Sam was a mere tropical depression Thursday morning, but by early Friday it stood at hurricane strength after a textbook rapid intensification. It’s set to make a run at major hurricane status by Saturday, potentially becoming a Category 4 hurricane as it churns across the open Atlantic.

No tropical storm or hurricane alerts are in effect for any place yet, since the storm will likely avoid land. However, there is still plenty of uncertainty in the eventual track of Sam, and the northern Leeward Islands and Puerto Rico should keep close tabs on Sam’s progress.

Sam is likely to join the ranks of Grace, Ida and Larry, which all attained Category 3 or greater status this year. Category 4 Ida brought devastation to southeast Louisiana with winds topping 150 mph upon landfall.

The season to date has been slightly above average, with Accumulated Cyclone Energy, or ACE, running about 12 percent ahead of the norm. ACE is a metric that describes how much energy tropical cyclones expend in their winds and hinges on storm intensity, longevity and size.

Because Sam is expected to remain near or at peak strength for several days and likely persist into the first week of October, there is a decent chance it will generate much more ACE than Ida did.

There are signs to support an additional slew of tropical systems developing in the coming weeks, which would exhaust 2021′s preset list of conventional hurricane names. At present, only three names remain — Teresa, Victor and Wanda.

Unlike in previous cases, during which the Greek alphabet was used to name storms once a season’s main list was cycled through, the World Meteorological Organization this year has created a supplemental list that will be dipped into. Adria, Braylen and Caridad are first up on that list.

Sam currently

As of 11 a.m. Friday, Sam was a 75 mph Category 1 hurricane as it churned west at 14 mph. It was located roughly halfway between the west coast of Africa and the Lesser Antilles. The Hurricane Center noted it was taking a “momentary pause” in strengthening but would likely pick up its intensification soon.

Rapid intensification has been ongoing since the storm’s inception due to a very favorable environment. Like an atmospheric powder keg, the combination of extremely warm sea surface temperatures in the Atlantic’s Main Development Region and relaxed upper-level winds has allowed Sam to quickly organize.

On infrared satellite, a cluster of intense convection, or shower and thunderstorm activity, can be seen blossoming near the center, while high-altitude outflow clouds radiate away from the storm’s periphery. That marks the exhaust of the storm; clearing “spent” air away from the system is instrumental in allowing it to ingest more moisture-rich air at the low levels.

An early-morning microwave satellite image (shown above), which more efficiently resolves processes occurring below cloud level inside the storm, indicated attempts at an inner core to form. It was present primarily only on the eastern half of the storm, but will probably fill in during the remainder of the day. The National Hurricane Center notes it wouldn’t be impossible for Sam to reach Category 3 strength, which begins at 111 mph, by the second half of Saturday.

What to expect

Sam will be a monstrous hurricane before long — probably sometime in the next 48 hours. There isn’t much holding it back, which makes forecasting its whereabouts so vitally important.

The main wild card at first was how quickly Sam would develop, since a stronger, taller storm initially would feel winds at the upper levels that would tug it a bit farther north. That appears to be on track.

The strength and placement of Atlantic high pressure, which stretches from the Azores to east of Bermuda, is also a key player since it acts like a steering guardrail. Instead of continuing to suppress Sam to the south, the storm will probably escape northward before getting too close to the Leeward Islands, recurving out to sea.

That said, there are still at least five or six days left before Sam makes its closest pass, during which a lot can change — so it’s too early to let our guard down.

Other systems to watch

Elsewhere in the Atlantic, a couple other systems are being monitored for potential tropical development. A cluster of thunderstorms near Bermuda has a 40 percent chance of acquiring tropical characteristics in the medium range, but its long-term prospects don’t suggest much more than a marginal system at most.

Earlier forecasts also suggested the disintegrated remnants of Odette would have a chance at redeveloping over some of the warmer waters of the north central Atlantic, but the window for that to occur is quickly closing.

There is another tropical wave exiting Africa west of Guinea-Bissau that has a medium chance at development, but it’s possible that Sam could set processes in motion that would limit its potential. Once Sam drifts north toward the jet stream in early October, it could induce the formation of a “tropical upper tropospheric trough,” or TUTT, which would enhance upper-level winds enough to potentially tear a fledgling system apart.