Hurricane Sam

Hurricane Sam was a powerful and long-lived Cape Verde-type hurricane that formed in the tropical Atlantic Ocean during the 2021 Atlantic hurricane season, lasting from September 22 to October 5 and reaching Category 4 intensity on the Saffir–Simpson scale with maximum sustained winds of 155 mph (250 km/h).^[1] Originating from a tropical wave that emerged off the west coast of Africa on September 17, Sam developed into a tropical depression on September 22 about 575 nautical miles west-southwest of the southernmost Cabo Verde Islands, rapidly intensifying into a tropical storm later that day and becoming a hurricane on September 24 due to favorable environmental conditions including low wind shear and warm sea surface temperatures.^[1] Over the next several days, the storm followed a west-northwestward track, undergoing multiple eyewall replacement cycles that temporarily slowed its intensification but allowed it to reach its peak intensity of 155 mph and a minimum central pressure of 927 millibars on September 26, making it the strongest hurricane of the 2021 season at that point.^[1] Sam maintained major hurricane status for nearly eight days—specifically 7.75 days—becoming the longest-lived major hurricane in the Atlantic basin since Hurricane Ivan in 2004, a record attributed to its position in a region of persistent warm waters and minimal vertical wind shear.^[1] As it approached Bermuda, the cyclone recurved northward and then northeastward under the influence of a mid-level trough, passing approximately 185 nautical miles east-southeast of the island on October 2 as a Category 4 hurricane, resulting in only tropical-storm-force winds up to 38 knots sustained and gusts to 46 knots, along with less than 2 inches of rainfall and no significant damage or casualties reported.^[1] The hurricane continued to weaken gradually as it moved into cooler waters and encountered increasing wind shear, transitioning into an extratropical cyclone by October 5 about 600 nautical miles south of Greenland, after which its remnants were absorbed by a larger extratropical low.^[1] Despite its exceptional longevity and intensity, Sam had no direct impacts on landmasses beyond minor effects on Bermuda, highlighting the variability of tropical cyclone paths in the open Atlantic.^[1]

Meteorological history

Formation and intensification

A strong tropical wave emerged off the west coast of Africa late on September 19, 2021, initially exhibiting a disorganized state with scattered showers and thunderstorms.^[1] As it progressed westward across the eastern tropical Atlantic, curved convective banding became evident in satellite imagery by September 20, while the system passed south of the Cabo Verde Islands.^[1] Favorable environmental conditions supported gradual organization, including sea surface temperatures of 28–29°C, very low vertical wind shear, and a moist mid-level atmosphere.^[1] By early September 22, a well-defined low-level circulation had developed amid increasing convective activity, prompting the National Hurricane Center (NHC) to initiate advisories on the system as Tropical Depression Eighteen at 1800 UTC, when maximum sustained winds reached 30 kt (35 mph) centered about 575 n mi west-southwest of the southernmost Cabo Verde Islands.^[1] The depression intensified steadily, and satellite interpretations showed improved convective organization with a central dense overcast forming over the center.^[1] Six hours later, at 0600 UTC on September 23, it was upgraded to Tropical Storm Sam, with winds increasing to 35 kt (40 mph).^[1] NHC forecasts at this stage indicated a high probability of further development, with the initial track projected westward toward the Lesser Antilles.^[1] Sam continued to strengthen over the warm waters of the central tropical Atlantic, attaining hurricane status by 0600 UTC on September 24, with maximum sustained winds of 65 kt (75 mph) centered approximately 1,100 n mi east of the Windward Islands.^[1] A subtropical ridge to the north helped steer the hurricane on a west-northwestward path while maintaining conducive conditions for intensification.^[1] Later that day, at 1800 UTC, winds had risen to 70 kt (80 mph), marking the onset of a rapid intensification phase driven by robust convective development and minimal inhibitory factors.^[1] By 1800 UTC on September 25, Sam had escalated to Category 3 major hurricane intensity with winds of 115 kt (130 mph), having increased by 45 kt over the preceding 24 hours.^[1]

