The Sky Holds More Water Now. Debby Is Dropping All of It On the Southeast.

One of the fundamental truths of a warmer planet is that there is more moisture up in the atmosphere — a lot more. A piece of sky at 68 degrees F can hold twice the amount of water the same piece holds at 50 degrees. As the climate has warmed, that means there is on average a whole lot more moisture above our heads than there used to be.

Today, a bunch of it is coming down all at once.

Hurricane Debby made landfall on the Gulf Coast of Florida on Monday, before quickly being downgraded to a tropical storm. But the wind speeds with this particular storm are not the real issue; it’s the water. Flooding in Florida is already widespread as more than 10 inches of rain has fallen; at least four people have died in the storm, and more than 230,000 customers in the state are without power.

Debby will now spend the next two days meandering damply across the Georgia and South Carolina coastal regions, dropping an ungodly amount of rain. According to the National Hurricane Center, an area encompassing Savannah and Charleston is likely to see as much as 20 inches of rain through the next few days; a portion of that area may see up to 30 inches fall. In an average August, Charleston gets less than seven inches of rain in the whole month.

This is, technically speaking, too much rain. Flooding is already rampant in Florida, and it will be worse as Debby moves to the northeast. The NHC gives an entire swath of Florida, Georgia, and South Carolina, from the coasts to more than 100 miles inland, an “at least 70%” chance of flash flooding over the coming days. At a certain point, the water simply doesn’t have anywhere to go but up.

“The heavy rains and flooding currently predicted this week are historic in scope and scale,” said Kim Stenson, South Carolina’s Emergency Management Division director, according to NBC News. The NHC warns of “catastrophic flooding” across Georgia and South Carolina. Savannah’s mayor wanted people as far away as possible: “If you don’t have to be in these Savannah streets tonight — don’t.”

Savannah’s city manager Jay Melder went on: “This is a once-in-a-thousand-year potential rainfall event.” And as far as immediate public health and disaster messaging goes, that’s a good thing to remember — only, it probably isn’t, not anymore. What used to be a thousand-year event — meaning a flood or heat wave or drought with a 0.1 percent chance of happening in any given year in a particular place — is now something else. Maybe this particular event is down in the 1oo-year event range, or 50-year, or 25. Or if it’s still above that today, tomorrow it won’t be; warmer air, more water. Physics doesn’t lie.