Enhanced satellite image of 96L from 10:45 am EDT. Yellow clouds are low-level, while white are upper-level. (NOAA)

An area of low pressure that moved off the coast of Africa late last week is being monitored for tropical cyclone development, with the potential to become the fourth storm of the Atlantic hurricane season.

The National Hurricane Center is giving this area of disturbed weather a 50 percent chance of formation within the next 48 hours, and a 70 percent chance within the next five days. If it continues to organize, it would be called Tropical Depression Four or Tropical Storm Danny, depending on the intensity. For now, forecasters are calling the disturbance Invest 96L.

The area of thunderstorms — an African easterly wave — can be traced all the way back to eastern Africa on Aug. 9, then it traversed the continent and entered the Atlantic Ocean on Friday. As of Monday morning, it was located approximately 600 miles southwest of the Cape Verde islands in the far eastern Atlantic, and still about 2,000 miles east of the Windward Islands.

These “Cape Verde” storms become the dominant variety in August, with origins over continental Africa moving westward across the deep tropics. Many of the most intense hurricanes on record are Cape Verde storms. Climatologically, it would not be unusual for this type of disturbance to develop into at least a tropical storm, if not the first hurricane of the 2015 season.


Red dots mark the tropical cyclone genesis locations for the 10-day period spanning August 11-20 (using data from 1851-2009). The cyan dot is the present location of the disturbance for reference. (NOAA)

While the current large-scale environment around this tropical wave is relatively benign, there is a lot of vertical wind shear and mid-level dry air ahead of it — two things that could seriously inhibit the development of the system.

It’s tricky to try and forecast how these elements will evolve over the next few days. The subtropical jet stream, which is causing the strong shear, may nudge northward as this disturbance stays to the south, thus making the environment more favorable for tropical cyclone development. The same can be said for the dry air — if the envelope of moisture associated with the disturbance can remain isolated, then the surrounding area of dry air will play much less of a role.


Analysis of the vertical wind shear (top) and the mid-level relative humidity (bottom) from the Monday morning GFS model. In the top image, warmer colors indicate higher wind shear. On the bottom, brown colors indicate areas of dry air. (tropicaltidbits.com)

The forecast from the suite of global and regional models is very mixed. Some of the models do not even produce a coherent, trackable area of low pressure, while others are rather aggressive with the intensification and take it on a west-northwest trajectory.

The model guidance will sharpen up if the disturbance develops into a tropical cyclone. Until then it’s extremely difficult to determine the storm’s peak intensity.

The track forecasts shown below are from 20 versions of the GFS model — the main GFS run is labeled AVNO, the average of all the runs is labeled AEMN, and the rest are each of the individual model runs. As of Monday morning, the GFS appears to be favoring a track toward the Caribbean, with the possibility that it will miss Puerto Rico and Hispaniola to the north.


Tracks from the 20 members of the GFS ensemble, from the 06Z run today. (UCAR)

Since 1851, we have records of seven tropical cyclones that developed within 200 miles of where this disturbance is currently located. Four of those storms went on to become major hurricanes — category 3 or higher. Roughly 90 percent of all African easterly waves never actually become tropical cyclones, and this one could certainly join the ranks of its many anonymous predecessors.

As of today, the Atlantic accumulated cyclone energy — a way to measure a season’s total activity — is at just 30 percent of where an average season would be by this date.


Tracks of past storms that formed in August and within 200 miles of where 96L is located. (NOAA)