Updated at 9:05 am on July 7th
Updated at 7:14 am on July 8th
I have been seeing rumblings from meteorologist colleagues about something "tropical" forming in the Gulf of Mexico next week. It remains to be seen whether anything pans out. However, there is something about the potential system grabbing the attention of the weather community. If a tropical disturbance does form, it may develop from a weather system over land right now. While this may be odd, Weather Channel meteorologist Greg Postel tweeted:
There is nothing phenomenologically unusual about tropical development from mid-altitude MCVs. Whether they come from continents, or the tropics, matters not. It just takes the proper setting for them to evolve in the right way, and extend that circulation to the surface.
I decided to take a closer look at what is going on.
A trough of low pressure over western Kentucky, Tennessee and northern Mississippi is forecast to move over the northeastern Gulf of Mexico where a low pressure area could form early next week.Some gradual development of the system is then possible as it drifts westward over the northern Gulf of Mexico through midweek.
I have to confess that upon first glanced at the graphic above, I assumed the system was drifting from the Gulf of Mexico into the southeastern U.S. It's actually the other way around. (Update: The Hurricane Center has upped chances of development to 80% within 5 days)
At this point, I want to revisit Dr. Postel's Tweet. A mid-altitude MCV is a mesoscale convective vortex present in the middle portion of the atmosphere. Meteorologist Jonathan Belles wrote in Weather.com:
These swirling areas a few thousand feet above the ground are produced by thunderstorm clusters. Because they already have spin, moisture and energy, they can occasionally become tropical systems once over water.
Computer models are hinting at the possibility that a couple of old frontal boundaries may interact in the Gulf of Mexico to enhance convergence needed in the genesis of a tropical disturbance.
A 2003 study published in the Journal of Atmospheric Sciences described how a cyclonic vortex in the middle troposphere, which is often a lingering feature of rain-producing cloud clusters called mesoscale convective systems, can evolve into a warm corm tropical system. The study points out that deep thunderstorms with cyclonic spin in their cores (called vortical hot towers) can play an important role in priming the lower, middle, and upper atmosphere for tropical cyclogenesis, particularly if pre-existing triggers or convergence are in place. A 2006 study in the journal Monthly Weather Review reviewed the multiple scale nature of tropical cyclogenesis. The authors reviewed several storms, including Tropical Storm Allison (2001, Texas) and Tropical Cyclone Ed (1993, Guam), that likely developed because of smaller-scale mesoscale vortices interacting with or enhancing lower level circulations required for a tropical system to strengthen. A more recent 2017 study in the same scientific journal describes how a mid-tropospheric MCV associated with a mesoscale convective system that drifted offshore in the Philippines produced Tropical Storm Mekkhala. They posited that land-based convection can play an important role in focusing the vortex and spinning up a low-level circulation.
National Hurricane Center tropical meteorologist Eric Blake posted an interesting Tweet on July 6th:
Well I was walking in Memphis.... One of my favorite outlooks that I’ve issued due to its weirdness. It will be a true model triumph if it develops (some could be correct 7 days in advance!)
