Unusual eastern Atlantic Tropical Disturbance looking more likely to develop

The robust disturbance in the deep tropics that moved off the coast of Africa a few days ago is moving toward atmospheric conditions that appear to be conducive for development. The National Hurricane Center is giving the system a good chance of becoming at least a tropical depression early to mid next week in the waters east of the southernmost Caribbean islands.

It’s quite unusual to have an Africa-originated disturbance develop into a tropical depression or tropical storm in the Atlantic east of the islands in June. There are only four in the record book. Usually, hostile upper winds and dry air keep systems from organizing in the tropical Atlantic until later in the summer.

The most recent system to pull it off was Elsa, just last year, however. It was named a depression in the 11:00 pm advisory on June 30th. So it just snuck under the July wire.

Like this disturbance, it formed unusually far south, so it avoided the hostile conditions just to its north. Elsa became a hurricane just as it passed over the southeastern Caribbean islands of St. Lucia and St. Vincent in early July. Eventually, a weakened version brushed the Lower Keys and the west coast of Florida before coming ashore southeast of Tallahassee.

This year’s disturbance came off Africa far enough south that it was able to tap into the band of tropical moisture that always resides just north of the equator. The extra moisture has allowed it to hold off the very dry air just to its north. The consensus of the computer forecast models is that the disturbance is wrapped in enough moisture that it will be able to overcome its low-latitude start in life and survive its trip to the Caribbean.

A sprawling high-pressure system is well established across the Atlantic – the so-called Bermuda high. This will block the system from turning abruptly to the north, at least until it gets near the Caribbean.

The current schedule has the disturbance developing into an organized system and passing across the Caribbean islands Tuesday or Wednesday. How far north it gets will likely be determined by how much it can organize and how strong it gets while it’s in the Atlantic. A stronger system would likely track a bit more to the north.

The long-range computer models generally keep the system in the southern Caribbean, well away from the U.S., blocked by the persistent heat-dome high-pressure system over the Central and Southeastern parts of the country. But there are scenarios that would track it at least a bit farther north.

The rule that forecasts for disorganized or just-developing systems are always subject to larger-than-average errors applies in this case. So it’s not good to project too far ahead.

Other early-season disturbances are set to follow this one off Africa into the tropical Atlantic. At this point, the computer forecast models show the normal June inhibiting factors in the atmosphere keeping them from organizing. But we’ll see.

It might be that this disturbance is a one-off that was able to tap into just enough moisture to hold off the dry air and make it across the Atlantic.

Other than this disturbance, nothing appears likely to develop in the tropics through the middle of next week, at least.