Tropical disturbance in Atlantic remains weak, encountering more dry air


Visible satellite image from 7:45am EDT. The wave is very disorganized with limited thunderstorm activity this morning.

An area of strong thunderstorms in the Atlantic is facing some challenges on Wednesday on its path to development. While chances still seem high that this system could become Tropical Depression Three, the surrounding environment is keeping the disturbance weak. The National Hurricane Center is giving the area of thunderstorms a 70% chance of development over the next five days.

Thunderstorm activity in the system, which is centered about 1,080 miles east of Trinidad, appears limited on satellite. Enough dry air has evidently worked its way into the wave’s circulation to keep thunderstorm activity at a minimum. The low vertical wind shear and warm sea surface temperature are both conducive for tropical cyclone development, but the dry air is enough to put the brakes on any intensification.

In a storm-relative sense, the circulation is ingesting low-to-mid level environmental air from the west and and north, which is exactly where the driest air is right now. Models forecast the system to become more isolated from the dry air by Thursday morning, which should allow for thunderstorm activity to become more vigorous and for strengthening to commence.


Infrared satellite image of the easterly wave (lighter shades are higher cloud tops associated with thunderstorms). The yellow-red coloring is the strength of the Saharan Air Layer (SAL)… and the dustiest and driest air is colored red here. The circulation is wrapping in dry air from the north and the west. (CIMSS)

Model guidance has come into better agreement on the long-term track. It is still expected to pass over the Leeward Islands on Saturday, and close to Puerto Rico on Sunday morning. Then, it’s looking more and more likely that it will recurve near or east of the Bahamas. However, the longer it remains weak, the further west it can advance before starting to recurve. So if the models are all too bullish on the intensification, they’ll also be too quick to nudge it north.


Track forecasts from 2 global and 3 regional models. These runs are from 2am EDT today, so 120h would be 2am on Monday, for example. (U.Albany)

Now, here’s an interesting tidbit. If I search the records for all tropical cyclones that passed within 100 miles of where this disturbance is now during July and August, the result is a bit surprising: I find just four storms and they remained weak and far south. However, if I take the same location but search September and October instead, the result is quite different – we see a number of storms strengthen and turn to the north. If this system does indeed strengthen and turn to the north, it would be the first time any storm has done that from this location this early in the season.


All tropical cyclones that passed within 100 miles of where the current tropical wave is located during July-August (top) and September-October (bottom).
Brian McNoldy works in cyclone research at the University of Miami’s world-renowned Rosenstiel School of Marine and Atmospheric Science (RSMAS). His website hosted at RSMAS is also quite popular during hurricane season.
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Ian Livingston · July 30, 2014