Hurricane Sam surged back to Category 4 strength on Tuesday as it churned across the central tropical Atlantic, well away from the Leeward Islands. Sam could complete an entire week at major hurricane strength (Cat 3 or stronger) by Saturday, when it will be passing east of Bermuda.

At 11 a.m. EDT Tuesday, Sam’s top winds were 130 mph, making it a low-end Cat 4. Sam weakened to Cat 3 strength on Monday during an eyewall replacement cycle, a process in which a hurricane’s eye degrades and is replaced by a larger concentric eye, over about a day’s time. An eyewall replacement cycle usually results in a larger hurricane, as the wind field gets spread out … exactly what’s happened with Sam. The radius of hurricane-force winds expanded from 28 miles on Monday morning to 40 miles on Tuesday morning, and the radius of tropical storm-force winds grew from 104 to 127 miles.

Sam’s strength and longevity is helping it rack up an impressive total of accumulated cyclone energy, which takes into account the duration and peak winds over the lifetime of a tropical cyclone. As of Tuesday, Sam had already achieved more than twice as much accumulated cyclone energy as Hurricane Ida.

Straightforward forecast for Sam

The long-term outlook for Sam is remarkably straightforward, with little change since Monday. Sam’s new eye was sharpening on satellite imagery at midday Tuesday, and it’s possible that Sam will strengthen further through the day. Rapid intensification is not expected. The atmosphere around Sam is on the dry side, with midlevel relative humidity only around 45-50%, so Sam’s rebound could easily be interrupted by an intrusion of dry air.

Overall, Sam likely will hold its own as a potent category 3 or 4 hurricane until at least Friday as it moves steadily northwest, angling more northward by late in the week. Wind shear will remain light to moderate (5-10 knots), and Sam will have sea surface temperatures of around 28 degrees Celsius (82°F), with warm waters extending well below the surface.

As Sam moves steadily closer to Bermuda later this week, it will be swept up by a strong upper-level trough of low pressure parked across the Canadian Maritimes and New England. The trough is expected to keep Sam just east of Bermuda and well away from the U.S. East Coast, although there is a modest chance Sam will angle slightly leftward and reach Newfoundland or even Nova Scotia, as predicted by a few members of the GFS and European model ensembles. Increased wind shear and cooler waters will eventually trigger Sam’s conversion into a powerful post-tropical storm over the far north Atlantic.

Disturbance 91L in eastern tropical Atlantic expected to develop

An area of low pressure located in the eastern Atlantic near 9°N, 33°W, several hundred miles southwest of the Cabo Verde Islands, was headed west at between 5 and 10 mph on Tuesday afternoon. Satellite images showed that this low, designated 91L, had changed little since Monday, and was poorly organized with only a modest amount of heavy thunderstorms.

Figure 1. Tropical disturbance 91L (center), tropical disturbance 90L off the west coast of Africa (right), and Hurricane Sam (upper left), as depicted in visible satellite imagery at 10 a.m. EDT Tuesday, September 28, 2021. (Image credit: RAMMB/CIRA/CSU)

The low’s position so close to the equator will slow development, as will the fact that it must separate itself from the Intertropical Convergence Zone (the band of semi-permanent thunderstorms that circles the globe near the equator); the separation process is often slow to occur. However, all other factors the next three days appear favorable for development: light to moderate wind shear of 5-15 knots, warm ocean temperatures of at least 29 degrees Celsius (84°F), and a reasonably moist atmosphere. But wind shear may increase by late in the week, hampering development. The low has strong model support for development and is predicted to move west to west-northwest at 5-10 mph over the next week, potentially arriving in the Lesser Antilles Islands in seven to eight days. However, 91L may never make it to the islands if it gets entangled with disturbance 90L to its east, which is moving at a faster speed and may intrude into 91L’s space.

In its 8 a.m. EDT Tuesday Tropical Weather Outlook, the National Hurricane Center gave 91L two-day and five-day odds of development of 70% and 80%, respectively. The next name on the Atlantic list of storms is Victor.

Tropical wave off coast of Africa expected to develop

NHC has designated a tropical wave off the coast of Africa as 90L. Satellite images showed 90L slowly growing more organized on Tuesday afternoon, with a moderate amount of spin and heavy thunderstorm activity. The wave was moving west-northwest at 10-15 mph and was at a very low latitude, near 7°N. This position so close to the equator will slow 90L’s development, but all other factors over the next three days appear favorable for development.

90L has strong model support for development, but the models show a mostly west-northwest track for the wave, which would carry it into the central Atlantic where it would not threaten any land areas. In its 8 a.m. EDT Tuesday Tropical Weather Outlook, NHC gave 90L two-day and five-day odds of development of 80% and 90%, respectively.

The next name on the Atlantic list of storms after Victor is Wanda – the last name on the 2021 list of storms. If 90L becomes a tropical storm south of latitude 7.7°N, as suggested by the 6Z Tuesday run of the HMON model, it would be the southernmost tropical storm on record in the Atlantic, beating out a storm from way back in 1902.

Peter may regain its name

After petering out last Wednesday, Tropical Storm Peter is making a bid to regain its name over waters about 350 miles east-northeast of Bermuda. Ex-Peter will have marginal conditions for development on Tuesday as it moves northeast into the open central Atlantic at roughly 10 mph, but increased wind shear and cooler ocean temperatures on Wednesday likely will halt development. Ex-Peter is not a threat to any land areas. In its 8 a.m. EDT Tuesday Tropical Weather Outlook, NHC gave ex-Peter two-day and five-day odds of development of 50%.

Watching the waters south of Mexico’s Baja Peninsula

In the Eastern Pacific, a tropical wave located a few hundred miles south-southwest of Manzanillo, Mexico, was moving slowly west-northwestward away from Mexico. This wave had lower model support for development than it did on Monday, and in its 8 a.m. EDT Tuesday Tropical Weather Outlook, NHC gave this system two-day and five-day odds of development of 0% and 40%, respectively.

Figure 2. GeoColor nighttime satellite image of Typhoon Mindulle south of Japan at 11 a.m. EDT Tuesday, September 28, 2021. (Image credit: NOAA/RAMMB/Colorado State University)

Typhoon Mindulle predicted to brush Tokyo

After peaking as a category 5 super typhoon with 165 mph winds on September 26, Typhoon Mindulle was a weaker category 3 storm with 115 mph winds as of the 11 a.m. EDT Tuesday advisory from the Joint Typhoon Warning Center. Mindulle is expected to accelerate and recurve to the northeast this week, passing about 150 miles offshore from the Japanese main island of Honshu on Friday. At that time, the typhoon is expected to be a category 2 storm with 100 mph winds. Tokyo will be on the weak (left) side of the storm, but will be within the region of tropical storm-force winds predicted for Mindulle.

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Jeff Masters

Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from 1986-1990. After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a...

Bob Henson

Bob Henson is a meteorologist and journalist based in Boulder, Colorado. He has written on weather and climate for the National Center for Atmospheric Research, Weather Underground, and many freelance...