Savannah's more-frequent flooding fueled by climate-induced extreme rain

Late-day thunderstorms and the accompanying torrential rain are rites of summer in coastal Georgia.

But as heat-trapping pollution from the burning of fossil fuels continues to warm the atmosphere, extreme rain events that were once relatively rare are becoming more commonplace.

On Saturday and then again Monday, rainfall intensities in Savannah reached levels that statistical modeling from the National Oceanic and Atmospheric Administration suggests the area should experience about once every quarter-century.

Both storms dumped as much as 4 inches of rain per hour on the city, Mayor Van Johnson said in his weekly press briefing Tuesday.

That was after more than 4 inches of rain already had fallen in the seven-day period leading up to Saturday, “which, as you know, primes the ground for flooding,” Johnson noted.

The National Weather Service estimated 12-hour rain totals of up to 6 inches in Savannah on Saturday and again Monday, a level the city is expected to hit once in a decade, according to NOAA.

Both events quickly swamped many of the city’s flood-prone streets. Saturday’s storm coincided with high tide, adding to the inundation near waterways, while Monday’s deluge hit Savannah’s core during evening rush hour.

Seventeen areas of the city experienced “extreme” flooding, Johnson said.

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Heat is on

Experiencing two flood-inducing storms in a three-day period is not as unexpected as it once was.

After all, Johnson said, 70% of Savannah’s most-extreme rain events ever recorded have been in the past nine years.

“Climate change is real,” he added.

Summers in Savannah are about 2 degrees warmer than they were a half-century ago. Through Monday, this month has been 3 degrees hotter than the typical July, according to National Weather Service records dating back to 1948.

That makes the coastal city more susceptible to flood-inducing rainfall, said Bill Murphey , Georgia’s state climatologist.

“A warming climate allows more water to evaporate into the atmosphere from the ocean and water surfaces, which can help intensify rain where it is falling,” Murphey explained, adding that conditions were especially ideal for Monday’s torrent.

That was happening in Savannah before the most recent storms.

The city already had hit projected once-in-a-decade one-hour rain totals three times over the past 10 years, with two of those – 3.69 inches in 2014 and 3.65 inches in 2017 – at or near NOAA’s 50-year standard.

Before this past weekend, Savannah also had experienced 50-year rainfall totals for a six-hour period three times over the last decade: 6.34 inches in 2014, 5.48 inches in 2016 and 5.52 inches in 2017.

Areas of Savannah likely hit that threshold again Saturday and Monday, according to estimates from the National Weather Service.

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Give and take

Like many symptoms of climate change, heat-induced rainfall can fuel seemingly counter-intuitive conditions.

“Precipitation patterns are changing, bringing flooding to some areas and drought to others, such that wetter areas get wetter and drier areas get drier,” Murphey explained. “Storms are indeed becoming stronger with more intense rainfall events, but rapid onset droughts are coming on quicker as well, especially during recent years.”

Much of Georgia is still recovering from such a drought that threatened much of state’s corn crop.

However, more periods of intense rain could be on the way for the Savannah area.

“In a nutshell we have ample southwesterly flow and deep moisture transport from the Gulf and Atlantic, so (there are) plenty of moist, unstable conditions for keeping (daytime summer) convection intact,” Murphey said. “As long as this pattern holds, expect more of the same, with the best chances of showers and storms in the afternoon and early evening.”

Those conditions heighten the possibility for more rounds of flooding in and around Savannah, the mayor cautioned.

“We know that wherever you are, if there's a lot of rain or a short period of time, it’s going to flood,” said Johnson, adding that the city has 16 storm-water improvements underway.  "Savannah, we know ... has always been low land and so we know that one of our challenges is rain. Most days we're able to handle significant amounts of rain (but) we are challenged to handle significant amounts of rain over repeated periods.”

That makes the city potentially primed for more flooding. Storms are likely daily from Thursday through the weekend, the National Weather Service said Tuesday.

John Deem covers climate change and the environment in coastal Georgia. He can be reached at 912-652-0213 or jdeem@gannett.com.

This article originally appeared on Savannah Morning News: Savannah's more-frequent flooding fueled by climate-induced extreme rain