Tropical Storm Sebastian continues to gather strength as it churns northeast over the open Atlantic. (RAMMB/CIRA/NOAA)

Tropical Storm Sebastien is “expected to become a hurricane by tonight,” according to the National Hurricane Center. The unusually late-forming storm has 60 mph winds as it swirls 445 miles north-northeast of the northern Leeward Islands.

A look at satellite imagery shows Sebastien is a bit disorganized. It “continues to produce a large area of deep convection over the eastern semicircle of the circulation,” the National Hurricane Center wrote, noting the blossoming burst of thunderstorm activity right of the center. But to the west, the system’s low-level circulation is largely exposed. Moreover, “banding features are not very well defined,” according to the NHC. Sebastien is holding steady.


An animation of Sebastien shows the lack of convection (shower and thunderstorm activity) on the storm's southwest flank. This is a factor limiting its strength, but could soon change. (TropicalTidbits.com/NOAA)

But it’s expected to strengthen to an 80 mph Category 1 hurricane. That’s most probable late Thursday night or early Friday. The intensification owes to favorable high altitude “diffluence,” or the spreading apart of air at the upper levels of the atmosphere. That creates a sort of vacuum effect, encouraging more air to rise from the surface to fill the void, boosting the storm.


Infrared satellite imagery of Sebastien shows a more complete attempt at building an arcing line of thunderstorm activity back around the storm's possible center of circulation. This could be the first step toward organization and strengthening. (TropicalTidbits.com/NOAA)

In addition, water temperatures are plenty warm to foster Sebastien’s anticipated strengthening. So long as upper-level winds don’t intervene, late season storms can occur if storms have warm enough sea-surface temperatures to work with.


Sebastien is forecast to zip out to sea and be a storm impactful only to fish. (NOAA/NHC)

On the forecast track, Sebastien is expected to zip northeastward, accelerating Thursday and Friday before transitioning into an “extratropical” cyclone. After that, it will slip near or west of the Azores before potentially being absorbed into the overall circulation of a larger mid-latitude cyclone. That would come sometime late this weekend or early next week.

Sebastien’s development in late November is rather atypical, but not unheard of. According to Colorado State University hurricane researcher Philip Klotzbach, seven other Atlantic systems have developed into hurricanes after Nov. 20 since the satellite era began in 1966.

Among these were Karl, a puny hurricane that developed out of the center of a nontropical cyclone on Nov. 25, 1980, while situated about 610 miles south-southwest of the Azores. It’s the same process by which an unnamed hurricane formed in the middle of the “Perfect Storm” in early November 1991.


Olga spins on Nov. 27, 2001. (NASA/Worldview)

Since 2000, only three Atlantic hurricanes have formed after Nov. 20. Hurricane Olga, which formed on Nov. 26, 2001, was the second November hurricane of that season. It also became the largest Atlantic hurricane on record, as judged by the diameter of its tropical storm force (39 mph) winds, and even lasted as a tropical storm into December.

If you’re looking for a really late bloomer, try Hurricane Epsilon in 2005. Its name came from the Greek alphabet because the National Hurricane Center had already exhausted its list of 22 preset names, ending with Wilma. Epsilon developed on Nov. 29 as a tropical storm well east of Bermuda, defying the odds and becoming a hurricane on Dec. 2.


A shot of Epsilon as an annular hurricane taken from the International Space Station on Dec. 3, 2005. (NASA)

Epsilon became an “annular hurricane.” These are hurricanes that are largely symmetric, often featuring a large eye and one big ring of cool cloud tops and intense thunderstorm activity. Outside of that region of central dense overcast, annular hurricanes don’t have much in the way of spiral rain band or thunderstorm activity. They resemble doughnuts on infrared satellite.

The 2005 season did feature one more tropical cyclone, albeit below hurricane strength. Tropical Storm Zeta formed on Dec. 30 and lasted a little over a week, into 2006! Only one Atlantic cyclone on record — Alice in 1954 — developed later in the year than Zeta (by a few hours), and is also the only other Atlantic cyclone to “live” during two years.


Otto makes an extremely rare continental crossover on Nov. 24-25, 2005. (NOAA Naval Research Laboratory)

Most recently, we had Otto — which made landfall in Nicaragua as a major Category 3 hurricane on Nov. 24, 2005. Things got even weirder when Otto retained its eye structure as it crossed the continent, emerging over the Gulf of Papagayo in the Eastern Pacific on Nov. 25. It was the first Atlantic storm on record to cross Central America and keep its Atlantic-assigned name.

So while Atlantic Hurricane Season technically ends Nov. 30, don’t tune out quite yet. Hurricane Season only truly ends when the atmosphere is good and ready.