Hurricane Sam is still a Cat 4; Tropical Depression 20 forms off the coast of Africa »Yale Climate Connections


Hurricane Sam remained at Category 4 strength on Wednesday as it crossed the central tropical Atlantic, well away from any land area. Sam could complete a full week of major hurricane force (Cat 3 or greater) by Saturday, when it passes east of Bermuda.

At 11 a.m. EDT Wednesday, Sam was a low-end Category 4 hurricane with winds of 130 mph and a center pressure of 949 mb, headed northwest at 9 mph. Sam looked like he was about to undergo another eye wall replacement cycle, a process in which a hurricane’s eye breaks down and is replaced by a larger concentric eye, over about a day. An eye wall replacement cycle typically results in a larger hurricane, as the wind field expands, but the peak winds decrease.

Sam’s long duration as a Category 3 or stronger storm made it a major wave machine, and its heavy swells were affecting parts of the Lesser Antilles and Greater Antilles on Wednesday. Large waves will spread north and east to affect much of the east coast of the United States and the coasts of the Canadian Maritime provinces by this weekend, bringing high waves and dangerous back currents. . NOAA hurricane chasers measured significant wave heights of up to 25 feet on Monday, and the GFS model predicts that waves in Sam’s heart will drop to 35 feet by Saturday.

Figure 1. Prediction of significant wave height (in feet) at 8:00 p.m. EDT on Saturday October 2, 2021, as projected by running the GFS model on Wednesday September 29 at 06Z. (Image credit:

Little change in the prognosis for Sam

Sam will likely continue as a powerful Category 3 or 4 hurricane until at least Saturday as it moves steadily northwest, moving further north at the end of the week. Windshear 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 rather dry, with an average relative humidity only around 45-50%, which will limit the chances of her experiencing a rapid intensification.

As Sam gradually moves closer to Bermuda later this week, he will be swept by a strong trough of high altitude depression stationed across the Canadian Maritimes and New England. The trough should keep Sam just east of Bermuda and well away from the east coast of the United States, but there is a slight chance that Sam will tilt slightly to the left and reach Newfoundland, as some predict. members of the GFS and European model sets (mainly the GFS, which has outperformed the euro in its predictions of Sam so far).

Increased wind shear and colder waters will eventually trigger Sam’s conversion into a powerful post-tropical storm over the far north Atlantic around Tuesday next week.

Figure 2. Tropical disturbance 91L (center), TD 20 off the west coast of Africa (right) and Hurricane Sam (top left), as shown in satellite images visible at 10:15 am EDT on Wednesday, September 29, 2021 (Image credit: RAMMB / CIRA / CSU)

Tropical Depression 20 forms off the coast of Africa

Tropical Depression 20 (TD 20) formed off the coast of Africa, about 500 miles south of the Cabo Verde Islands, at 11 a.m. EDT Wednesday. Satellite imagery showed that the TD 20 was becoming more organized, with many intense thunderstorm activities and a low-level traffic center becoming more defined. The TD 20 was moving west-northwest at 14 mph at an unusually low latitude, 8.3 ° N: the lowest latitude on record for an Atlantic tropical storm is 7.7 ° N, per storm unnamed from September 1902.

Positioning the TD 20 so close to the equator will slow its development, but all other factors over the next three days appear favorable for development. According to forecasts from the National Hurricane Center, TD 20 would become Tropical Storm Victor Wednesday evening at latitude 8.8 north and Hurricane Victor Thursday evening at latitude 10.3 north; the two locations would be exceptionally close to the equator for an Atlantic system. The southernmost position on record for an Atlantic hurricane was 9.5 ° N, by Hurricane Ivan in 2004.

The TD 20 is moving clockwise around a high pressure ridge to its north, a directing flow that will carry the system northwest and then north at the weekend, taking it into the Central Atlantic, far from any land area. By Saturday, the TD 20 is expected to move into an area of ​​dry air and high windshear of 20 to 40 knots, associated with an upper low. This meeting should considerably weaken the TD 20 this weekend.

Disturbance 91L in the eastern tropical Atlantic is unlikely to develop

An area of ​​low pressure in the eastern Atlantic near 7 ° N, 33 ° W, several hundred miles southwest of the Cabo Verde Islands, was drifting west-northwest at about 5 mph Wednesday . Satellite images showed that this low, designated 91L, had changed little since Monday, and was poorly organized with few severe thunderstorms. The low position so close to the TD 20 significantly interfered with development, a factor that alone made development of the 91L unlikely. In its tropical weather forecast on Wednesday at 8 a.m. EDT, the National Hurricane Center gave 91L a 30% chance of development over two and five days. The next name on the Atlantic storm list after Victor is Wanda.

Peter shouldn’t get up

The remnants of Tropical Storm Peter, located several hundred miles south of Newfoundland, Canada, experience a windshear of over 30 knots and are increasingly disorganized. The ex-Peter will move northeast into the open central Atlantic at around 10 mph, and this is not a threat to any land area. In its tropical weather forecast on Wednesday at 8 a.m. EDT, the National Hurricane Center gave ex-Peter a 10% two- and five-day development chance.

Figure 3. MODIS visible satellite image of Typhoon Mindulle in southern Japan on Tuesday afternoon September 28, 2021 (Image credit: NASA Worldview)

Typhoon Mindulle is expected to hit Tokyo

After peaking as a Category 5 super typhoon with winds of 165 mph on September 26, Typhoon Mindulle was a weaker Category 4 storm with winds of 130 mph at 11 a.m. EDT Wednesday notice from the Joint Typhoon Warning Center. Mindulle is expected to accelerate and curve back northeast this week, passing about 150 miles off the main Japanese island of Honshu on Friday. By that time, the typhoon is expected to be a weakening Category 2 storm with winds of 105 mph. Tokyo will be on the weak (left) side of the storm, but will be in the region of tropical storm force winds forecast for Mindulle.

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