Western Wildfires Spark Stronger Storms in Downwind States

That heat creates a strong difference in air pressure. Near the fires, air pressure is high. In the stormy Central U.S., air pressure is generally lower. As high pressure builds near the fire, the surrounding air flows toward lower pressure air, strengthening wind that already flows west to east.

Those stronger, westerly winds then carry smoke aerosols from western to central states. Along their journey, the winds pick up and carry atmospheric moisture, too. Now transported into storms brewing above the Central U.S., the greater concentration of moisture and aerosols kicks off a series of storm-strengthening reactions.

Like water droplets pooling on the needles of a redwood tree, the aerosols provide extra surface area on which water vapor can condense. As the water condenses, it releases heat. This added heat provides energy that strengthens storms. When a storm is sufficiently strong, the condensed water droplets freeze and start to form hailstones.

Inside the storm, strong updrafts repeatedly lift the hailstones up. Every second a hailstone spends inside the storm is another moment it can collect more cooled water droplets, creating a bigger and bigger hailstone, like string after string added to a ball of twine. Once the stones grow too heavy to be lifted by the storm’s updrafts, they drop down, dealing damage to crops, buildings, cars and occasionally people. 

Local wildfires in the Central U.S. also strengthened the same storms, according to the study, but to a less significant degree. Those wildfires are much weaker than their western counterparts. Fan’s group plans to look for similar connections in other regions.

The findings could help inform future severe weather forecasts, said Yuwei Zhang, first author of the new study and a postdoc in Fan’s research team.

“The cost of the storms we studied exceeded $100 million in damage,” said Zhang. “If we know that distant wildfires contribute to stronger storms, that information could bring about better projections, which might help avoid some degree of destruction.”

The Carr Fire, which claimed a quarter of a million acres in California, and the Mendocino Complex Fire, which dealt $257 million in damage and burned 280 structures in the same state, were among the fires under study.

Hail in a Changing Climate
Many areas in the United States will see increased wildfires—that means more aerosols from wildfires will be lofted into the Earth’s atmosphere and influence its climate in ways that scientists are working to understand. In addition to enhancing severe storms through wildfires, how will the warming climate directly affect severe weather, particularly storms that produce hail?

In a separate study published in the journal Earth’s Future, Fan explored how climate change could alter hailstorms in the Central U.S. Fan found that some storms are sensitive to climate change, resulting in more frequent large hail, while other storms don’t hold that same sensitivity. Those sensitive storms are associated with a large-scale weather pattern, which is different from that of the storms insensitive to a changing climate.

“By linking the impacts of climate change on hailstorms to the easier-to-model large-scale weather patterns, this study advances our knowledge of hailstorm predictability with important implications for risk management,” Fan said.