Mobile, Ala. (WKRG) – During hurricane season you might hear broadcasters or forecasters say “subtropical storm,” “tropical storm,” “extra tropical storm,” or “post tropical storm.” It can be a bit confusing, but here is the breakdown.
News 5’s Caroline Carithers spoke to Lead Forecaster at the National Weather Service in Mobile, Don Shepherd, about this topic. Shepherd explains, “The two primary low pressure systems that people are used to hearing about is extra-tropical, which are the bigger winter systems, and tropical cyclones.” The difference in these two is primarily in the way they develop. Extra-tropical systems derive their energy from a temperature difference from cold/warm fronts where tropical systems develop from getting energy from warm ocean waters. Extra-tropical cyclones often have larger wind fields with the heaviest rain/snow concentrated fairly far away from the center of circulation. Tropical cyclones are more symmetrical and compact with the strongest winds occurring near the center (eye/eyewall).
Sometimes, we get a hybrid of the two. These systems are called “subtropical” and have characteristics of both extra-tropical cyclones (fronts/large wind field) and tropical cyclones (symmetrical/warm core).
Extra-tropical cyclones (can be nor’easters or called mid-latitude cyclones as well) usually form between 30 degrees and 60 degrees latitude. Tropical and subtropical cyclones normally form over warm ocean waters closer to the equator.
Shepherd expresses, “The main takeaway is that you don’t need to get caught up in the definitions (tropical and subtropical) when they are weaker like that; the impacts are similar.”
Finally, post-tropical cyclones form when a tropical system moves north and loses its tropical characteristics and becomes more extra-tropical with associated fronts and a larger/asymmetrical wind field.
For more details and visual explanations, check out the video!
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