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Hurricane Sam

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Hurricane Sam
Category 4 major hurricane (SSHWS/NWS)
Hurricane Sam near peak intensity on September 26
FormedSeptember 22, 2021
DissipatedOctober 9, 2021
(Extratropical after October 5)
Highest winds1-minute sustained: 155 mph (250 km/h)
Lowest pressure929 mbar (hPa); 27.43 inHg
FatalitiesNone
Areas affectedWest Africa, Lesser Antilles, Puerto Rico, East Coast of the United States, Bermuda, Iceland
Part of the 2021 Atlantic hurricane season

Origins, development, and peak intensity

Map plotting the track and the intensity of the storm, according to the Saffir–Simpson scale
Map plotting the storm's track and intensity, according to the Saffir–Simpson scale
Map key
Saffir–Simpson scale
  Tropical depression (≤38 mph, ≤62 km/h)
  Tropical storm (39–73 mph, 63–118 km/h)
  Category 1 (74–95 mph, 119–153 km/h)
  Category 2 (96–110 mph, 154–177 km/h)
  Category 3 (111–129 mph, 178–208 km/h)
  Category 4 (130–156 mph, 209–251 km/h)
  Category 5 (≥157 mph, ≥252 km/h)
  Unknown
Storm type
triangle Extratropical cyclone, remnant low, tropical disturbance, or monsoon depression

Hurricane Sam originated from a westward tracking tropical wave that departed from the western coast of Africa on September 19.[1] The National Hurricane Center (NHC) had first forecast the possibility of tropical development earlier that day.[2] Although thunderstorm activity was somewhat organized, the system lacked a closed surface circulation. Upper-level winds hindered the development of the system over the next day,[3][4] causing fluctuations in organization.[5][6] Thunderstorms became increasingly organized,[7] and an elongated low-level surface circulation developed on September 22. This led to the formation of a tropical depression around 21:00 UTC that day. At that time, a mid-level ridge located to the north and west was steering the system westward. The nascent depression was located in a favorable environment both below and aloft, with sea surface temperatures of 82–84 °F (28–29 °C) and low vertical wind shear.[8]

Intensification was initially slow; the cyclone's center was still attached to the northern edge of a low-pressure trough and the former's surface circulation was only narrowly closed.[9] Although the depression had curved rainbands across its northern and western sides, dry air was entraining across its eastern flank.[10] The depression was upgraded to Tropical Storm Sam at 15:00 UTC on September 23, after its structure improved significantly on both satellite and microwave imagery; the latter showed a banding feature wrapping around 75 percent of the center. The combination of a favorable environment, Sam's structural improvement, and the alignment of the cyclone's low- and mid-level centers signaled that rapid intensification was imminent.[11] The storm rapidly intensified shortly after, with an eyewall forming on microwave imagery,[12] as well as, banding features that wrapped around most of its center. Sam became a Category 1 hurricane around 09:00 UTC on September 24.[13]

Sam's strengthening then halted for a brief period as dry air degraded the western portion of its core.[14] The cyclone began to intensify again late on September 24 as the dry air's influence lessened, however, microwave imagery displayed concentric eyewalls, indicating that Sam was undergoing an eyewall replacement cycle. The storm continued to track west-northwest, however, its forward speed began to decrease as a result of the mid-level ridge moving to the northwest of Sam.[15] A small, 12-mi (19-km) wide eye emerged on infrared satellite imagery early on September 25, surrounded by a symmetric ring of cold, −94 °F (−70 °C) cloud tops.[16][17] Sam was upgraded to a Category 3 major hurricane[nb 1] at 15:00 UTC on September 25 after its eye became mostly cloud-free.[19] The cyclone further strengthened to Category 4 status by 21:00 UTC.[20] A NOAA Hurricane Hunters reconnaissance aircraft surveying the storm later that day indicated that multiple mesovortices, including some tornadic-scale ones, were present in both the southeastern and northwestern portions of the eyewall; the wind directions were entirely opposite to the normal wind direction in some regions. Radar from the aircraft displayed that dry air was entraining into the storm, thinning the eyewall in numerous locations.[21] The hurricane continued to intensify through September 26, with the NHC estimating that Sam's peak probably occurred between 19:00 and 22:00 UTC, likely attaining maximum sustained winds of 155 mph (250 km/h) and a minimum central pressure of 929 mbar (27.4 inHg). During that time, the eye had shrunk to a diameter of 8 mi (13 km).[22] The cold convection surrounding the eye had also grown in size. Likewise, a microwave pass showed a compact, strong eyewall surrounding the small eye. Despite Sam's intensity, it remained a small storm, with tropical storm-force winds reaching out 80 mi (130 km) from its center.[23]

