Tropical Storm Debby to unleash ‘historic’ rainfall, ‘catastrophic’ flooding in Southeast: 5 things to know

Heavily populated and low-lying counties of South Carolina and Georgia are bracing for deadly floods as Tropical Storm Debby lumbers across the Southeast.

Since making landfall Monday morning in Florida, where it killed five people and left more than a 100,000 households without power, Debby has been downgraded from hurricane status. But the storm remains a serious threat.

In large part, that’s because of the heavy rain it’s dropping, like a colossal swirling sponge squeezing itself out over the already flood-prone coastal areas around Savannah, Ga., and Charleston, S.C.

Before dawn on Tuesday, the National Hurricane Center warned of “potentially historic heavy rainfall” across the coastal flanks of the two states, which “will likely result in areas of likely catastrophic flooding.”

For both regions, Debby represents the intersection of acute and chronic crises: the slow, implacable legacy of sea-level rise meeting the sudden outburst of storms fueled by the water-trapping fury of a heating climate.

Here’s what you need to know.

Where is Debby — and what impact has it had so far?

Debby is currently moving at the speed of a racewalking human above the coastal city of Savannah, which it hit late Tuesday morning, bringing what local officials called some of the worst flooding they had ever seen.

Debby, creeping northeast at just 5 miles per hour, has lost a lot of power since crashing into Florida’s Big Bend — the part of the state’s Gulf Coast that makes a 90 degree turn southwest of the state capitol of Tallahassee.

Like Hurricane Beryl, which left millions in Houston without power for a week following its July onslaught, Debby was a Category 1 hurricane when it made landfall.

However, Debby’s impact on the Florida power system was relatively minor compared to the crisis Beryl wrought in Texas, with just more than 100,000 households without power as of Tuesday morning — a reflection both of the storm’s positioning and years of work hardening Florida’s electric system , which have made its power grid the envy of its fellow Gulf Coast red states, Texas and Louisiana .

When it hit the Big Bend at 7 a.m. Monday, Debby brought winds in excess of 80 miles per hour. By Tuesday morning, those speeds had dropped to 45 miles per hour.

But the deadliest aspect of hurricanes isn’t their winds: It’s the flooding caused by their rain , as the Weather Channel reported.

And over the next three days, Debbie is going to bring a lot of rain.

How much water will it bring to the region?

“Potentially historic” amounts, the National Hurricane Center forecast Tuesday morning.

Debby’s center is expected to cross back over the Atlantic by Tuesday afternoon. The fact that it’s leaving land is not especially good news. Water, and particularly the unusually warm waters of the Atlantic, is hurricane fuel — all the more so as rising fossil fuel pollution in the atmosphere traps solar heat in the oceans in record levels.

According to the National Hurricane Center, Debby’s lack of a defined inner core will hinder its ability to gain strength over the next day — and it is expected to stay far enough from the heat of the Gulf Stream to avoid a significant increase in strength.

But by late Wednesday or early Thursday, as the storm banks back toward land, it should begin to strengthen again.

Some parts of coastal Georgia and the Carolinas can expect a staggering 25 inches of rain over the following three days, with less impacted areas receiving a still-considerable 10 to 20 inches.

That’s in addition to storm surges of up to 4 feet along the South Carolina coast, and up to 3 feet between the North Carolina state line and the Cape Lookout peninsula.

That surge means a piling up of ocean water that will make urban flooding worse as it overwhelms the ability of sewer systems and local waterways to shunt record rainfall into the oceans.

The risk of flooding underscores a somewhat paradoxical danger posed by Debby’s loss of power.

For all that hurricanes are seen as more serious than tropical storms — and although a Category 5 is more dangerous than a Category 1 — the difference between the classifications is largely a function of wind speed.

And by and large, while high winds wreck power systems, houses and infrastructure, it’s water that kills people — and more water falls when storms are slow.

Why are slow storms more dangerous?

Because they have more time to dump water over a given area.

According to the National Hurricane Center , rain-driven flooding killed two-thirds of the 41 people lost in 2001’s Tropical Storm Allison and 95 percent of the 68 people who died in 2017’s Hurricane Harvey and was responsible for 77 percent of the 22 deaths in 2018’s Hurricane Florence .

To be clear, “slow” in this case refers to how quickly the storm system itself moves — its “translational motion” or “forward speed,” in meteorology jargon — rather than the wind speed. For example, Hurricane Dorian hit Grand Bahama in 2019 as a Category 5 with wind gusts up to 220 miles per hour — but the system as a whole moved at a shambling 1.3 miles per hour .

Slower storms in the midlatitudes — which include most of the United States — appear to be a function of a climate being heated by fossil fuel emissions, according to 2020 research out of Princeton University.

Higher temperatures drive stronger “westerly” winds, which blow through the midlatitudes and toward the poles, effectively creating a brake on the movement of storms, the research showed.

And 2019 research published in Environmental Research Letters suggested that extreme precipitation from Harvey — a Category 4 storm that stalled above Houston and southeast Texas for several days in 2017 — was made about 20 percent worse by climate change, and that such stalling storms would become more prevalent.

For the Carolina Lowcountry, a slow Debby means more inches of rain, which likely means more floods, more infrastructure destroyed and, potentially, more deaths. If Debby were moving at, say, 20 miles per hour instead of its current 5, it would have far less time to dump water over a city like Savannah or Charleston.

