Enhanced satellite image from Wednesday morning depicting mid-upper level water vapor — cool colors indicate dry air. (NRL-MRY)

The tropical Atlantic Ocean, usually buzzing with storms at this point in September, looks more like it should in June, when hurricane season is just starting.

Although there are two named storms active across the Atlantic, dry air and strong wind shear are dominating the basin in a pattern unfitting of the climatological peak of the season.

Reconnaissance aircraft have been flying into, over and around Karl to assess its intensity and structure, and they have found that is weakening. It was recently downgraded to a tropical depression, and it may even get downgraded again to an open wave later today given the trend. The circulation is centered about 900 miles south-southeast of Bermuda and tracking toward to the west at 14 mph.


Visible satellite image of Tropical Depression Karl northeast of the Leeward Islands. (NASA)

Forecast models suggest that Karl will make a comeback by the end of the week. It is expected to turn toward Bermuda and intensify to a hurricane as it interacts with a mid-latitude trough and acquires characteristics of a non-tropical weather system. Bermuda can expect Karl’s closest approach and worst weather on Saturday, though it may get spared hurricane conditions.

Bermuda averages one damaging hurricane encounter every six to seven years, but the tiny island has had some recent bad luck, with about six storms passing close enough to produce hurricane conditions in just the past decade.  As of the latest forecast, there is still a slight chance that Bermuda could experience hurricane-force winds as Karl passes by this weekend (assuming it is able to regain some organization).


Tropical Storm Lisa is located about 1600 miles east of Karl and is faring slightly better while immersed in a hostile environment. Lisa began as Tropical Depression 13, which formed Monday afternoon, and is now approximately 600 miles west of the Cape Verde islands. It has strengthened recently, and sustained winds are up to 50 mph.


Enhanced infrared satellite image of Tropical Storm Lisa. (CIRA/RAMMB)

Models generally agree that Lisa will continue its track to the northwest and find itself in a very hostile environment this weekend, so it is forecast to weaken and dissipate in the central Atlantic.

The large-scale pattern across the Atlantic would suggest that a lull in tropical storm activity may last for a while.

The anomalous dry air and strong wind shear prevalent across the basin may well combine to either choke off development of storms that manage to form or to prohibit formation altogether.

The graphic below shows the extent of the anomalously dry air. While Karl and Lisa show up as moist pockets, the drier-than-normal mid-level air is lurking in the deep tropics and working its way into their circulations.


Total precipitable water normalized anomaly, where browns indicate dry anomalies and greens indicate moist anomalies. (Tropicaltidbits.com)

Another atmospheric featur,e known as the Madden-Julian Oscillation, or MJO, is in a phase that typically suppresses Atlantic tropical cyclone activity, and it may remain in that phase through early October.

While the number of named storms this hurricane season is impressively above average for this date, the Accumulated Cyclone Energy (ACE) stands at about 70 percent of average because the storms that have formed have generally been weak and/or short-lived. One storm, Hurricane Gaston, contributed half of the season’s ACE, while the other half came from the other 11 storms.