When should NYC worry about Lee’s path? Making sense of hurricane forecasts.

Hurricane Lee rapidly intensified last week — growing from tropical storm to Category 5 in a matter of hours — as it crept westward across the Atlantic.

Weather buffs know that rapid intensification is nothing new for major hurricanes. It’s a natural part of their progression, and four of every five major hurricanes do it.

What’s changing is the pace.

“[Lee] basically jumped several Saffir-Simpson scale categories in the span of hours,” Jamie Rhome, deputy director of the National Hurricane Center, told Gothamist. “Normally, you would get about 30-to-35 miles per hour of intensity gain over the span of a day. [With Lee], you basically got double that.”

Lee’s strength is fluctuating between the major categories — between 3 and 5 — as it continues onward, and the storm is part of a pattern of a North Atlantic hurricane season that suddenly seems busier than an overbooked music festival in New York City.

Nine cyclones, ranging from tropical storms to major hurricanes, have developed in August and September so far — more than double what occurred earlier in hurricane season, which began on June 1.

In the spring, initial predictions called for “near-normal” hurricane activity this year, but in early August, the National Weather Service’s Climate Prediction Center upgraded the forecast to above normal. Hurricane numbers tend to ramp up as autumn arrives, but meteorologists said climate change is likely adding intensity to what’s been a wacky summer of weather.

As New Yorkers and New Jerseysans doom scroll pictures of Lee, refreshing to see the latest predictions of its path, they might wonder why the sudden shift in storm fortunes and strength. Hurricane Idalia also went through a noteworthy bout of rapid intensification in late August, before slamming into the South, causing four deaths and creating billions in damages.

They may also have questions about if or when the Northeast should start worrying.

Watch what happens live…with Hurricane Lee

It’s too early to tell if Lee will hit the U.S., but the storm is currently tracking in the general direction of the Middle Atlantic states. Rhome said from Tuesday to Wednesday this week, Lee is going to encounter weaker steering currents — the atmospheric forces that are shoving it along.

“Think of a leaf in a stream…that leaf is moving along, and then it hits like an eddy or a weak spot in the stream and just kind of slows down and stops — as it awaits the next thing to push it,” Rhome said.

The hurricane maps released since Friday show the storm’s path going from a steady northwest advance to barely crawling northward about 900 miles east of South Carolina.

And while some headlines seem mesmerized by the storm’s path and intensity, Rhome said the thing people may be overlooking is its physical size. On Friday, Lee measured about 200 miles across — in terms of the reach of its tropical storm force winds. Rhome said the extent of those strong winds will likely double in size.

“This might not sound significant to the average person, but it has extremely huge impacts on the ability of that system to generate these waves that I'm talking about,” he added, referring to swell waves that can travel hundreds of miles of ocean.

When those swells hit the coastline, they can cause dangerous rip currents, which are considered the third-leading cause of weather fatalities in the U.S.

Rhome predicted Florida could start feeling the swells over the weekend, despite the storm still being far from the U.S. mainland — and that the rip current effects would start spreading northward along the East Coast. The National Weather Service in Miami called for Lee’s swells to hit South Florida starting Sunday night.

“By midweek, we'll be smack in the middle of a big rip-current risk,” Rhome said.

He added that around Tuesday and Wednesday, forecasters will have a much better idea of Lee’s northward destination once the storm gets caught in new steering winds and begins moving again.

Why rapid intensification matters

Rapid intensification happens when the winds of a major hurricane increase 30 knots (or an additional 35 miles per hour) over a 24-hour period. Recent studies show the pace at which major hurricanes intensify has ramped up since the early 1980s— and that more of these events are happening globally over time.

This 40-year rise in rapid intensification is considered largely due to hotter sea temperatures, and the National Oceanic and Atmospheric Administration describes the pattern as a “possible emerging human influence” on tropical cyclones. But climate experts say more research is needed to determine the degree to which human activity and climate change are responsible. (Note: Climate change connections to the “overall” intensity of hurricanes are not in doubt — namely, scientists expect tropical cyclones to produce more hazardous rainfall and storm surge.)

All that said, Atlantic hurricanes are also experiencing more “extreme” rapid intensification over that same time period, Lauren Casey, a meteorologist at Climate Central explained. That’s when maximum sustained winds grow by 50 knots or greater, or about 58 miles per hour, in a 24-hour period.