Eyewall replacement cycles, threat to Bermuda, and secondary peak

File:Hurricane Sam at secondary peak.png
Hurricane Sam at its secondary peak on October 1

A subtropical ridge located to the north and northeast of Sam caused it to begin tracking northwestward early on September 27. At the same time, a second eyewall replacement cycle was causing the cyclone to weaken. The storm's eye had doubled in size and was cloud-filled while convection in the eyewall was irregular and had degraded across Sam's eastern side.[22] Soon after, Sam's eye disappeared entirely from infrared and visible satellite imagery as a result of the replacement cycle and mid-level dry air intrusions from the west.[24][25] The cyclone weakened to Category 3 status around 15:00 UTC,[25] and bottomed out at 120 mph (195 km/h) around 21:00 UTC. By that time, a new eye had emerged on satellite imagery, surrounded by a ring of −85 °F (−65 °C) cloud tops.[26] A reconnaissance aircraft found that the cyclone's wind field had increased following the eyewall replacement cycle,[27] with the radius of hurricane-force winds expanding to 40 mi (60 km) and the radius of tropical-storm-force winds increasing to 125 mi (205 km).[28][29]

Sam restrengthened into a Category 4 hurricane around 09:00 UTC on September 28, after a ring of colder clouds formed around its eye. The concentric eyewalls that were priorly apparent had fused together in a single eyewall.[27][30] Later that day, the storm's structure degraded, with its eye becoming less pronounced in satellite imagery. Upper-level outflow was reduced in Sam's southwestern region as a result of impinging mid-level dry air, and multiple arcus clouds were radiating outward from the western side of the storm.[31] Sam strengthened to 140 mph (220 km/h) early on September 29, while its eye warmed and convection in its eyewall cooled.[32] Shortly after, the cyclone experienced slight weakening as a result of moderate southwesterly wind shear, with Sam's eye becoming cloud-filled and the circulation slanting along the southwest-to-northeast. However, the convection surrounding the eye had cooled.[33] Later that day, Sam's overall cloud pattern had improved, however, a Hurricane Hunters reconnaissance aircraft reported that another eyewall replacement cycle was underway.[34][35]

The Bermuda Weather Service issued a tropical storm watch for Bermuda at 09:00 UTC on September 30 due to the potential for the cyclone to bring tropical storm conditions to the island.[36] The watch was increased to a tropical storm warning at 21:00 UTC.[37] Early on September 30, Sam began another strengthening trend as the eyewall replacement cycle concluded, located over a warm ocean eddy and within a region of low wind shear.[38] The cyclone exhibited a warm 50–68 °F (10–20 °C), 29-mi (46-km) wide eye surrounded by cold, −76 to −94 °F (−60 to −70 °C) cloud tops from September 30–October 1.[39][40] Early on October 1, Sam turned towards the north-northwest as it began to round the western edge of the subtropical ridge.[40] Sam reached its secondary peak around 09:00 UTC, with maximum sustained winds of 150 mph (240 km/h) and a minimum central pressure of 934 mbar (27.6 inHg).[41]

Weakening and demise

Sam as an extratropical cyclone on October 6

See also

Notes

  1. ^ A major hurricane is one that ranks at Category 3 or higher on the Saffir–Simpson scale.[18]

