Tropical Storm Lee forms, forecast to become major hurricane

A new tropical storm midway between Africa and South America is set to become the season’s next major hurricane, organizing and intensifying markedly in the coming days as it churns west toward the Bahamas and the Caribbean. As it draws closer to North America, its path is highly uncertain.

But forecasters are unusually confident that the storm will become extremely intense — raising the stakes of the track forecast. The National Hurricane Center predicts that the storm, named Lee, will explosively strengthen into an “extremely dangerous” Category 4 hurricane by Saturday.

Naturally, the concern arises for any possible effects on land. Current projections suggest the storm could come perilously close to the northern Leeward Islands — the island chain that divides the Atlantic and Caribbean — before it turns northward out to sea.

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However, these projections could change, and there is reason for the East Coast of the United States to monitor this storm. Past hurricanes, such as Florence, Irma and others, were at first projected to go out to sea before model simulations changed and the storm track shifted west toward the United States. While this scenario is improbable, Lee is still about five to six days away from its closest approach to the Leeward Islands, and the forecast could shift.

Residents along the U.S. East Coast and the Canadian Maritimes should closely follow the system. Bermuda should also keep watch, although model projections indicate that it probably wouldn’t affect the island territory for at least eight or nine days.

Regardless of where the storm goes, the soon-to-manifest powerhouse system will probably burn through considerable ACE, or Accumulated Cyclone Energy — a figure that represents how much energy storms extract from warm ocean waters and transform into ferocious winds. That could help propel the 2023 Atlantic hurricane season into well-above-average territory, matching the recent expectations of many forecasters.

Tropical Storm Lee now

On Tuesday afternoon, Lee was a little more than 1,300 miles east of the Leeward Islands, moving west-northwest at 16 mph. That placed it midway between Gambia in Africa and Venezuela in South America.

Tropical Storm #Lee has formed in central tropical Atlantic - the 13th named storm of 2023 season (subtropical storm formed in January). 4 other years on record have had 13+ Atlantic named storms by Sept. 5: 2005, 2011, 2012, 2020. pic.twitter.com/kozmzJEMQy

The Hurricane Center noted that it was “improving in structure,” with thunderstorms flaring up near its center while sustained winds climbed from 35 to 45 mph. These indicators of intensification prompted it to upgrade the system from a tropical depression to a named storm.

Explosive intensification likely

During the second half of this week, the storm will drift over incredibly warm oceans. Sea surface temperatures range between 84 and 86 degrees. That’s high-octane fuel for strengthening.

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In addition, minimal wind shear, or a disruptive change of wind speed and/or direction with height, is predicted. That lack of wind shear means the storm won’t be knocked off kilter, and should have an easier time organizing vertically.

Moreover, the broad wind flow at the upper levels is clockwise. That works in tandem with the storm’s clockwise outflow, or high-altitude exhaust. The more air a system can evacuate away from its center aloft, the easier it is for it to ingest warm, humid air from below and intensify.

There aren’t many limiting factors to the storm’s strengthening in the coming days.

“It is becoming a question of when and not if rapid intensification (RI) occurs with Lee,” the Hurricane Center wrote. It is forecasting that Lee will be a Category 4 hurricane with 145 mph winds by Sunday.

Wild cards

US is *not* in the clear from what's likely to become Hurricane Lee, but a dip in the jet stream has many model tracks bending out to sea before reaching our coast. It's an undeveloped system 3000 miles & 7+ days out. Lots of ?s. Not an imminent concern, just need to watch trends pic.twitter.com/3Gl9KN60KG

The key question forecasters have at this point is where the storm “recurves,” or takes its northward turn. It could come precariously close to, or even impact, the Leeward Islands late Saturday into Sunday. While it’s probable that the storm will clear the islands to the north, they should remain on guard.

If the storm remains weaker in the short term, it may not “feel” the upper-level winds that would draw it northward, meaning it could continue heading west farther than models anticipate.

Thereafter, there will be two main features steering the storm: a clockwise-spinning high-pressure system to the east over the west-central Atlantic, and an approaching trough, or low-pressure system nestled in a jet stream dip, to the west. Troughs sometimes are able to capture tropical systems and tug them westward, which would bring it closer to the East Coast of the United States.

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The strength of that trough, and subsequent speed and timing of its trek across the Lower 48, is unclear. The upper-air dynamics that will give rise to the trough are offshore of British Columbia on Canada’s West Coast, so it will probably be a day or two before we have a better idea of how the trough will behave.

If the trough does manage to pull the storm to the west, the Canadian Maritimes or U.S. East Coast could be threatened by the storm around Sept. 13-15.

Past surprises

Hurricane Florence : a lesson in never assuming anything.

Full archive of the NHC 5 day cone. This was a “guaranteed fish storm” aka out to sea track.

Until it actually goes OTS, we watch them all. #95L pic.twitter.com/FVxfolAKuC

While most storms in Lee’s current position turn to the north before hitting the United States, some don’t. Florence, which brought catastrophic flooding to the Carolinas in mid-September of 2018, caught meteorologists off guard by simply not recurving. It was supposed to, based on forecasts; instead, it barreled into the Southeast and dropped up to 35 inches of rain.

The models remain very close with potential threats to the northeast Caribbean, the Bahamas, and the US, but it really is too early to tell if a potential threat will turn into an impactful reality. Below are not models, but past major hurricane tracks (cat 3+) since 1855.
We… pic.twitter.com/nyUhFJnATE

Then there was Irma, which lashed the Keys and Gulf Coast of Florida in 2017 as a Category 4 hurricane. It, too, continued west instead of having an earlier recurvature.

Simply stated, it’s too early to know whether Lee will threaten the Lesser Antilles or have any effects on North America.

Jason Samenow contributed to this report.

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