A tropical wave exited Africa into the Atlantic on September 19. Finding favorable conditions, it began to organize, and a spin was soon evident on satellite imagery. It stayed pretty far south, missed Cabo Verde, and moved west over the next few days. By the 21st, a low had formed, but the disturbance didn't have the closed circulation center necessary to be called a tropical cyclone. One more day was enough for the system to become Tropical Depression Eighteen.
It took another day or so for the new tropical cyclone to start intensifying, but after that it was off to the races. On September 23, it strengthened into Tropical Storm Sam, which was the second earliest an eighteenth named storm had ever formed on record in the Atlantic (Sally of 2020 was the record-holder). Sam had a small core, which allowed it to quickly vault to hurricane strength on the 24th. There was little wind shear in the system's vicinity, and waters were plenty warm to support development; the moderately dry mid-level air around the storm was unable to penetrate it and slow down the intensification process. By the afternoon of September 25, Sam reached category 4 intensity.
After having moved west for a few days, Sam had slowed down and turned west-northwest under the influence of a weak subtropical ridge. Indeed, the ridge was so weak that Sam's forward motion was unusually slow for a hurricane in its location. The storm was still small enough that cold water upwelling didn't present a significant issue even with the lower forward speed. The cyclone had a small, symmetric eye surrounded by extremely cold cloud tops and managed to reach a peak intensity of 155 mph winds and a central pressure of 929 mb during the afternoon of September 26. Right afterward, the eye contracted and dry air forced its way into the center from the east during an eyewall replacement, causing sudden rapid weakening. This brought Sam down to category 3 by the 27th.
As the storm continued northwest, it fought back, restrengthening to category 4 the next day. Its structure was quite different, with a larger, less symmetric eye, and a larger radius of maximum winds. On September 29, it made its closest approach to the Lesser Antilles, passing well to the northeast. The next day, Sam begin to speed up and turn north as it rounded the subtropical ridge. Remarkably, the storm continued intensifying, reaching its secondary peak intensity of 150 mph winds and a central pressure of 934 mb early on October 1.
Overnight, Sam made its closest approach to Bermuda to the south-southeast; the island was far enough away to be spared significant impacts. Gradual weakening commenced as the storm recurved, but its tropical storm force windfield continued to expand. Early on October 3, Sam finally lost major hurricane strength after being category 3 or above for 7.75 consecutive days, joining only a small handful of Atlantic hurricanes to achieve this feat. Later that day, as it moved northeast toward the north wall of the Gulf stream, the eye cleared out again and became symmetric, with well-defined concentric eyewalls. This resulted in some restrengthening that evening near 40° N! The storm's structure was exceptional for that latitude. At last, on October 4, Sam encountered much colder water and began extratropical transition. As the inner core decayed, Sam weakened to a category 1. The storm became a hurricane-force extratropical low early on October 5 and merged with another low over the far north Atlantic soon after.
The above image shows Sam at its peak intensity on September 26.
Sam's long track as a major hurricane made it one of the largest Accumulated Cyclone Energy (ACE) producers recorded. Despite its longevity, Sam didn't affect any landmasses except Bermuda, to which it only delivered a glancing blow.
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