References

  1. ^ Brown, Daniel (20 September 2021). Tropical Weather Outlook: Atlantic [800 PM EDT Sun Sep 19 2021] (Report). NHC Graphical Outlook Archive. National Hurricane Center. Archived from the original on 5 October 2021. Retrieved 5 October 2021.
  2. ^ Hagen, Andrew; Latto, Andrew (19 September 2021). Tropical Weather Outlook: Atlantic [800 AM EDT Sun Sep 19 2021] (Report). NHC Graphical Outlook Archive. National Hurricane Center. Archived from the original on 2 October 2021. Retrieved 5 October 2021.
  3. ^ Brown, Daniel (21 September 2021). Tropical Weather Outlook: Atlantic [800 PM EDT Mon Sep 20 2021] (Report). NHC Graphical Outlook Archive. National Hurricane Center. Archived from the original on 2 October 2021. Retrieved 5 October 2021.
  4. ^ Brown, Daniel (22 September 2021). Tropical Weather Outlook: Atlantic [800 PM EDT Tue Sep 21 2021] (Report). NHC Graphical Outlook Archive. National Hurricane Center. Archived from the original on 5 October 2021. Retrieved 5 October 2021.
  5. ^ Papin, Philippe (21 September 2021). Tropical Weather Outlook: Atlantic [800 AM EDT Tue Sep 21 2021] (Report). NHC Graphical Outlook Archive. National Hurricane Center. Archived from the original on 5 October 2021. Retrieved 5 October 2021.
  6. ^ Papin, Philippe (21 September 2021). Tropical Weather Outlook: Atlantic [200 PM EDT Tue Sep 21 2021] (Report). NHC Graphical Outlook Archive. National Hurricane Center. Archived from the original on 5 October 2021. Retrieved 5 October 2021.
  7. ^ Papin, Philippe (22 September 2021). Tropical Weather Outlook: Atlantic [800 AM EDT Wed Sep 22 2021] (Report). NHC Graphical Outlook Archive. National Hurricane Center. Archived from the original on 5 October 2021. Retrieved 5 October 2021.
  8. ^ Papin, Philippe (22 September 2021). Tropical Depression Eighteen Discussion Number 1 (Report). National Hurricane Center. Archived from the original on 24 September 2021. Retrieved 5 October 2021.
  9. ^ Berg, Robbie (23 September 2021). Tropical Depression Eighteen Discussion Number 2 (Report). National Hurricane Center. Archived from the original on 6 October 2021. Retrieved 5 October 2021.
  10. ^ Cangialosi, John (23 September 2021). Tropical Depression Eighteen Discussion Number 3 (Report). National Hurricane Center. Archived from the original on 23 September 2021. Retrieved 5 October 2021.
  11. ^ Papin, Philippe (23 September 2021). Tropical Storm Sam Discussion Number 4 (Report). National Hurricane Center. Archived from the original on 24 September 2021. Retrieved 6 October 2021.
  12. ^ Berg, Robbie (24 September 2021). Tropical Storm Sam Discussion Number 6 (Report). National Hurricane Center. Archived from the original on 24 September 2021. Retrieved 6 October 2021.
  13. ^ Cangialosi, John (24 September 2021). Hurricane Sam Discussion Number 7 (Report). National Hurricane Center. Archived from the original on 25 September 2021. Retrieved 6 October 2021.
  14. ^ Papin, Philippe (24 September 2018). Hurricane Sam Discussion Number 8 (Report). National Hurricane Center. Archived from the original on 24 September 2021. Retrieved 6 October 2021.
  15. ^ Papin, Philippe (24 September 2021). Hurricane Sam Discussion Number 9 (Report). National Hurricane Center. Archived from the original on 5 October 2021. Retrieved 6 October 2021.
  16. ^ Berg, Robbie (25 September 2021). Hurricane Sam Discussion Number 10 (Report). National Hurricane Center. Archived from the original on 25 September 2021. Retrieved 6 October 2021.
  17. ^ Cangialosi, John (25 September 2021). Hurricane Sam Discussion Number 11 (Report). National Hurricane Center. Archived from the original on 6 October 2021. Retrieved 6 October 2021.
  18. ^ "Saffir–Simpson Hurricane Wind Scale". National Hurricane Center. Archived from the original on 20 June 2020. Retrieved 22 June 2020.
  19. ^ Latto, Andrew (25 September 2021). Hurricane Sam Discussion Number 12 (Report). National Hurricane Center. Archived from the original on 27 September 2021. Retrieved 6 October 2021.
  20. ^ Beven, Jack (25 September 2021). Hurricane Sam Discussion Number 13 (Report). National Hurricane Center. Archived from the original on 25 September 2021. Retrieved 7 October 2021.
