Hurricane Sam put on a burst of rapid intensification overnight, reaching hurricane status at 5 a.m. EDT Friday. Sam is expected to be a powerful long-track Cape Verde-type hurricane that could threaten the Leeward Islands and Bermuda next week. It is too early to know if Sam might threaten the U.S. or Canada.

At 11 a.m. Friday, Sam was in the central tropical Atlantic about 1,365 miles east-southeast of the northern Leeward Islands, headed west at 14 mph with top winds of 75 mph and a central pressure of 993 mb. Satellite images showed Sam becoming well-organized, with a concentrated area of heavy thunderstorms with cold cloud tops near its core. Sam’s low-level spiral bands were on the increase, but no eye was yet visible.

Sam is the seventh Atlantic hurricane of 2021, bringing this year’s tallies to 18 named storms, seven hurricanes, and three major hurricanes. The 1991-2020 averages were 14.4 named storms, 7.2 hurricanes, and 3.2 major hurricanes. On average, we should expect to see five to six more named storms, two to three more hurricanes, and one additional major hurricane after September 24.

Six of this year’s hurricanes have intensified by at least 35 mph in 24 hours – the National Hurricane Center’s definition of rapid intensification. Only Henri failed to do so:

Sam: 40 mph ending 9Z Sep. 24;
Nicholas: 35 mph ending 3Z Sep. 14;
Larry: 45 mph ending 3Z Sep. 4;
Ida: 65 mph ending 15Z Aug. 29;
Grace: 55 mph ending 6Z Aug. 21; and
Elsa: 35 mph ending 12Z Jul. 2.

Intensity forecast for Sam

Sam has strong model support for intensification, and conditions appear quite favorable for it to intensify throughout the next three days. Wind shear is predicted to be light (5-10 knots), and sea surface temperatures (SSTs) will be 28.5-29 degrees Celsius (83-84°F), which are unusually warm (see Tweet below). The atmosphere is predicted to be on the dry side, though, with a mid-level relative humidity of 50-60%. The National Hurricane Center (NHC) forecast is aggressive, calling for Sam to become a major hurricane on Saturday night. The 12Z Friday run of the SHIPS model gave a 23% chance that Sam would intensify into a 115-mph category 3 hurricane by Saturday morning; the experimental DTOPS model gave that prospect a 30% chance of occurring.

This coming Monday through Wednesday Sam may encounter slightly less favorable conditions for intensification, with wind shear rising to a moderate 10-15 knots, and the atmosphere surrounding the storm getting dry, with a mid-level relative humidity of 50%. The NHC predicts that Sam will top out as a category 4 storm with 130 mph winds on Sunday and Monday. Sam is a small hurricane, and small storms are more prone to rapid changes in strength.

Figure 1. Track forecasts out to 10 days for Sam from the 0Z Friday, September 24, run of the European ensemble model. Individual forecasts of 51 ensemble members are lines color-coded by the wind speed in knots they predict for Sam; red colors correspond to a category 1 hurricane. The heavy black line is the ensemble mean. Times in hours from the model initialization time are in grey text. Many of the more southerly ensemble members predicted that Sam would pass through the Leeward Islands five or six days from initialization time; these tended to show a weaker storm. The forecasts showing a strong hurricane had Sam missing the islands. (Image credit: weathernerds.org)
Figure 2. Track forecasts out to 10 days for Sam from the 6Z Friday, September 24, run of the GFS ensemble model. Individual forecasts of 31 ensemble members are lines color-coded by the wind speed (in knots) they predict for Sam; red colors correspond to a category 1 hurricane. The heavy black line is the ensemble mean forecast. The times in hours from model initialization time are in grey text. Only one of the forecasts showed Sam passing through the Leeward Islands. (Image credit: weathernerds.org)

Track forecast for Sam

Sam is predicted to track west to west-northwest at 10-15 mph and gradually slow down over the next five days, potentially making it a threat soon after to the Leeward Islands. These islands were just outside the NHC’s 5-day cone of uncertainty as of the 11 a.m. EDT Friday advisory. Sam is likely to make its closest approach to the northeastern Leeward Islands on Wednesday and Thursday. Friday’s model runs showed less of a threat to the Leeward Islands than they did on Thursday.

