Hurricane Debby is dangerously rainy because it formed over abnormally warm water

MARY LOUISE KELLY, HOST:

Hurricane Debby made landfall in Florida, has now weakened to a tropical storm. The most dangerous thing about the storm is the rain. Rebecca Hersher from NPR's climate desk explains why a hotter planet is partly to blame for flooding from Debby.

REBECCA HERSHER, BYLINE: Debby is carrying an absolutely tremendous amount of moisture. Some areas, especially in South Carolina, are forecast to get 20 to 30 inches of rain in the next few days. William Cogswell is the mayor of Charleston, S.C.

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WILLIAM COGSWELL: For perspective, we typically get between 50 and 60 inches of rain annually. So our system is not designed to handle this kind of volume in this short of a period. The risk of intense flooding cannot be overstated.

HERSHER: Which is dangerous. Flooding from rain kills more people in the U.S. than any other hurricane-related hazard.

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COGSWELL: This unprecedented amount of rainfall poses a real risk of life-threatening flash flooding across Charleston.

HERSHER: But why is Debby so rainy? Where'd all that moisture come from? Well, one reason is that Debby formed over abnormally warm water in the Gulf of Mexico. The water off the west coast of Florida is about five degrees Fahrenheit hotter than usual for this time of year. Warmer water evaporates more readily and that adds more moisture to storms. So why is the ocean water in the Gulf abnormally warm?

ANDREW DESSLER: We kind of know what the answer is.

HERSHER: Andrew Dessler is a climate scientist at Texas A&M.

DESSLER: The two primary things are obviously global warming and El Ninos.

HERSHER: The oceans soak up most of the extra heat that humans are trapping on earth by burning fossil fuels. And the El Nino that just wrapped up also makes the oceans extra warm. All of which means more moisture for hurricanes, which means more rain from storms like Debby. Rebecca Hersher, NPR News. Transcript provided by NPR, Copyright NPR.

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