The Hurricane Imposter: A Non-Tropical Low
You likely heard the news over Memorial Day weekend as a non-tropical low-pressure system caused quite a stir. As the system was approaching land, a Carnival cruise ship sailed directly into its path and was met with hurricane-force winds (74+ MPH). Not only were winds present, but there were massive waves over 15 feet. The strong combination led to flooding and much damage to the ship as it attempted to get back to port.
That same system would soon track inland into the Carolinas, disrupting plenty of Memorial Day Weekend plans, including NASCAR’s Coca-Cola 600.
The National Hurricane Center (NHC), located in Miami, Florida, was monitoring the system prior to its declaration in the Carolinas. The agency gave the low a 10% chance of developing into a tropical system while warning of the expected impacts.
Given the development and chatter this storm carried, many are quick to question why this wasn’t a tropical system. From above, it resembles a tropical cyclone, and it has hurricane-force winds. The main understanding relates to the definition of a tropical cyclone. According to the National Weather Service, a tropical cyclone is defined as:
A warm-core non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center…
This system resembles a synoptic-scale cyclone with notable circulation about an identifiable “eye,” albeit not the center of the storm. However, the NHC has stated that the system is not a warm-core cyclone, and it did not form from a tropical (easterly) wave.
While it is a low-pressure system (all tropical cyclones are areas of low pressure, which helps builds strength and causes wind), it was a cold-core storm as opposed to a warm-core cyclone. Due to the lack of a warm core within the storm, there simply is not enough available energy for this system to have grown and developed into a tropical cyclone.
In instances of a warm-core cyclone, warm air eventually encompasses the entire system, from the surface to high levels within the atmosphere. Due to the warmth, a feedback loop is created, and the storm strengthens as a result of an abundance of warm moisture. A strengthening storm would grow symmetrically outward, with the strongest winds and rains of the storm centered around the storm's eye.
In this instance of a cold-core storm, there is not enough “juice” to strengthen the storm. Although it can still circulate and grow in size, it won’t be symmetrical and will have more strength on its outskirts due to forming fronts. This low is just that.
In summary, a non-tropical low-pressure system formed off the coast of the Carolinas, leading to many disrupted Memorial Day plans. Despite the hurricane-force winds, the system was non-tropical as it was not a warm-core cyclone. So, while it resembled and acted like a tropical cyclone, it failed to strengthen and organize itself to become one.
