By Dr. Jeff Masters  From Weather Underground

A strong tropical wave (92L) located a few hundred miles west-southwest of the Cabo Verde Islands at 8 am EDT Thursday was headed west to west-northwest at about 10 mph, and has the potential to affect the Caribbean next week. Favoring development of the wave were warm sea surface temperatures (SSTs) near 28°C (82°F), a moist atmosphere, and light wind shear near 5 – 10 knots. Satellite images on Thursday morning showed that the wave had a broad area of rotation, but 92L appeared to be ingesting some dry air from the Saharan Air Layer to its northwest, keeping heavy thunderstorm activity limited.

Figure 1: Invest 92L as seen at 8:35 am EDT Thursday, September 6, 2018. Image credit: Levi Cowan, tropicaltidbits.com.

Forecast for 92L

The 12Z Thursday run of the SHIPS model predicted that SSTs would slowly cool along 92L’s path through Saturday, slowing development, but wind shear was predicted to remain light, counteracting the cooler SSTs. SSTs will begin to warm by Sunday and wind shear is predicted to remain light, and the system should have increased chances for development early next week. The 0Z Thursday runs of our top three models for predicting tropical cyclone development—the European, GFS and UKMET models—all predicted development within five days. In their 8 am EDT Thursday Tropical Weather Outlook, NHC gave the system 2-day and 5-day odds of development of 70% and 90%, respectively. The next name on the list of Atlantic storms is Helene.

Figure 2. Predicted tracks for 92L from the 50 members of the 0Z Thursday run of the European ensemble forecast (left) and the 20 members of the 0Z Thursday GFS ensemble forecast (right). About 42% of the lower-resolution European model ensemble forecasts predicted that 92L would eventually enter the Caribbean, along with 68% of the lower-resolution GFS ensemble members. The European model’s high-resolution operational forecast (red line) predicted that 92L would make a sharp turn and go directly north before reaching the Caribbean, in response to the steering influence of a low pressure system associated with the remnants of Tropical Storm Gordon. Image credit: CFAN.

Steering currents favor a slow west to west-northwesterly track at about 10 mph over the next week, and 92L could arrive in the Lesser Antilles Islands as early as Thursday, September 13, as predicted by the 0Z and 6Z Thursday operational runs of the GFS model. About 42% of the 50 ensemble members of the 0Z Thursday European model and 68% of the 20 ensemble members of the 0Z Thursday GFS model showed 92L entering the Caribbean next week.

However, the 0Z Thursday run of the European model predicted that the remains of Tropical Storm Gordon would move off the coast of Newfoundland, Canada next Wednesday, and help create a large non-tropical low pressure system that would exert a profound steering influence on 92L, yanking it sharply to the north before reaching the Caribbean islands. The intensity of the ex-Gordon low pressure system that the European model is depicting over the Atlantic next week is likely to be overdone, though, given the uncertainties in the evolution of Gordon’s remains; I doubt that 92L will make a sudden northward turn next week.

Hurricane Florence may also end up playing a role in the steering flow that will end up dictating where 92L goes, further complicating the forecast. In summary, we are still several days from being confident of whether or not 92L is likely to be a significant threat to the Caribbean.

Another African wave coming on Friday

Another tropical wave that is predicted to emerge from the coast of Africa on Friday has had intermittent support for development from the European and GFS models; the 0Z Thursday runs of these models did develop the system, though the 6Z Thursday run of the GFS model did not. This wave is expected to follow 92L’s slow west to west-northwesterly path, passing a few hundred miles south of the Cabo Verde Islands on Sunday. In their 8 am EDT Thursday Tropical Weather Outlook, NHC gave the system 2-day and 5-day odds of development of 10% and 50%, respectively. The next name on the list of Atlantic storms after Helene is Isaac. The models have been consistently showing that this system is not likely to affect any land areas save for the Cabo Verde Islands.

We’ll have a post on Hurricane Florence, which is becoming of increasing concern to Bermuda and the U.S. East Coast, later today.

IMAGE:
Saharan Air Layer (SAL) analysis for 5 am EDT September 6, 2018. The dry air of the SAL (orange colors) lay between 92L and Hurricane Florence, and a tropical wave that will likely be designated 93L this weekend was on the coast of Africa. Image credit: University of Wisconsin CIMSS.

For more on this this story go to: https://www.wunderground.com/cat6/Two-Eastern-Atlantic-Tropical-Waves-May-Develop-Weekend