Peak intensity and eyewall replacement cycles

Hurricane Sam reached its peak intensity as a Category 4 hurricane at 1800 UTC on September 26, 2021, with maximum sustained winds of 155 mph (135 kt) and a minimum central pressure of 927 mb, as determined by NOAA Hurricane Hunter aircraft reconnaissance data.^[1] This marked the culmination of a period of rapid intensification that began around 1800 UTC on September 24, during which the storm's winds increased by 52 mph (45 kt) over 30 hours, fueled by favorable environmental conditions including low vertical wind shear and sea surface temperatures exceeding 28.5°C (83.3°F).^[1] Satellite imagery from this phase revealed a compact, pinhole eye approximately 7 nautical miles in diameter, surrounded by a ring of intense deep convection, with objective Dvorak technique estimates peaking at T6.6, consistent with the observed structure.^[1] Aircraft observations, including dropsonde measurements, confirmed peak flight-level winds near 162 kt, underscoring the storm's extreme inner-core dynamics at this zenith.^[2] The storm's first eyewall replacement cycle (ERC) commenced shortly after peak intensity, around 0000 UTC on September 27, 2021, triggered by increasing southwesterly vertical wind shear that disrupted the symmetric convection and led to the formation of a secondary outer eyewall.^[1] This process involved the outer eyewall contracting inward while the inner eyewall decayed due to reduced moisture inflow and thermodynamic instability, causing a temporary weakening to 121 mph (105 kt) by 1200 UTC on September 27.^[1] Radar and satellite data showed the eye becoming obscured and asymmetric convection developing as the inner core reorganized, a classic manifestation of vortex dynamics where the outer rainband complex strangled the primary eyewall's ventilation.^[2] By early September 28, the cycle completed, allowing Sam to reintensify to 132 mph (115 kt) as the new, larger eyewall stabilized, contributing to the storm's prolonged major hurricane status by enhancing its resilience to environmental stresses.^[1] A second ERC unfolded between September 30 and October 1, 2021, further exemplifying the internal fluctuations that sustained Sam's intensity over warm subtropical waters averaging 29°C (84.2°F).^[1] During this event, the outer eyewall formation again led to transient disruption, but the process resulted in a more robust structure with a well-defined eye expanding to about 25 nautical miles, enabling a secondary intensity peak of 150 mph (130 kt) at 0600 UTC on October 1.^[1] Aircraft penetrations revealed persistent high winds in the reorganized eyewall, while satellite observations highlighted symmetric deep convection, illustrating how these cycles mitigated shear-induced decay through improved radial ventilation and energy redistribution within the vortex.^[2] Overall, the ERCs, driven by interactions between convective bursts and the storm's azimuthal asymmetries, played a key role in Sam's exceptional longevity, allowing it to fluctuate but not collapse despite recurring structural upheavals.^[1]

Decline and dissipation

After reaching a secondary peak intensity on October 1, 2021, Hurricane Sam began to weaken early on October 2 due to increasing southwesterly wind shear exceeding 20 knots (37 km/h) and sea surface temperatures dropping below 26°C (79°F), which disrupted the storm's inner core structure.^[1] The lingering effects of prior eyewall replacement cycles further contributed to structural degradation, though the system retained a well-defined eye amid the unfavorable conditions. By 0600 UTC on October 3, maximum sustained winds had decreased to 90 kt (104 mph; 167 km/h), prompting the National Hurricane Center to downgrade Sam to a Category 2 hurricane.^[1] As Sam tracked north-northeastward, its center passed approximately 185 nautical miles (213 mi; 343 km) east-southeast of Bermuda between October 1 and 2, with the closest approach occurring around 0300 UTC on October 2; this proximity influenced the storm's path without resulting in a direct landfall.^[1] Continued exposure to hostile upper-level winds and marginal ocean temperatures led to further erosion of intensity. Sam continued to weaken, reaching 75 kt (86 mph; 139 km/h) by 0000 UTC on October 5, equivalent to Category 1 intensity, before completing extratropical transition later that day. By 0600 UTC on October 5, near 49°N, 40°W in the central North Atlantic, the hurricane had lost its central convection and adopted a more asymmetric, elongated appearance characteristic of mid-latitude systems.^[1] In its post-tropical phase, the remnants of Sam accelerated northeastward under the influence of a mid-latitude trough, which steered the system away from major landmasses and hastened its overall demise.^[1] Sam finally dissipated on October 7 east of Greenland after merging with another extratropical low over the far North Atlantic.^[1]

Preparations and impacts

Preparations and warnings

As Hurricane Sam strengthened into a major hurricane in the central tropical Atlantic, the National Hurricane Center (NHC) issued early advisories noting the potential for dangerous swells to affect the northern Leeward Islands and portions of Puerto Rico beginning around September 25, 2021. Although no tropical storm watches or warnings for direct wind impacts were required in those areas due to the storm's distant track, local meteorological services monitored conditions for hazardous surf and rip currents.^[3] The primary focus of official responses centered on Bermuda, where the Bermuda Weather Service (BWS) issued a Tropical Storm Watch for the island on September 30, 2021, at 0900 UTC. This was upgraded to a Tropical Storm Warning at 2100 UTC on September 30, which was discontinued on October 2 at 1200 UTC as Sam recurved northeastward well offshore.^[1] NHC and BWS forecasts during this period highlighted the risk of tropical-storm-force winds, and swells potentially causing life-threatening surf and rip currents in Bermuda, while acknowledging track uncertainty stemming from Sam's ongoing eyewall replacement cycles. Public advisories urged residents to secure outdoor items, stock emergency supplies, and stay informed via official channels.^[4]^[5] Bermuda's government, through the Emergency Measures Organisation, activated standard hurricane preparedness protocols, including coordination with utility providers for potential power outages and dissemination of safety guidelines for high winds and rough seas. No widespread school closures or shelter activations were necessary, as forecast models increasingly favored a passage more than 150 nautical miles east of the island.^[6]^[7] Farther afield, minor preparations addressed indirect threats from swells propagating outward from Sam. Along the U.S. East Coast, the National Weather Service issued high surf advisories and rip current warnings from Florida to Maine starting September 30, 2021, anticipating life-threatening beach conditions through early October due to waves up to 10-15 feet. Similar swell impacts prompted general cautions in the Azores, though no formal warnings were issued by Portuguese meteorological authorities.

