The Atlantic tropics are beginning to heat up as we get into mid-June. This morning, the National Hurricane Center (NHC) designated the African wave we have been tracking for the last few days Invest 92L. 92L is the first in-season “invest” of the 2017 Atlantic season. In their 8:00 p.m. EDT Friday Tropical Weather Outlook (TWO),  the NHC gave 92L a 40 percent chance of development within 48 hours and a 60 percent chance within five days. In addition to 92L, there is a developing area of low pressure in the Northwestern Caribbean Sea with a chance to develop into a disorganized tropical storm. The NHC gave this system a 20 percent chance of development within 48 hours, and a 60 percent chance within five days. The Atlantic has a chance to see simultaneous named storms in the month of June for the first time since 1968.

Invest 92L: A potentially historic storm in the making

At 00:00 UTC Saturday (8:00 p.m. EDT Friday), the tropical wave 92L was centered near 6.2°N 35.8°W. The wave had maximum sustained winds near 25 knots (30 mph) and an estimated minimum pressure of 1009 mb. 92L’s circulation has become better defined today, with microwave passes showing evidence of developing banding features. However, 92L lacks much central convection – likely as a result of another tropical wave to its east flaring up with thunderstorm activity. 92L needs to develop persistent deep convection near the center in order to be classified as a tropical depression. Model support for 92L is largely confined to the GFS model. The GFS model develops 92L into a fairly strong tropical storm passing south of the Windward Islands by next Tuesday. More than half of the GEFS ensembles develop 92L into a tropical cyclone as well. However, the ECMWF and CMC models do not really show any development with 92L.

The conditions ahead of 92L seem mostly favorable for development for the next few days. Sea surface temperatures are about 28°C (84.2°F), more than warm enough to support a tropical cyclone. Wind shear is currently light to moderate, but is expected to increase when 92L emerges into the Southeastern Caribbean by Tuesday. 92L is embedded in a fairly moist environment, although another tropical wave to its east is limiting 92L’s potential to develop its own thunderstorms. 92L’s development prognosis is much better than yesterday, in which I only gave it a 10 percent chance of development. If 92L develops, it will likely have a small circulation that will be easily disrupted by wind shear.

92L has potential to develop into a historic storm if it does develop. If it acquires tropical storm status, it would become the first June Atlantic named storm to develop east of the Lesser Antilles since Ana in 1979. In addition, 92L has potential to become the lowest latitude Atlantic tropical cyclone on record. At present, Hurricane Isidore of 1990 is the lowest-latitude Atlantic storm on record, forming at a low latitude of 7.2°N.  Based on the current structure of 92L, I agree with the NHC’s 40 percent chance in 48 hours and 60 percent chance in 5 days.  Since model support remains somewhat limited and the cyclone lacks persistent central convection, I do not think 92L warrants a “high” (70%+) chance of development at this time.

Northwestern Caribbean AOI: A potential messy tropical storm, track very uncertain

A broad area of low pressure has begun to develop over the Western Caribbean. This low pressure area is part of a large monsoonal gyre, and also has a chance to develop into a tropical cyclone over the next several days. The 12z Friday ECMWF and CMC model runs, as well as the 18z Friday GFS run, develop this system into a very disorganized tropical cyclone by Tuesday. The forecast track for this system is extremely uncertain. The GFS and CMC models show development occurring sooner and in the Northeastern Gulf of Mexico with a landfall in Northwestern Florida. The ECMWF disagrees, and predicts the cyclone to emerge over the western Gulf of Mexico with a landfall near the Mexico-Texas border. This is a similar situation to Tropical Storm Debby in June 2012, in which the ECMWF model predicted a Texas landfall while the GFS model predicted a Florida landfall. A United States landfall is definitely a possibility and this system will need to be monitored very closely.

An alternate possibility, suggested by the ECMWF model, is that a separate area of low pressure forms over the East Pacific in addition to the developing low over the Gulf of Mexico. This scenario has not been suggested by other models, and the NHC has not highlighted the development possibility in the East Pacific. The large size of the gyre, in addition to expected moderate to strong wind shear should keep this system weak and very disorganized if development occurs. The primary threat from this system will be heavy rain, even if development does not occur. I give this system a 70 percent chance of development within five days, slightly higher than the 60 percent chance from the NHC, since it has more model support than 92L.

I will be back for another post tomorrow about 92L and the Caribbean low pressure system, which should be designated Invest 93L soon.