“Tropical cyclones always produce heavy rain, but normally as they’re moving, you know, it doesn’t accumulate that much in one place,” Richard Pasch of the National Hurricane Center told WKYC on Tuesday .

“But when they move very slowly, that’s the worst situation.”

As things stand, Debby’s relatively pokey speed and the bad luck of its trajectory keeping it in an arc just off the Southeast’s coast over the next three days are set to more than make up for its tropical storm status, leading to equivalent rainfall damage to a Category 3 or 4 storm , according to Yale Climate Connections.

On Monday alone, Debby dumped 6 inches of rain on Savannah — more than the city received all last August. Still more rain is expected until the weekend.

That’s bad news for a region that already suffers from significant flooding risk even when it isn’t raining .

Why is the Southeast particularly vulnerable to flooding?

A mixture of geography, sea-level rise, a heating planet and a legacy of maladaptive development decisions.

To start with, the coastal Southeast is low, flat and at the constant intersection of both big waterways and tidal flux — all of which would lead to a significant flood risk even if sea levels weren’t rising as global temperatures climb.

Hotter water doesn’t just fuel hurricanes or melt ice off the poles — it also takes up more space than cooler water, which translates to sea levels creeping up into sewers and waterways, and also (because the geometry of stormwater drainage relies on seawater levels being lower than the water being drained) causes rain to pool and back up.

The southern U.S. is a global hot spot of sea-level rise, with levels in Charleston up an average of 7 inches since 2010, and water levels along the East Coast rising by about 4 millimeters per year.

That rise makes “sunny day flooding” a continual problem in cities like Savannah. And compounding the issue is the fact that, like East Coast cities from Miami to Boston, the Georgia city’s demand for water is worsening its drainage problem .

Every gallon of groundwater pulled from these underground aquifers causes the overlying city to sink, making it even harder to get rid of stormwater — a dynamic that, along the East Coast as a whole, is lowering cities by about 2 millimeters per year.

Put those trends together, and the effect is sea levels rising along the East Coast by more than 2 inches per decade — a rate that a 2023 Journal of Climate study suggests is accelerating , particularly in the Southeast.

Experts told The Washington Post that this slow, creeping erosion from saltwater — which blocks roads and corrodes sewers and asphalt even on sunny days — is a greater long-term danger than the more dramatic hurricanes.

“We are preparing for the wrong disaster almost everywhere,” Rob Young, a geology professor at Western Carolina University, told the newspaper.

“These smaller changes will be a greater threat over time than the next hurricane, no question about it.”

But as with so many consequences of planetary heating, the crises aren’t separate; they build on one another. Storms like Debby are sudden, potentially devastating disasters that are worsened by the ways they interact with the slow-moving one, as higher seas and bigger storm surges combine with more dramatic rainfalls to lead to higher water levels in the streets.

In the case of 2012’s Hurricane Sandy, the combination of rising seas and a hurricane led to billions of dollars in additional damage and more than a hundred thousand additional flooded households , according to 2021 study in Nature Communications.

Such dynamics don’t require a hurricane: For example, New York City saw some of its highest water levels ever in the wake of a winter storm last December, experiencing more than 4 feet of water above average, according to federal statistics.

In a troubling mark of the rapid pace of sea-level rise, that last 2023 nonhurricane storm drove up sea levels by nearly as much as 2017’s Hurricane Irma, which hit Charleston with the third-highest storm surge ever recorded.

Debby’s entrance into Georgia’s coastal region worsens this already unfavorable math with a new deluge. With the storm still 80 miles from Charleston — and Debby not expected until Thursday morning — water levels at federal coastal stations are already at more than 2 feet above average .

These trends suggest a grim outlook for the Southeast coast — so grim that they’ve prompted suggestions that the most vulnerable communities in the region should be abandoned . Coastal development is continuing, however, and a March study in Nature Communications found that government at all levels is effectively subsidizing the rising cost .

The study’s authors acknowledged that these subsidies are keeping communities from facing the price crash that would come from a frank reckoning with the reality of sea-level rise — and holding off climate gentrification. But they argued that these benefits are only temporary.

They only “delay precipitous declines as total inundation approaches,” the authors wrote.

Where is Debby going next?

The prevailing story for the next two days will be “deja vu,” with the storm’s limited movement leading to “the same areas getting impacted over the course of the forecast,” the National Weather Service wrote Tuesday.

It’s hard to know precisely where Debby will go and when, in part because it’s moving so slowly that it is much more subject to atmospheric conditions, which are causing it to wander more than a storm with a strong forward momentum.

But current federal predictions assume that after a wide 24-hour swing around Charleston, Debby will make landfall again near the city on Thursday morning.

It is then forecast to head up through the North Carolina Piedmont and the mid-Atlantic, where it is expected to dump 4 to 8 inches of rain, with some municipalities possibly getting as much as 12 inches.

From there, the cone of uncertainty spreads to encompass the entire Northeastern seaboard, potentially including severely flood-prone communities from Chesapeake Bay and the Delaware Valley to the Jersey Shore, Hudson Valley and Long Island — though the National Weather Service currently thinks the most likely targets are northern New Jersey and Central or Eastern Pennsylvania.

Current projections suggest between 50 and 80 percent of communities in that area — which butts up against the west side of New York’s metro area — could get an excess of 3 inches of rain by this weekend.

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