5 takeaways from hurricanes Helene and Milton and what it all means for Long Island

Two powerful hurricanes rampaged through the southeastern United States in the past month, leaving behind catastrophic flooding, washed-out roads, downed power lines and more than 200 deaths across six states, from Florida through the southern Appalachians.

According to climate scientists, fierce, quickly accelerating storms such as Helene and Milton are developing more often as the planet heats up. Along the East Coast and northern parts of the North Atlantic, temperatures are running close to 9 degrees Fahrenheit above the 1971-2000 average, according to the National Oceanic and Atmospheric Administration.

This year’s hurricane season has been a ferocious one. Hurricane Beryl, which tore through the Caribbean, the Yucatán Peninsula and the U.S. Gulf Coast in late June and early July was "the earliest category-5 Atlantic hurricane on record," NOAA administrator Rick Spinrad announced in August. Milton was the fifth-largest ever in the Atlantic Basin.

So far, Long Island has been spared, and peak hurricane season is winding down. Matt Rosencrans, NOAA's lead hurricane forecaster, said Thursday that there's about a 30% chance of any tropical storm forming in the next few weeks, and a lower chance that such a storm could reach the Northeast. Officially, hurricane season in the Atlantic Basin continues through the end of November.

Here are five takeaways from Helene and Milton, and what these storms can tell residents of Long Island, a 118-mile-long land mass surrounded by water.

Hotter air, oceans generate bigger storms

Hurricanes form over tropical oceans, drawing their power from warm, damp air and warm ocean water.

Last year, NOAA records show, the Earth's average air and sea temperatures were the hottest since the agency began collecting data in 1850. This year is likely to break that record. And according to the Environmental Protection Agency, "Sea surface temperatures have been consistently higher during the past three decades than at any other time since reliable observations began in 1880." June 2024 marked 15 consecutive months of record-breaking global sea surface temperatures, according to NOAA.

Hotter oceans are generating more intense tropical storms. In the past 100 years there have been 40 Category 5 hurricanes in the Atlantic Basin; in just the past five years, there have been five.

Some of these big storms form in the Caribbean "then curve up the coast," said Brian Colle, professor of atmospheric science at Stony Brook University, who studies climate and coastal meteorology. "Those are the ones that affect Long Island."

Near-tropical temperatures in North Atlantic

Traditionally, hurricanes that develop in tropical waters weaken as they move north over cooler water. Hurricane Gloria, 1985's "storm of the century," was a Category 4 in the Bahamas, a 3 when it reached the Carolinas and a 2 when it reached New York.

But ocean temperatures are rising in the North Atlantic, including the waters around Long Island, as well as in the tropics.

Although there’s variation from year to year, the Atlantic is "very warm — in most cases, at near record temperatures," said Kevin Reed, a professor in marine sciences at Stony Brook and the founder of the university’s Climate Extremes Modeling Group.

"If we have above-normal temperatures in August and September off our coast, that can sustain the storms a little bit longer," Colle said. "Instead of a Category 1 or 2, it might arrive as a 3."

The last Category 3 storm to hit Long Island was the so-called Long Island Express of 1938, which killed about 60 people in Suffolk County, most in Westhampton Beach. Superstorm Sandy, the most destructive storm since, had been downgraded to a post-tropical cyclone by the time it hit the mid-Atlantic coast in late October 2012.

When stronger hurricanes move up the coast, Long Island’s particular geography — a long finger of land extending into the Atlantic — makes it more vulnerable, Reed said. "The way in which we stick out increases our chances of getting clipped by a hurricane," he said. And "there’s the risk of damaging storm surges from both sides," north and south.

Warm air brings more rainfall

Another effect of global heating: Hot air holds more water vapor, which then pours down at an alarming rate. Even regular thunderstorms are carrying and dumping more rain, as Long Islanders experienced in August when parts of Suffolk County were deluged with more than 10 inches of rain.

"We’ve looked at rainfall associated with specific storms or with the whole season," Reed said, "and we typically see anywhere from a 5 to 15% increase in rainfall rates, or accumulation — depending on the metric you’re looking at — due to climate change.

"Whether we can say this storm happened because of climate change" — Helene or Milton or another tropical cyclone — "that’s a very complex scientific question. But we can certainly say it’s wetter than it would have been in a world without climate change," Reed concluded.

Hurricanes are classified according to wind speed, but flooding actually causes more deaths and damage than the winds. With heavier rainfalls, the category numbers don’t necessarily align with the damage they can cause. Increasingly, hurricane flooding will come not just from storm surge at the coasts but from downpours.

Hurricane Henri reached Long Island as a mere tropical storm in 2021, but nevertheless dropped 4 inches of rain in short order. In a warmer world, Long Islanders living well away from either shore will be more at risk for hurricane damage. 

Rapidly intensifying storms

Both Helene and Milton quickly grew from relatively standard tropical storms to Category 4 and 5 hurricanes within a matter of days, though Milton had slowed to a Category 3 by the time it made landfall.

Climate scientists say it’s hard to predict if a newly developing storm will undergo this type of "rapid intensification" — defined as an increase in the maximum sustained winds of 30 knots (about 35 mph) or more within 24 hours. But experts say warmer air and oceans will bring more of these fiercely accelerating storms in the near future.

"There’s a very clear link between the likelihood that a hurricane can undergo rapid intensification and the temperature of the ocean below it," Reed said.

But it's warm temperatures deep in the ocean — hundreds or even a thousand feet down — that drive rapid intensification, as Rosencrans explained. "Those temperatures there are the real energy reservoir for these storms to intensify" as the high winds churn up the water from below.

The deep ocean in the Northeast is still cool, even as the surface temperatures are rising, so it's not likely a rapidly intensifying hurricane will make landfall in the Northeast, Rosencrans said. "You’re more likely to see a storm de-amplifying" but dropping "a wall of water" when it reaches the South Shore of Long Island.

More power after landfall

Historically, once a hurricane made landfall, it quickly "decayed," or weakened, as the roiling weather system was severed from its source of heat and moisture. But a 2020 study published in the scientific journal Nature found that the intense, rapidly accelerating storms seen recently are maintaining their strength much longer after they come ashore.

"In the late 1960s, a typical hurricane lost about 75% of its intensity in the first day past landfall," the researchers, Lin Li and Pinaki Chakraborty, found. "Now the corresponding decay is only about 50%."

Over the past 50 years, the time it takes for a hurricane in the North Atlantic to weaken after landfall has increased by 94%.

"The stronger a storm is when it makes landfall, it’s going to have a greater inland penetration rate," Colle said, "especially if it’s going at a good clip as [Helene and Milton] did. And so you take that swath of heavy rainfall and move it way inland."

Areas hundreds of miles from the coasts and never considered part of hurricane country — such as eastern Tennessee and western North Carolina during Helene — are now in the danger zone. A similar hurricane could easily churn across Long Island’s mere 23-mile width without losing any intensity at all.