  21. ^ Stewart, Stacy (26 September 2021). Hurricane Sam Discussion Number 14 (Report). National Hurricane Center. Archived from the original on 6 October 2021. Retrieved 7 October 2021.
  22. ^ a b Stewart, Stacy (27 September 2021). Hurricane Sam Discussion Number 18 (Report). National Hurricane Center. Archived from the original on 27 September 2021. Retrieved 27 September 2021.
  23. ^ Latto, Andrew (26 September 2021). Hurricane Sam Discussion Number 17 (Report). National Hurricane Center. Archived from the original on 6 October 2021. Retrieved 8 October 2021.
  24. ^ Reinhart, Brad (27 September 2021). Hurricane Sam Discussion Number 19 (Report). National Hurricane Center. Archived from the original on 6 October 2021. Retrieved 10 October 2021.
  25. ^ a b Hagen, Andrew; Pasch, Richard (27 September 2021). Hurricane Sam Discussion Number 20 (Report). National Hurricane Center. Archived from the original on 5 October 2021. Retrieved 10 October 2021.
  26. ^ Latto, Andrew (27 September 2021). Hurricane Sam Discussion Number 21 (Report). National Hurricane Center. Archived from the original on 1 October 2021. Retrieved 10 October 2021.
  27. ^ a b Blake, Eric (28 September 2021). Hurricane Sam Discussion Number 22 (Report). National Hurricane Center. Archived from the original on 5 October 2021. Retrieved 10 October 2021.
  28. ^ Latto, Andrew (27 September 2021). Hurricane Sam Advisory Number 21 (Report). National Hurricane Center. Archived from the original on 5 October 2021. Retrieved 10 October 2021.
  29. ^ Blake, Eric (28 September 2021). Hurricane Sam Advisory Number 22 (Report). National Hurricane Center. Archived from the original on 28 September 2021. Retrieved 10 October 2021.
  30. ^ Reinhart, Brad (28 September 2021). Hurricane Sam Discussion Number 23 (Report). National Hurricane Center. Archived from the original on 1 October 2021. Retrieved 10 October 2021.
  31. ^ Pasch, Richard (28 September 2021). Hurricane Sam Discussion Number 25 (Report). National Hurricane Center. Archived from the original on 5 October 2021. Retrieved 10 October 2021.
  32. ^ Blake, Eric (29 September 2021). Hurricane Sam Discussion Number 26 (Report). National Hurricane Center. Archived from the original on 5 October 2021. Retrieved 10 October 2021.
  33. ^ Brown, Daniel (29 September 2021). Hurricane Sam Discussion Number 27 (Report). National Hurricane Center. Archived from the original on 5 October 2021. Retrieved 10 October 2021.
  34. ^ Pasch, Richard (29 September 2021). Hurricane Sam Discussion Number 28 (Report). National Hurricane Center. Archived from the original on 6 October 2021. Retrieved 10 October 2021.
  35. ^ Pasch, Richard (29 September 2021). Hurricane Sam Discussion Number 29 (Report). National Hurricane Center. Archived from the original on 6 October 2021. Retrieved 10 October 2021.
  36. ^ Brown, Daniel (30 September 2021). Hurricane Sam Advisory Number 31 (Report). National Hurricane Center. Archived from the original on 30 September 2021. Retrieved 10 October 2021.
  37. ^ Pasch, Richard (30 September 2021). Hurricane Sam Advisory Number 33 (Report). National Hurricane Center. Archived from the original on 6 October 2021. Retrieved 10 October 2021.
  38. ^ Blake, Eric (30 September 2021). Hurricane Sam Discussion Number 30 (Report). National Hurricane Center. Archived from the original on 1 October 2021. Retrieved 11 October 2021.
  39. ^ Pasch, Richard (30 September 2021). Hurricane Sam Discussion Number 33 (Report). National Hurricane Center. Archived from the original on 1 October 2021. Retrieved 11 October 2021.
  40. ^ a b Papin, Philippe (1 October 2021). Hurricane Sam Discussion Number 34 (Report). National Hurricane Center. Archived from the original on 5 October 2021. Retrieved 11 October 2021.
  41. ^ Berg, Robbie (1 October 2021). Hurricane Sam Discussion Number 35 (Report). National Hurricane Center. Archived from the original on 1 October 2021. Retrieved 11 October 2021.

Public Domain This article incorporates public domain material from websites or documents of the National Weather Service.

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