Peak hurricane season dawned Monday with a seething tropical Atlantic overrun with three hurricanes, two areas pulsing with cyclonic ambition and Florida out of the fray — for now.
While mid-August through mid-October is the busiest period for Atlantic hurricanes, Sept. 10 is the pinnacle — a time when warm water and low wind shear conspire in earnest to turn tropical waves into menacing storms.
This week, an atmospheric oddity is steering dangerous Hurricane Florence toward the east coast while different weather patterns tug at far-off hurricanes Helene and Isaac, spinning one harmlessly into the North Atlantic and shooting the other on an arrow-straight path into the Caribbean. Isaac, which was downgraded to a tropical storm overnight, is expected to restrengthen today.
Troughs and ridges steer upper-air flows like a snake’s slither across the Atlantic and are responsible for much of where hurricanes go. Only a few degrees of latitude or longitude can make all the difference.
“It’s a big ocean and each hurricane is responding to the weather flow that immediately surrounds it,” said James Franklin, former chief of forecast operations for the National Hurricane Center. “There are highs and lows all over the globe so the features steering Florence are different than Helene and Isaac.”
The National Hurricane Center is also watching areas near the Yucatan Peninsula and in the northeastern Atlantic for possible development. Each was given a 50 percent chance of forming over five days as of 2 p.m. Monday.
Florence, which intensified to a powerful Category 4 hurricane Monday, is being steered by two high pressure areas on a unique path toward the Carolinas and Virginia. Other hurricanes that were in Florence’s location have curved north away from the U.S. coast, according to records that date to the 1800s.
While one high is pushing Florence west, a seasonally abnormal area of high pressure is developing over the far northeast that will block the storm from re-curving north.
Trapped between the two clockwise flows, forecasters fear Florence will stall over the coast, dumping as much as 15 inches of rain on North Carolina and Virginia through the weekend.
“That’s the really scary scenario with Florence,” said Michael Bell, an associate professor for science at Colorado State University. “Certainly, we’re not expecting a Hurricane Harvey, which was almost eight days of rain. But even a few days of tropical rainfall can cause flooding.”
Florence’s threats of storm surge and freshwater inland flooding were called “life threatening” by the National Hurricane Center. Evacuations began Monday in coastal areas of the Carolinas with the official forecast putting Florence at the coast Thursday as a Category 4 hurricane with sustained winds of 140 mph and gusts to 165 mph.
South Florida’s escape so far from direct hurricane impacts is just luck — a quirk in weather patterns that are less predictable than global shifts such as El Nino, said Franklin.
“If Florence had formed 5 degrees south, then we may be worrying about it gently curving on a path more like Irma’s,” Franklin said.
While Helene and Isaac are at nearly the same latitude, their 20-degree separation in longitude is what’s taking them in sharply different directions.
Helene’s location closer to Africa means it is expected to be pulled between a subtropical ridge and upper-level trough, with the opposing clockwise and counterclockwise flows working together to shoot it north.
Isaac’s location farther west keeps it from getting caught in that area of weakness, allowing it to stay on a path west.
“These subtle differences are part of what can make long-range track forecasting difficult,” Bell said. “Sometimes there are saddle points in the atmosphere where a slight difference in position will mean heading in a different direction.”
Bell believes a periodic visit by the Madden-Julien Oscillation, or MJO, is also helping stir up the Atlantic. The MJO is a global player in the tropics — an eastward moving grouping of clouds, showers and winds that traverses the planet and returns to its starting point in 30 to 60 days.
Bob Henson, a meteorologist for Weather Underground, isn’t convinced the MJO is the culprit for the busy week.
“Sometimes the upper level conditions just align in a way that is very favorable,” he said.
Henson noted that there could be three tropical cyclones hitting U.S. states and territories this week with Typhoon Mangkhut sweeping over Guam, Hurricane Olivia hitting Hawaii and Florence reaching the east coast.
“It’s just the way the weather is unfolding,” Henson said. “It’s quite a week.”