“Taking Hurricane Lee, for example, the maximum sustained winds with Lee increased 80 miles an hour in 18 hours,” Casey said. “So doubling or nearly doubling the kind of standard rapid intensification rate or definition.”

The consequences of rapid intensification are clear. Climate Central states that “of the 56 tropical cyclones that have caused at least $1 billion in damage in the U.S. from 1980-2021, 73% underwent rapid intensification.”

Why the 2023 hurricane season started out as average (Spoiler: It wasn’t El Niño)

Warmer ocean temperatures are a big contributor to whether hurricanes develop in the first place — and the Atlantic’s waters have been abnormally toasty most of this year.

Part of this pattern was due to the Saharan air layer — large masses of dusty, dry air that spawn over North Africa and move west, high over the Atlantic ocean at about 5,000 to 10,000 feet above sea level. Those dust particles reflect back some sunlight before it can beam down and heat up seawater, said Matthew Rosencrans, the lead hurricane season forecaster with the Climate Prediction Center.

Doing so typically cools the Atlantic in June and July, but Rosencrans said there was very little Saharan air layer this year. Likewise, trade winds had lower than normal activity. Trade winds help stir up the ocean, churning up deeper cooler water, so when they’re absent, the sea surface is hotter.

“We saw some of those signals even back in March and April,” Rosencrans said. He added that Atlantic sea surface temperatures in June and July were the hottest they’ve been since 1950.

Yet the Climate Prediction Center’s annual outlook — released on May 25 — still called for a near-normal hurricane season. That was because El Niño — a climate event that manifests naturally in the Pacific Ocean every few years — often counteracts the birth of hurricanes far away in the Atlantic basin. Doing so can reduce the number of hurricane days by 60% and the intensity of storms.

El Niño typically causes a shift in the jet stream, creating extra wind across the section of Atlantic Ocean that stretches from Florida and the Caribbean over to West Africa. Air masses and thunderstorms rolling off of Africa often birth the hurricanes that reach North America.

“The Gulf of Mexico and then off the Carolina Coast — those are your favorite hurricane areas during July,” Rosencrans said. “And then August, September and October, it moves down into the core of the tropics — in the Caribbean.”

El Niño’s jet stream shift increases wind shear — the change in wind speed or direction between two different points.

Those points could be places — the difference in wind speed between Newark, New Jersey and New York City on a summer day. But when it comes to hurricanes, meteorologists are referring to “vertical” wind shear, or the difference in wind speeds and directions at 35,000 feet, versus 5,000 feet above sea level.

To understand why, think of a hurricane as being like a toy top spinning on a table between two people. If one person could blow just on the upper part of the toy, while the other blows on the bottom, the top might start to wobble until it ultimately falls over. That’s what happens when tropical cyclones try to develop and a lot of wind shear is present.

“It's not the hurricane itself, per se. It's the individual [smaller] storms that would kind of develop and then kind of morph into becoming a tropical storm,” Rosencrans said. “If those have a lot of wind shear, each one of those storms, they get tilted. They're not able to have what's called a feedback cycle.”

While El Niño emerged this year, it didn’t create the wind shear that kept hurricane activity average in June and July, say Rosencrans and other atmospheric scientists.

The big player was the heat dome that sat over much of the South for several weeks earlier this summer.

“When we're talking about a heat dome, what we're really talking about is a high pressure system in the upper levels of the atmosphere. And in the northern hemisphere, the [wind] circulation around a high pressure system is clockwise,” said Anthony Broccoli, a distinguished professor of atmospheric science at Rutgers University.

So if a heat dome centers over the South, the air flow around that heat dome is coming primarily from the Northwest and sliding over the East Coast, he said. That’s why temperatures in June were cooler than usual. And this airflow also set up a buffer that kept Atlantic hurricanes from traveling toward the U.S., pushing away their storm tracks and potentially keeping any storms out to sea.

“The wind shear was quite high across much of the Gulf of Mexico and Western Atlantic during July and even June, and right now the hypotheses are that it's linked to mid-latitude circulation, so things coming from the north and west,” Rosencrans said.

Both Rosencrans and Rhome said forecasters have not seen the influence of El Niño increasing the wind shear yet, but it could happen later in the hurricane season.

“El Niño is far more nuanced than people give it credit for,” Rhome added. “There's often a lag between the onset of El Niño and its impact or its influence on the season. So while El Niño is absolutely ramping up, you don't really feel the full effects of its suppression on storms until several weeks or several months after.”