednesday.

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