Hurricane Sam remained at category 4 strength on Thursday as it churned across the central tropical Atlantic, well away from any land areas. Sam by Saturday could complete an entire week at major hurricane strength (Cat 3 or stronger), when it will be passing east of Bermuda.
At 11 a.m. EDT Thursday, Sam was a category 4 hurricane with 145 mph winds and a central pressure of 937 mb, headed northwest at 13 mph. Sam appeared poised to undergo another 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, but the peak winds decrease. Sam has experienced at least three eyewall replacement cycles, greatly expanding its wind field. The radius of hurricane-force winds expanded from 30 miles on Monday morning to 60 miles on Thursday morning, and the radius of tropical storm-force winds grew from 105 to 150 miles.
Sam’s long duration as a category 3 or stronger storm has made it a major wave machine; a NOAA buoy in Sam’s path measured a significant wave height of 39.7 feet early Thursday morning, and a peak wind gust of 98 mph.
Sam’s large swells were affecting portions of the Lesser Antilles and Greater Antilles on Thursday, and large waves will propagate northward and eastward to much of the U.S. East Coast and the coasts of the Canadian Maritime Provinces by Saturday, bringing high surf and dangerous rip currents.
Sam expected to brush Bermuda, miss the U.S. and Canada
Sam likely will hold its own as a potent category 3 or 4 hurricane until at least Saturday as it moves steadily northwest and then north. 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. The atmosphere around Sam is on the dry side, with midlevel relative humidity only around 45-50%, limiting chances for it to undergo rapid intensification.
Sam is being 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 and Canadian Maritime Provinces. The 11 a.m. EDT Thursday wind probability forecast from the National Hurricane Center gave Bermuda a 22% chance of experiencing tropical storm-force winds from Sam.
Increased wind shear and cooler waters eventually will trigger Sam’s conversion into a powerful post-tropical storm over the far north Atlantic around Tuesday.
Tropical Storm Victor forms off coast of Africa
Tropical Storm Victor formed off the coast of Africa, about 500 miles south of the Cabo Verde Islands, at 5 p.m. EDT Wednesday. Victor formed at an unusually low latitude, 8.3°N: The lowest latitude on record for an Atlantic tropical storm is 7.7°N, by an unnamed September 1902 storm.
At 11 a.m. EDT Thursday, Victor, with top sustained winds of 45 mph, was moving west-northwest at 14 mph. Satellite images showed Victor was a large storm that was steadily growing more organized, with plenty of heavy thunderstorm activity and low-level spiral banding.
Victor’s position so close to the equator will slow its development, as will its large size, but all other factors through Saturday appear favorable for development. The National Hurricane Center predicted Victor will approach hurricane strength on Friday at latitude 11.1°N. The farthest south position on record for an Atlantic hurricane was 9.5°N, by Hurricane Ivan in 2004.
Victor is moving clockwise around a ridge of high pressure to its north, a steering flow that will carry the system to the northwest and then north this weekend, taking it into the central Atlantic, far from any land areas. By Saturday night, Victor is expected to move into an area of dry air and high wind shear of 20-30 knots, associated with an upper-level low. This encounter should significantly weaken Victor.
Atlantic likely to run out of names for 2nd year in a row
Victor was the 20th named storm of 2021. According to Phil Klotzbach of Colorado State University, only one other season, 2020, has had as many as 20 named storms by September 29. In that insanely active year, we’d by then already burned through the full list of 21 names, and were two letters into the Greek alphabet (Beta). So far in 2021, the Atlantic has had 20 named storms, seven hurricanes, and four major hurricanes. By comparison, the 1991-2020 averages for an entire season were 14.4 named storms, 7.2 hurricanes, and 3.2 major hurricanes.
On average, we can expect to see five more named storms, two to three more hurricanes, and one additional major hurricane. This makes it very likely that we will run through the full list of 21 names in the Atlantic for the second year in a row. Only one name remains in the 2021 list: Wanda. There are currently no Wanda candidates being discussed in NHC’s Tropical Weather Outlook, but the Caribbean is ominously warm at present, and a La Niña event appears to be developing in the Eastern Pacific – both harbingers of an active final two months of Atlantic hurricane season. The 22nd named storm of 2021 would be named Adria, as the season cycles through a new alphabetical list of 21 supplemental names (naming storms after Greek letters has been permanently discontinued).
Category 1 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 much-weakened category 1 storm with 90 mph winds as of the 11 a.m. EDT Thursday advisory from the Joint Typhoon Warning Center. Mindulle is expected to accelerate and recurve to the northeast, passing about 150 miles offshore from the Japanese main island of Honshu on Friday. At that time, the typhoon is expected to be a weakening category 1 storm with 80-85 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.
Nagoya University and the University of the Ryukyus teamed up and did a reconnaissance mission using a Gulfstream IV aircraft into Mindulle on September 29, penetrating the eye and releasing approximately 20 dropsondes (see this news article on the flight). The researchers found Mindulle had a central minimum pressure of 932 mb, which matched up well with estimates from the Joint Typhoon Warning Center and Japan Meteorological Agency.
Editor’s note: Dr. Jeff Masters will present a webinar – Another Year of Weather Extremes: Outliers… or Omens? – beginning at noon eastern on Friday, October 1. Register at this link.
Yale Climate Connections soon afterward will provide a recording of the 60-minute webinar for those unable to attend.
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