Sam’s future track depends most strongly on the timing and strength of troughs of low pressure passing to the north next week. One wild card in this forecast is the potential for perturbations to the jet stream created by Tropical Storm Mindulle in the Northwest Pacific. Mindulle is predicted to intensify into a powerful category 4 typhoon that will recurve to the north near Japan next week. When Mindulle merges with the jet stream, it is likely to cause a major ripple effect in the jet that could influence the steering of Sam. However, Mindulle may not recurve in time to influence the steering pattern for Sam.

Sam’s future track will depend also on how quickly it intensifies. A stronger storm is more likely to feel steering currents aloft that will tug the system farther to the north and miss the Leeward Islands; a weaker and slower-to-organize storm will track farther to the west and potentially pass through the islands.

Late next week, the models agree on a northward turn for Sam, which could potentially bring the hurricane close to Bermuda on Friday. It is possible that Sam could be a threat to the Northeast U.S. or Canadian Maritime Provinces in 8-10 days, but it is impossible to judge this threat with any confidence so far in advance.

Figure 3. Track forecasts out to five days for 99L from the 6Z (2 a.m. EDT) Friday, September 24, run of the GFS ensemble model (GEFS). The black line is the mean of the 31 ensemble members; individual ensemble member forecasts are the thin lines, color-coded by the central pressure they predict for 99L. Many of the members predicted a northeastward track near or over Newfoundland on Monday. (Image credit: Tropical Tidbits)

New low-pressure system near Bermuda could develop into Teresa

A surface area of low pressure developed overnight beneath an upper-level low located a few hundred miles east of Bermuda. This system, designated 99L by NHC, was headed north-northwest at 10-15 mph. 99L had marginal conditions for development, with high wind shear of 20-30 knots, sea surface temperatures of 27 degrees Celsius (81°F), and a dry surrounding atmosphere.

99L appeared well-organized on satellite imagery Friday afternoon, with an intense band of heavy thunderstorms to the north of the center. The displacement of this band away from the center gave 99L the appearance of a subtropical cyclone, rather than a tropical cyclone. While 99L did not yet have a well-defined surface circulation, it does have enough time to develop one before high wind shear and cool waters less than 25 degrees Celsius (77°F) squelch further development by Sunday morning. On Sunday, 99L is predicted to accelerate to the northeast, and it could bring strong winds and heavy rain to Newfoundland, Canada, on Monday.

In its 8 a.m. EDT Friday Tropical Weather Outlook, the NHC gave 99L 2-day and 5-day odds of development of 40%. The next name on the Atlantic list of storms is Teresa.

Next week, watch for tropical wave to emerge from coast of Africa

A new tropical wave is expected to emerge from the coast of Africa by Sunday. This wave has strong model support for development next week, when the system will be moving west at 10-15 mph over the eastern tropical Atlantic. In its 8 a.m. EDT Friday Tropical Weather Outlook, the NHC gave the new wave 2-day and 5-day odds of development of 0% and 40%, respectively.

Ex-Odette waning

The remnants of Tropical Storm Odette, located about 600 miles west-northwest of the Azores Islands, now appear unlikely to regenerate into a tropical storm. The system, located over cool waters of 23-24 degrees Celsius (73-75°F), has not been able to sustain enough heavy thunderstorm activity to qualify as a tropical storm, as seen on satellite imagery.

In its 8 a.m. EDT Friday Tropical Weather Outlook, the NHC gave ex-Odette 2-day and 5-day odds of regeneration of 20%. On Friday evening, Odette is expected to encounter high wind shear that will finally end any chance the system has to regain tropical storm status.

Bob Henson contributed to this post.

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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...