|Above: Saharan Air Layer (SAL) analysis for 11 am EDT September 5, 2018. The dry air of the SAL (orange colors) lay between 92L and Hurricane Florence. Image credit: University of Wisconsin CIMSS.
A strong tropical wave (92L) located a few hundred miles south-southwest of the Cabo Verde Islands at 2 pm EDT Wednesday was headed west to west-northwest at 10 – 15 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 moderate wind shear near 15 knots. Satellite images on Wednesday afternoon showed that the wave had a moderate amount of spin and was growing more organized, but the dry air of the Saharan Air Layer to the northwest of 92L was keeping its heavy thunderstorm activity limited.
|Figure 1. Invest 92L as seen at 1:20 pm EDT Wednesday, September 5, 2018. The system's heavy thunderstorms were being disrupted by dry air to the northwest. Image credit: Levi Cowan, tropicaltidbits.com.
Forecast for 92L
The 12Z Wednesday run of the SHIPS model predicted that SSTs would slowly cool and the atmosphere would grow drier along 92L’s path through Saturday, slowing development, but wind shear was predicted to fall to the light range, counteracting the cooler SSTs and drier air. SSTs will begin to warm by Sunday and wind shear is predicted to be light, and the system should have increased chances for development early next week. The 0Z and 12Z Wednesday 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 2 pm EDT Wednesday 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 Wednesday run of the European ensemble forecast (left) and the 20 members of the 0Z Wednesday GFS ensemble forecast (right). About 25% of the lower-resolution European model ensemble forecasts (grey lines) along with the high-resolution operational forecast (red line) predicted that 92L would eventually enter the Caribbean, along with 18% of the lower-resolution GFS ensemble members. Image credit: CFAN.
Steering currents favor a slow west to west-northwesterly track at 5 - 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 12Z Wednesday operational runs of the GFS and European models. However, a threat to the Caribbean is by no means a sure thing, as the large majority of the 70 ensemble members of the European and GFS models show 92L embarking on more of a northwesterly track by early next week, missing the Caribbean islands (Figure 2). The evolution of Hurricane Florence may end up playing a role in the steering flow that will end up dictating where 92L goes.
93L coming this weekend?
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 latest 12Z Wednesday GFS and UKMET models did not develop the system, though the 12Z European model run did. 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 2 pm EDT Wednesday Tropical Weather Outlook, NHC gave the system 2-day and 5-day odds of development of 0% and 30%, respectively. The next name on the list of Atlantic storms after Helene is Isaac.
For information on Hurricane Florence, see our 11:30 am Wednesday post. Still to come today: Hawaii needs to pay attention to Hurricane Olivia.