Hurricane Sam strengthens to a Category 4 storm

·7 min read

Sam underwent rapid intensification and became the seventh hurricane of the 2021 Atlantic tropical season on Friday morning. Sam has continued to rapidly strengthen and is now a Category 4 monster hurricane with sustained winds of 145 miles per hour. AccuWeather forecasters warn that there is a risk that the hurricane could turn toward the United States during the first few days of October after it navigates near the Leeward Islands early this week.

Sam was a Category 1 hurricane with maximum sustained winds of 75 mph on Friday morning, which was an increase of 40 mph in less than 24 hours. Meteorologists define rapid intensification as an increase in a storm's maximum sustained winds of 35 mph or more within 24 hours, and Sam surpassed that benchmark.

By Saturday morning, the maximum sustained winds had increased to 120 mph, making it a Category 3 storm. Once a storm reaches category 3 status, it is considered to be a major hurricane. By Saturday afternoon, Sam strengthened further to 140 mph, making it a Category 4 storm.

The cyclone's development wasn't showing any signs of slowing down any time soon. High-level clouds were seen fanning out away from the storm on satellite images, which indicated strong outflow and what meteorologists refer to as "venting."

"This venting signature is typical of an intensifying hurricane," AccuWeather Hurricane Expert Dan Kottlowski said. He added that the counter-clockwise orientation of the clouds gives Sam a "classic Cabo Verde hurricane," which is a type of hurricane that forms in the eastern Atlantic near the Cabo Verde Islands. This type of hurricane can go on to become some of the strongest, longest-lived hurricanes in the basin.

Hurricane Sam was over the open waters of the Atlantic on Saturday morning, Sept. 25, 2021, about halfway between the Leeward and Windward islands of the Caribbean and the coast of Africa to the right. (NOAA/CIRA)

Sam was located 940 miles to the east-southeast of the northern Leeward Islands and was moving west-northwestward at 8 mph. A sweeping area of Saharan dust associated with dry air was visible near Hurricane Sam in satellite images on Friday. Dry air and dust can often act as inhibiting factors for tropical cyclone development, but Sam's rapid intensification had so far defied those elements.

"There's still lots of dry air associated with or in the path of Sam, but what it's doing is, in a sense, creating its own environment for it to develop," AccuWeather Chief On-Air Meteorologist Bernie Rayno said. "None of the dry air is coming in toward the storm's center itself. Instead, it's being pushed outwards away from the system, and, with little wind shear, that is light winds aloft, we expect Sam to continue to develop."

As Sam strengthens, the hurricane will produce progressively larger swells that will propagate outward from the center. These swells will generate building seas ahead of the storm that will cause large waves and rough surf along the east- and northeast-facing shores of the Leeward Islands that have direct exposure to the Atlantic.

"Sam has become a major hurricane Saturday morning and is expected to continue to strengthen on Sunday," AccuWeather Senior Meteorologist Rob Miller said Saturday morning.

Sam is predicted to begin a gradual turn more toward the north, which will take the hurricane on a northwestward path on Sunday. Precisely when Sam makes that turn will be critical in determining exactly how much impact the strengthening cyclone will have on the Leeward Islands.

The sooner Sam begins to track to the north the more likely the hurricane's dangerous eyewall, with its destructive winds, will avoid the islands on Monday and Tuesday. With an earlier northward turn, the outer rain bands from the storm would bring brief heavy, gusty squalls with rough seas and surf to the islands.

However, if Sam waits a few days to begin curving northward, then there is the potential for harsher impacts to the Leeward Islands, including stronger winds, coastal erosion from pounding waves and flooding rain that can bring substantial risk to lives and property.

Beyond a potential close encounter with the Leewards, Sam will either continue to turn progressively to the north or continue along on a west-northwest track later this week, which would take the hurricane closer to the U.S.

At this point, AccuWeather forecasters say a due west path through the northern islands of the Caribbean seems unlikely. However, there is still potential for Sam to drift toward the U.S. or toward Bermuda and Atlantic Canada.

The weather pattern over North America could dictate how close Sam approaches the U.S., Rayno explained.

If a southward dip in the jet stream sets up off the East Coast, then there is a good chance that Sam will be steered away from the U.S. "But, if that jet stream dip sets up farther west or meanders westward, then there is room for Sam to get very close to the U.S. next weekend," Rayno said.

The latter scenario played out during a notorious East Coast storm nearly nine years ago. During late October 2012, a pronounced southward dip in the jet stream helped to steer Superstorm Sandy westward into New Jersey with devastating consequences.

Even though AccuWeather meteorologists are not suggesting that a similar outcome to Sandy will occur with Sam, they are raising concerns that there is a range of possibilities for the hurricane's future track, including one scenario that could cause Sam to track very close to the U.S. in early October. Because of that, forecasters are urging those who live along the East Coast to pay close attention to the forecast for Sam.

In addition to Sandy, which originated from the southwestern Caribbean and peaked as a Category 3 hurricane just south of Cuba in October 2012, few "S" storms have had a direct impact on the U.S. simply because the number of S storms is limited in the first place.

One of the more recent S storms to make landfall in the U.S. occurred in September 2020. Category 2 Hurricane Sally strengthened over the northern Gulf of Mexico before making landfall in the Alabama Panhandle on Sept. 16.

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"Even if Sam remains east of the U.S. late this week through next weekend, seas will first build westward through the north- and east-facing islands of the northern Caribbean, the Bahamas and later along the Atlantic coast from central Florida to Maine with building surf and increasing rip currents," Miller said.

That increase in seas and surf can begin as early as the middle of the week along the southern part of the U.S. coast then expand northward into next weekend.

A non-tropical storm associated with the dip in the jet stream could add to the intensity of the surf along part of the Atlantic coast, as well as produce its own zone of rain by late this week.

Should a more northward turn occur, the next potential landmass that could experience significant impacts would be the islands of Bermuda next weekend. And, beyond Bermuda, people living as far north as Atlantic Canada were being urged by AccuWeather forecasters to monitor Sam for potential impacts during early October.

Sam is the 18th named storm of the 2021 season, which is on pace for 20-25 named storms prior to the end of the year. If the remaining designated names for 2021, Victor and Wanda, are exhausted, additional storms will be named using a supplemental list prepared by the World Meteorological Organization.

Adria, Braylen and Caridad are the first three names on the list, which is being used for the first time after the Greek alphabet was retired earlier this year. The Greek alphabet had been used to name storms during only two Atlantic seasons: the hyperactive 2020 and 2005 hurricane seasons. By late September in 2020, the National Hurricane Center had already started using the Greek alphabet to name storms, with Beta forming on Sept.18.

In addition to 20-25 total named systems predicted this year, AccuWeather is projecting up to 10 hurricanes and five to seven major hurricanes. Not including Sam, there have been three major hurricanes thus far with maximum sustained winds of 111 mph or greater. There have been eight landfalls in the U.S., and there may yet be another direct impact or two on the U.S. before the end of this season.

Sam is the fifth storm to undergo rapid intensification thus far this year, following Elsa, Grace, Ida and Larry.

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