Hurricane Lee weakened dramatically from Friday into Saturday in the western tropical Atlantic, but it remained a powerful major hurricane. Lee is predicted to restrengthen and make a sharp northward turn around the middle of next week, and it could still make a run for Atlantic Canada or even New England several days beyond that.
As of 11 a.m. EDT Saturday, Lee was near the bottom of the category 3 range, with top sustained winds of 115 mph, and was centered about 350 miles east-northeast of the northern Leeward Islands. Lee continued to chug west-northwest at about 12 mph. There were no advisories for any Caribbean islands, as Lee is expected to pass well to their northeast. However, large swells and rough surf will become an increasing concern over the next several days, especially on north-facing coasts of the Lesser and Greater Antilles and toward the Bahamas and Bermuda and eventually toward the U.S. and Canadian east coasts.
Intensity forecast for Lee
Lee’s weakening from Friday into Saturday was the result of moderate to strong wind shear (around 20 knots) that injected dry air into the heart of the storm and disrupted the circulation that had vaulted to 165-mph category 5 strength from Thursday into early Friday. Though attention at that point was understandably focused on Lee’s sheer power, the new state-of-the-art high-resolution HAFS-B model, which is especially important for diagnosing intensity, correctly insisted that some weakening was in store.
Lee’s once-distinct eye was not apparent on satellite on Friday night, and the southwesterly wind shear was pushing a large share of Lee’s showers and thunderstorms (convection) to the northeast of the center. By midday Saturday, reconnaissance data showed Lee’s formerly small eye had enlarged to 12-18 miles wide, perhaps signaling an eyewall replacement cycle (ERC). On infrared satellite imagery, convection appeared to be re-wrapping around Lee’s eye, which may herald an imminent restrengthening.
The leading large-scale forecast models differ on how wind shear will evolve over the next several days, so there is more uncertainty than usual in Lee’s intensity forecast. If and when the wind shear diminishes, Lee could easily take advantage of near-record-warm sea surface temperatures (around 29 degrees Celsius or 84 degrees Fahrenheit) and ample deep oceanic heat content to reintensify, perhaps quickly. In its forecast late Saturday morning National Hurricane Center predicted Lee would gradually restrengthen back to category 4 strength by Monday and Tuesday, followed by another weakening phase. Both short- and long-term ups and downs in strength are certainly possible, given the constellation of factors battling it out with Lee.
Track forecast for Lee
Lee is expected to move west-northwest at a gradually slowing forward speed through Tuesday, driven by the clockwise circulation around the Azores-Bermuda High to the north. On this track, the system will pass a few hundred miles to the northeast of the northernmost Leeward Islands on Saturday and Sunday, sparing them any major impacts from wind and rain.
On Wednesday, Lee is expected to reach the western edge of the high-pressure ridge steering it and begin to feel the steering influence of a trough of low pressure moving over the U.S. East Coast. This trough is likely to turn Lee fairly sharply toward the north by Thursday, as predicted by NHC and depicted in Figure 1 above. This turn would keep the core of Lee away from the southeastern Bahamas and Turks and Caicos Islands, sparing them from any major impacts from wind and rain. NHC predicts about a 5-10% chance of tropical storm-force winds in these islands.
One of the bigger and more crucial uncertainties in the long-range forecast for Lee, as evident in Figure 1, is how far west the hurricane will move before it takes the anticipated midweek turn north. The exact location and timing of Lee’s turn — and how fast it is moving at the time — will strongly determine Lee’s subsequent future. Lee is expected to make its closest approach to Bermuda Friday. Nearly all of the model ensemble members keep Bermuda on the stronger right side of Lee, so at a minimum the island will have a substantial risk of receiving tropical storm-force winds and heavy rains from Lee’s outer spiral bands.
After Lee passes Bermuda, the hurricane poses a landfall threat to the Canadian Maritime Provinces, and perhaps to the Northeast U.S. as well. The Saturday morning runs of the GFS and European models and their ensembles were showing that a landfall as far west as Massachusetts or as far east as Newfoundland were possibilities; a landfall farther to the south along the U.S. East Coast was looking less and less likely. A recurvature out to sea without making landfall is also a good possibility. The steering of Lee late next week will be determined by a trough of low pressure to the storm’s west, over eastern North America, combined with a ridge of high pressure extending to the northwest from the Azores-Bermuda High. If the ridge extends far enough to the northwest, it will block Lee’s path to the east, resulting in the hurricane making landfall in the Canadian Maritime Provinces or New England on Saturday or Sunday. A weaker and/or more southerly ridge would allow Lee to recurve out to sea without making landfall. The strength of this ridge may depend critically upon how close Tropical Storm Margot gets and how strong it is (see below). If Margot is strong enough and close enough to Lee, Margot will weaken the ridge and engage in a Fujiwhara interaction with Lee — a situation where two cyclones rotate counterclockwise around a common center. It also remains unclear exactly how the eastern North American trough will evolve, as several impulses will be entering and lifting out from the trough over the next few days.
Figure 2 above shows the differences in both trajectory and forward speed from the Friday-night European, GFS, and UKMET ensemble runs extending out to Saturday night, September 16. In the operational (deterministic) runs, shown in dark lines, Lee’s location ranges from inland Nova Scotia (GFS, in green) to the subtropical western Atlantic (ECMWF, in red), and the westernmost ensemble members are still a day or more from any potential landfall. In short, this remains a high-uncertainty forecast!
One impact is guaranteed: Lee will bring a prolonged period of high surf and dangerous rip currents to the northern shores of the islands of the northeastern Caribbean beginning this weekend and to much of the east coast of North America beginning early next week. High surf will begin to affect the Canadian Maritime Provinces by Wednesday.
Tropical Storm Margot in no hurry to organize
Even though forecast models have been persistently bullish on the future of Tropical Storm Margot, the system is taking its time to strengthen over the eastern tropical Atlantic, almost 1,000 miles west-northwest of the Cabo Verde Islands. As of 11 a.m. Saturday, Margot’s sustained winds were 45 mph. Margot continues to build a large but disorganized field of convection (even larger than Lee’s) that may help pave the way for the strengthening long predicted by models. NHC is predicting Margot to gradually intensify and reach hurricane strength by Monday night as it angles toward the north-northwest over open water.
Margot may end up being a long-lived storm, but it is not expected to move anywhere near populated areas. Its main importance will be as a player in the future of Hurricane Lee, as Margot will be pushing into the south side of the Bermuda-Azores High next week, potentially weakening and reshaping the high in ways that could affect Lee’s path far to the west.
The next Atlantic wave
GFS and European model ensembles continue to suggest development by the middle of next week in the central tropical Atlantic of a tropical wave predicted to move off the coast of Africa early next week. This wave is not yet being highlighted by NHC in its Tropical Weather Outlook.
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