Direct and indirect impacts

Hurricane Sam produced limited direct impacts as it tracked through the open Atlantic, with no significant effects on Puerto Rico beyond swells generating hazardous surf conditions. As the hurricane passed offshore of Bermuda on October 1–2, 2021, it generated sustained winds up to 38 kt (44 mph) and gusts up to 46 kt (53 mph) near the island, with little or no rainfall reported. Minor disruptions occurred, but the offshore track prevented significant damage, power outages, or injuries on Bermuda.^[1] Indirect effects from Sam were more pronounced along the U.S. East Coast, where the storm generated swells of 10–15 feet from September 27 to October 3, 2021, leading to hazardous surf conditions. These swells prompted 30 water rescues in New Hanover County, North Carolina, and caused beach erosion in Florida and the Carolinas.^[8] The extratropical remnant of Sam brought minor effects to Iceland and Greenland in early October 2021, including several inches of snow in Iceland and winds up to 50-60 mph, with no significant disruptions reported in either location. Overall, Hurricane Sam caused no fatalities and economic losses did not exceed $10 million, largely due to its track over open ocean that avoided direct landfalls.^[1]

Records and significance

Meteorological records

Hurricane Sam set several meteorological benchmarks during the 2021 Atlantic hurricane season, primarily due to its exceptional longevity and sustained intensity over open waters. It maintained major hurricane status (Category 3 or higher on the Saffir-Simpson Hurricane Wind Scale) for 7.75 days, from 1800 UTC September 24 to 1200 UTC October 4, marking the longest such duration for any storm that year. This period tied Sam for the ninth-longest duration as a major hurricane in Atlantic basin records dating back to 1851, and it was the longest since Hurricane Ivan in 2004.^[1] The storm's endurance contributed to its record accumulated cyclone energy (ACE) index of 54.0 × 10⁴ kt² for the season, the highest among all 2021 systems and ranking fifth overall in Atlantic history since reliable records began in 1966. The ACE, a measure of a tropical cyclone's energy potential calculated as the sum of the squares of its estimated maximum sustained wind speeds (in knots) every six hours while it is a tropical storm or hurricane multiplied by 10^{-4}, underscored Sam's prolonged high-intensity phase despite undergoing multiple eyewall replacement cycles. These cycles, which occurred around September 27 and again on September 29, temporarily disrupted intensification but ultimately enhanced the storm's resilience by allowing a larger, more stable outer eyewall to dominate.^[9]^[1] Sam's overall lifespan extended from its formation as a tropical depression at 1800 UTC September 22 until its remnants merged with a larger extratropical low after 0600 UTC October 7, placing it among the longest-lived Atlantic hurricanes on record. At its peak, the storm achieved sustained winds of 135 knots (155 mph) and a minimum central pressure of 927 millibars around 1800 UTC September 26, classifying it as a high-end Category 4 hurricane. These metrics positioned Sam as one of the strongest non-landfalling hurricanes in the Atlantic basin, with its peak intensity rivaling only a handful of similar open-ocean systems since the satellite era began.^[10]^[1]

Seasonal and broader significance

Hurricane Sam played a notable role in the above-normal 2021 Atlantic hurricane season, which produced 21 named storms, tying the total from 1933 and ranking third highest on record.^[11] The storm's exceptional longevity, with an accumulated cyclone energy (ACE) index of 54.0 × 10⁴ kt², contributed substantially to the season's overall ACE total of 145.6 × 10⁴ kt², which was 19% above the 1991–2020 average and marked the third-highest on record.^[1]^[11] The National Hurricane Center's post-season analysis highlighted how Sam's eyewall replacement cycles bolstered its durability, allowing it to maintain major hurricane intensity for 7.75 days despite environmental challenges like moderate wind shear.^[1] These cycles, which temporarily weakened the storm before reintensification, provided valuable data for refining intensity forecast models, particularly for open-ocean hurricanes that avoid landfall and experience prolonged structural evolution.^[1] Sam's prolonged strength was facilitated by broader climate conditions, including the emergence of La Niña during the season, which reduced vertical wind shear across the tropical Atlantic, and anomalously warm sea surface temperatures exceeding 28°C along its track.^[12]^[13] These factors favored the persistence of major hurricanes like Sam, which remained over the open ocean without significant land interaction, underscoring patterns of extended tropical cyclone lifespans in such environments.^[12] Due to its lack of significant human impacts, with no reported deaths or substantial damage, the World Meteorological Organization did not retire the name Sam following the 2021 season, permitting its reuse in the six-year rotating list starting with the 2026 season.^[14] Sam's near-miss of Bermuda, where its outer circulation produced tropical-storm-force winds and gusts up to 46 kt but caused no major disruptions, illustrated the island's vulnerability to such close approaches from powerful systems.^[1] Additionally, the hurricane generated extensive swells, with significant wave heights reaching 14 m in its right-front quadrant due to an extended fetch mechanism, affecting distant coastlines including the U.S. East Coast and prompting advancements in swell propagation models for better predicting indirect coastal hazards.^[10]