Visible satellite image of TD6 on Wednesday morning.

A disturbance that left the African coast this past Saturday has been gradually getting better organized. It was upgraded to Tropical Depression Six late Tuesday night and is very close to becoming the sixth named storm of the season, Fiona. If named, it would be the earliest “F” storm since 2012.

Tropical Depression Six is very far from land — centered about 1,700 miles east of the Lesser Antilles — and is tracking toward the northwest at 15 mph. Model simulations generally suggest that the system will not be a threat to land.

Over the next several days, the storm is expected to intensify modestly. As it enters a somewhat more hostile environment this weekend, drier air and higher vertical wind shear should put a brake on any strengthening, perhaps even keeping the system below hurricane intensity.

While the storm isn’t expected to affect land, there are some different future track scenarios.


Storm tracks from the 20 GFS ensemble members (colored lines) and the average of all of them (black line). (tropicaltidbits.com)

The GFS model is run every six hours in “deterministic” mode, with higher resolution and with more bells and whistles switched on. This same model run in its “ensemble” mode, which includes 20 slightly different runs at a reduced resolution (to save computing time), yields fairly different tracks and intensities. The black line is the average of those twenty runs.

Storms that form as far east as this one generally have faint landfall prospects. Of the 69 that have occurred in the past 30 years, only five made U.S. landfall as hurricanes. But those five were noteworthy: Gloria 1985, Hugo 1989, Georges 1998, Ivan 2004, and the most recent, Ike 2008. Category 5 Hurricane Dean from 2007 is also among the 69 but made landfall on the Yucatán Peninsula.

The vast majority of storms that form so far east end up curving to the north well before reaching land. If Tropical Depression Six is named within the next day or so, Fiona will join that majority in the history books.


Tracks of the 69 storms that reached tropical storm status east of 40°W from 1985-2015.

Another disturbance is expected to exit the African coast this weekend, so we’ll be watching it closely, too. With the exception of the area just west of Africa, water temperatures across the entire tropical Atlantic basin are still running slightly warmer than average for this date.