The odds for subtropical develop are low but not zero across the eastern Gulf of Mexico. An upper-level low is now near-stationary over the Gulf and will remain over the general area much of this week. Meanwhile at the surface, a weak surface low is expected to form in the northeastern Gulf of Mexico. By midweek, it may organize further into a subtropical low, therefore designating this area of disturbed weather as either a tropical depression or a tropical storm if sustained winds exceed 39 mph.
What is a subtropical low? Basically, it contains some characteristics of a tropical cyclone, and the National Hurricane Center therefore recognizes these systems and name them if warranted. Here’s their full definition: “A non-frontal low-pressure system that has characteristics of both tropical and extratropical cyclones. Like tropical cyclones, they are non-frontal, synoptic-scale cyclones that originate over tropical or subtropical waters, and have a closed surface wind circulation about a well-defined center. In addition, they have organized moderate to deep convection, but lack a central dense overcast. Unlike tropical cyclones, subtropical cyclones derive a significant proportion of their energy from baroclinic sources, and are generally cold-core in the upper troposphere, often being associated with an upper-level low or trough. In comparison to tropical cyclones, these systems generally have a radius of maximum winds occurring relatively far from the center (usually greater than 60 n mi), and generally have a less symmetric wind field and distribution of convection.”
Currently, the National Hurricane Center is giving this system a 10 percent chance for tropical development within the next 48 hours. It is expected to meander over the northeastern Gulf of Mexico before moving onshore and into the Florida Panhandle between Wednesday and Thursday.
The atmosphere will be somewhat favorable for the organization of this system to become a subtropical depression or storm. Wind shear will be light early in the week. Low wind shear, or light upper-level winds, is conducive for the organization of tropical cyclones. Unlike thunderstorms that rely on wind shear, tropical cyclones get ripped apart by shear. They also need warm sea surface temperatures, and we’re getting to that time of the year when temperatures become favorable and warm enough to support tropical systems. Temperatures are currently in the upper 70s to low 80s in the eastern Gulf of Mexico. The only negative is that there will be some dry air that entrains into the western side of the possible low-level circulation.
Regardless if this tropical cyclone forms, heavy rain will be the main threat. This storm will aid in the deep flow of tropical moisture from the Gulf of Mexico through this weekend.