Infrared satellite loop with lightning flashes overlaid. (weathernerds.org)

A disorganized low pressure system has been festering over the western Caribbean Sea all week, and the odds of it forming and affecting the Gulf Coast are increasing. It is not in any hurry though, so if it should develop, it would not reach the Gulf Coast until at least Wednesday. If it reaches tropical storm status, its name will be Michael.

Most, though not all, forecast models now predict a tropical storm will reach the northern Gulf Coast sometime between Wednesday and Saturday next week, depending on how soon it starts crawling out of the Caribbean.

If the storm directly affects the Gulf Coast, there is a wide range of possibilities about exactly where it comes ashore. Simulations suggest the landfall point could span from western Louisiana to South Florida. While it’s too soon to detail how strong the system will be or if and where it will strike, tropical systems that move through the Caribbean and Gulf of Mexico draw vast amounts of moisture northward and can become prolific rain producers.


Simulations of the development and track of a possible tropical storm from European and American modeling systems. The European system contains 50 simulations, while the American system has 20. (Alan Brammer/State University of New York at Albany)

The developing system could get a boost from another disturbance on the west coast of Honduras in the eastern Pacific Ocean. That system’s circulation could get drawn toward this area and enhance the chances of a tropical storm forming.

The National Hurricane Center is giving the system a 30 percent chance of becoming a tropical depression or storm by Sunday and a 60 percent likelihood by Wednesday. It’s famously hard to predict exactly when things will “click” for this type of system. They can sit in place for well over a week, and for one reason or another, the right ingredient falls into place at the right time and it becomes a tropical cyclone.

The western Caribbean is a favored location for tropical cyclone formation at this time of year. These broad circulations, called monsoon gyres, slowly form in and migrate from a zone of low pressure where the monsoon develops. This is an entirely different mechanism from the tropical waves we watch come off Africa from mid-August through mid-October, and they almost always track north toward the Yucatan Peninsula, the Gulf of Mexico or Cuba.


Historical tropical cyclone formation locations during the first third of October. (NOAA/NHC)

Looking back at the season as a whole so far, we have seen 12 named storms, six hurricanes (Category 1+), and one major hurricane (Category 3+). Of the 12 named storms, half were subtropical cyclones — holding characteristics of both tropical and nontropical storms — at some point in their lives, an unusually high percentage. In terms of Accumulated Cyclone Energy, or ACE, the season is at about 108 percent of average for the date.