Hurricane Sam strengthened overnight, lurching to a high-end Category 4 storm as it picks up pace while getting ready to sideswipe Bermuda. A tropical storm warning is in effect there, where Sam’s outermost bands may bring gusty showers to the island Friday night into Saturday. Sam should become a potent extratropical, or nontropical, system by early next week.
Sam is the strongest hurricane on record so far east in the Atlantic Ocean this late in the calendar year, according to Phil Klotzbach, a hurricane researcher at Colorado State University.
Tropical Storm Victor is also whirring through the central tropical Atlantic. It should remain a storm before fizzling toward the start of the workweek while drifting northwest.
Sam has been the most energetic hurricane of the season, expending more ACE, or Accumulated Cyclone Energy, than any other system this season. ACE is proportional to storm strength and duration. Sam beat out Ida, a potent but comparatively short-lived storm, and also edged out Larry, which tracked all the way from the tropics to near Greenland.
Sam, meanwhile, is far from over, and is expected to remain a major hurricane through at least Saturday. Thereafter, it will gradually decline in strength, though will continue to rack up ACE.
That puts the season thus far into very active territory, with ACE running 47 percent ahead of average. We’ve already surpassed the amount of activity typically seen in an average season, and, with two months left to go, it comes as no surprise we’re on the verge of running out of “conventional” hurricane names. Only Wanda remains on the National Hurricane Center’s preset list for 2021; after that, a supplemental naming list will be used instead of the Greek alphabet as in years past.
As of 11 a.m., Hurricane Sam was a 150 mph high-end Category 4 storm. It was located 380 miles south-southeast of Bermuda, and was barreling north at 21 mph. While Bermuda is under a tropical storm warning, its most severe winds should remain well offshore. Hurricane-force winds extend outward 70 miles from the center, while tropical-storm-force winds stretch out as far as 205 miles away, potentially bringing them close to the island territory.
That wind radius has been increasing all week following “eyewall replacement cycles,” during which an eyewall is knocked off-kilter and withers and dies while a new eyewall forms around it, contracts and takes its place. Eyewall replacement cycles frequently result in a decrease in wind speed, but a broadening in wind field.
An overnight aircraft reconnaissance flight into the storm found flight-level winds of 169 mph. The Hurricane Center often extrapolates winds to the surface using a 90-percent rule, meaning they estimated maximum eyewall surface winds at 152 mph. In addition to extreme winds in the eyewall, wave heights topping 50 feet are expected in Sam’s core.
On infrared satellite, Sam was showing mixed signals apropos to its strength. Cloud tops were warming acutely, connoting less robust updrafts, or vertical motion, transporting heat and moisture into the sky. Ordinarily, that would signify the start of a gradual weakening trend. That said, Sam’s inner core remained intense and symmetric, with a crystal-clear eye. On Tuesday night, those aboard a Hurricane Hunter aircraft were able to see stars through the tranquil eye.
The broader structure of the storm appeared a smidgen lopsided, the result of changing wind speed and/or direction with height. But, so long as Sam’s inner core remains in balance, it will likely remain stronger than other intensity forecasts would permit.
Where Sam is going
Sam is curving north around the periphery of clockwise-spinning high pressure centered over the Azores. Because the high is far enough east, it will allow Sam to slip out to sea. Like Larry, it’s entirely possible that Sam’s precipitation shield doesn’t even graze the island of Bermuda. The biggest concern for land-dwellers associated with Sam will be wind gusts, rough surf and rip currents.
“Swells are expected to reach Bermuda and the Bahamas today, and then spread to the United States east coast and Atlantic Canada this weekend,” wrote the National Hurricane Center. “These swells could cause life-threatening surf and rip current conditions.”
Into early next week, Sam will begin passing over waters substantially cooler than 79 degrees, which will spell its demise as a tropical system — yet the energetic ball of spin and moisture will interact with the jet stream to produce a nor’easter-like storm over the open North Atlantic.
Aside from Sam and Victor, there are, fortunately, no other disturbances to watch in the tropics at this point. During October, wind shear begins increasing over the open Atlantic, which usually results in a downward trend in the number of tropical storms and hurricanes. After all, disruptive winds inhibit their ability to form. As such, the Atlantic’s “main development region” between Africa and the Lesser Antilles begins to quiet down this time of year.
Instead, the focus turns to the Gulf of Mexico and the Caribbean, where hurricanes can threaten in October and even November. Lobes of spin on the tail end of fall cold fronts can occasionally serve as the seeds for hurricane growth, and seasonably warm waters often in the mid-80s can help fledgling storms to intensify.
While no tropical threats are immediately visible on the horizon, two months remain within the “official” bounds of hurricane season — and, as last year showed us, it’s not over until the atmosphere says it is.