Two systems in the Atlantic still have a chance to develop into a tropical cyclone. An elongated area of low pressure in the central tropical Atlantic, Invest 99L, has become slightly better organized today, but its chances of developing into a tropical cyclone are decreasing since the disturbance is expected to move into unfavorable conditions by early next week. A strong tropical wave over the central Caribbean, Invest 90L, has become a little better organized today, and has potential to develop into a tropical cyclone over the Gulf of Honduras or Bay of Campeche.

Invest 99L over Tropical Atlantic becoming less likely to develop in the short term

As of 00:00 UTC Sunday, Invest 99L was centered near 10.2°N 38.8°W, and was moving west-northwestward at about 15 mph. Maximum sustained winds were 25 knots (30 mph), with an estimated minimum pressure of 1010 mb. Although 99L has become a little better organized today with some deep convection, the showers and thunderstorms are not all that well organized. 99L also remains quite elongated. The National Hurricane Center (NHC) now gives 99L a 30 percent chance of development within 48 hours, and a 50 percent chance of development within five days. Model support for 99L has dropped today. The 18z Saturday GFS model run only showed limited development, into a tropical depression at most. The 12z Saturday UKMET model run predicted that 99L could develop north of the Lesser Antilles in about four to five days. The 12z Saturday ECMWF model run kept 99L as a weak low pressure system without development into a tropical cyclone. The most likely outcome is 99L approaches the Northern Lesser Antilles islands as an open tropical wave, although there is a chance it remains north of the islands. 99L is currently in a marginally favorable environment, with wind shear near 15 knots, mid-level relative humidity values near 60 percent, and sea surface temperatures of near 28°C (84.2°F). Wind shear is expected to increase to over 20 knots by Tuesday, and this should inhibit significant development. Although it is possible 99L briefly spins up into a tropical depression or weak tropical storm during the next few days, conditions do not currently support a strong tropical cyclone. However, if 99L manages to remain intact as a tropical wave without being disrupted by land interaction with the Greater Antilles, it will need to be watched for possible development in the long range. Last August, another tropical wave – also known as Invest 99L – took a similar track and struggled to develop in the tropical Atlantic, but eventually developed into a tropical depression over the Florida Straits. This depression eventually developed into Hurricane Hermine, which made landfall in the Florida Panhandle as a Category 1 hurricane. Hermine’s development is an reminder on why to never ignore tropical waves during the peak months of the season, even if development is not likely in the short term.

Invest 90L likely to develop over Gulf of Honduras or Bay of Campeche

As of 00:00 UTC Sunday, Invest 90L was centered near 14.1°N 77.8°W, and was moving west-northwestward at about 10 mph. Maximum sustained winds were 30 knots (35 mph), with an estimated minimum pressure of 1008 mb. The NHC gives 90L a 50 percent chance of development within 48 hours, and a 70 percent chance of development within five days. 90L has become better organized today compared to yesterday, with deep convection developing east of the center. Cloud tops have recently warmed east of the center, but some new convection has developed south of the estimated center. 90L continues to lack a well-defined surface circulation. The 18z Saturday GFS, as well as the 12z Saturday ECMWF and CMC models, predicted that 90L could develop into a tropical cyclone before the Yucatan Peninsula, and eventually strengthen over the Bay of Campeche. The 12z Saturday UKMET model run did not predict any development would occur. There is a low chance that 90L moves inland over Central America and does not reemerge over water, and development is very unlikely if this happens. 90L is expected to remain over waters near 30°C (86°F), with mid-level relative humidity values above 70 percent and wind shear less than 15 knots. Because of the favorable environment, 90L will have to be watched for rapid intensification if it can develop an inner core. In fact, the 18z Saturday GFS run predicted that 90L would become a Category 1 hurricane over the Bay of Campeche before landfall. I give 90L a 30 percent chance of developing into a hurricane over the Bay of Campeche, due to the very favorable conditions. An Air Force Reserve Hurricane Hunter aircraft is scheduled to investigate 90L beginning at 1:30 p.m. EDT tomorrow, if necessary. The Yucatan Peninsula, as well as the eastern coast of Mexico, should closely monitor the progress of 90L. Tropical storm watches or warnings may be required soon for the Yucatan Peninsula, and in that case, advisories would likely be initiated on a Potential Tropical Cyclone tomorrow if 90L still has not developed a well-defined surface circulation. The average formation date of the first Atlantic hurricane is August 10.

I will be back tomorrow with another post on 99L and 90L. The next two Atlantic tropical cyclone names are Franklin and Gert.