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Steady Sam on last legs as hurricane forecasters shift attention toward United States

October can be a key month for ‘homegrown’ hurricanes

Hurricane Sam pulls through the North Atlantic on Monday. (College of DuPage)

For nearly two weeks, Sam — the most energetic and enduring hurricane of 2021 — has wandered the open Atlantic Ocean. But it will lose its tropical characteristics over the next day as forecasters turn their attention to waters closer to the Caribbean, Central America and United States, where storm development is more probable as we head into October.

Sam has had quite the history. Meteorologists first became suspicious of a tropical wave emerging off the African coastline Sept. 19; by Sept. 24, it became Hurricane Sam. Now, it’s been a hurricane for the past week and a half, spending upward of a week as a Category 3 or greater major hurricane as it churned through the open ocean.

The relentless storm has brought high surf and rip currents to Bermuda and beaches along the eastern U.S. coastline, but little fanfare otherwise as it remains a hazard only for mariners.

Now slowly weakening, Sam will soon become a potent extratropical, or nontropical, low-pressure system and retain hurricane-force winds as it whirs through the North Atlantic. It could eventually approach Greenland and Iceland late this week.

Meanwhile, forecasters are tracking two other systems. The remnants of Tropical Storm Victor, currently a tropical depression, are present midway between the Leeward Islands and the African coastline. They should disintegrate in the coming days.

A tropical disturbance set to meander off the East Coast this weekend is unlikely to become a named storm but could contribute to heavy downpours in parts of the Southeast and Mid-Atlantic as the week wears on.

October is ordinarily a time of year when the jet stream slides south, increasing wind shear, or a change of wind speed and/or direction with height, over the central and eastern Atlantic. That proves disruptive to established tropical storms and hurricanes, and inhibitive to fledgling storms working to become organized. Instead, the focus ordinarily shifts to the Caribbean and Gulf of Mexico, where “homegrown” storms, or systems close to the U.S. coastline, can develop.

How tropical storms and hurricanes have hit U.S. shores with unparalleled frequency

Sam, currently

After peaking last weekend with winds of 155 mph, Sam is a shell of its former self. Maximum sustained winds in the eyewall have declined to 100 mph, and Sam is a Category 2 storm. Nevertheless, the National Hurricane Center described it as a “formidable high-latitude hurricane” Monday.

The storm is centered about 800 miles east of Cape Cod, Mass., or 460 miles southeast of Cape Race, Newfoundland, and is charging northeast at 32 mph.

On satellite, the first signs of “extratropical transition” were visible. Sam’s eye is beginning to fill in as the system weakens, and dry air is encroaching on the storm’s backside. Sam is still very much tropical but probably won’t be in 24 hours.

Gradual weakening is forecast with Sam, and the storm will likely become a powerful nor’easter-type system by Tuesday evening. Its wind field will broaden, too.

The storm will make a left turn around a high-altitude disturbance in the North Atlantic. That interaction will yank Sam north and eventually northwestward into Thursday and Friday, at which point the diminishing system will have tropical storm-force winds. Gusts may top 30 mph in south coastal Iceland and southeastern Greenland.

Sam’s legacy on the 2021 Atlantic hurricane season

In addition to being the season’s strongest storm, Sam was also the most long-lived. Ida, which made landfall in Southeast Louisiana, was a major hurricane for only one day; Sam was for 7.75 days.

Sam was also the season’s most energetic, evinced by its hefty tally of ACE, or accumulated cyclone energy. That’s a measure of how much atmospheric energy a named storm expends on its winds and is proportional to duration and intensity.

The average Atlantic ACE for an entire season is 105 units. Ida racked up 10.8 units of ACE. Sam has racked up at least 52.6 units, good enough to rank among the top 10 most energetic Atlantic hurricane on record. At present, the 2021 season is running 60 percent ahead of average.

Sam’s longevity is unusual. According to Philip Klotzbach, a hurricane researcher at Colorado State University, Sam is the longest-lived Atlantic hurricane to form this late in the season since Inez in 1966. Klotzbach also says that Sam alone has accounted for more ACE than 14 full hurricane seasons since then.

Even neglecting Sam, the 2021 season has been a busy one; we’re only one storm away from running out of “conventional” hurricane names and dipping into a supplemental list. That said, the season has been jam-packed with a record nine “shorties,” or storms that have remained named less than two days, allowing us to quickly move down the list.

The 2021 season will go in the books as the sixth busier-than-normal Atlantic hurricane season in a row.

Looking ahead

There are no immediate disturbances that are next to watch for in the 2021 hurricane season, but we can look to climatology, or past historical tendencies, to get an idea of where to keep an eye out.

Autumn is a time when tropical threats brew closer to the United States and can occasionally develop from ordinary nontropical processes. That can include a lobe of spin on the south side of a cold front or the leftover swirl of a nontropical low-pressure system.

Such a “cutoff low” over the Tennessee Valley and Southeast will transfer some of its energy off the Carolina coastline by late in the workweek. While the National Hurricane Center gives slim to no odds of development, heavy rain from the system could eye parts of the Mid-Atlantic and Northeast late this weekend or next week.

Toward the middle of the month, large-scale weather patterns may conspire to bring a slight increase in the potential for tropical development.