Behind the Forecast: Subtropical storms

Behind the Forecast: Subtropical storms
GF Default - Science Behind the Forecast: Subtropical Storms

LOUISVILLE, KY (WAVE) - It’s still hurricane season so talk of tropical storms is quite normal. The term “subtropical storm” sometimes pops up at this time of year. So what is it?

The National Hurricane Center defines a subtropical cyclone as “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.”

Rare Subtropical Storm off the Coast of Chile
Rare Subtropical Storm off the Coast of Chile (Source: NOAA)

Okay. So what does that mean in simpler terms.

Think of a subtropical cyclone as a weird weather hybrid. They form from extratropical storms which have colder temperatures in the upper levels of the atmosphere than a typical tropical storm. Extratropical storms are the type that we usually see sweeping across the country with warm and cold fronts. Subtropical storms do not have associated fronts but exist in the same latitude ranges as extratropical storms; think areas further north.

Since they have colder upper level temperatures, they can form in areas where the ocean is cooler. They can form earlier or later than normal tropical systems since very warm water is not a necessity. If one of these systems travels into a warmer area then it can transition into a tropical storm. Subtropical Storm Alberto formed in May 2018.

Tropical systems are quite symmetrical visually and with regards to temperatures and dewpoint spread within the storm. Subtropical systems have warmer temperatures, higher dewpoints and more moisture in their eastern half. Because of this, most of the cloud cover, showers and thunderstorms will be east of the center of the storm. Thunderstorms are also far away from the center of the system. Unlike tropical systems that have the strongest winds near their center, a subtropical storm will have their strongest winds away from the center. Like a strong tropical cyclone, a subtropical storm often has a cloud-free center.

Visible satellite image of Alberto on the morning of May 27, 2018.
Visible satellite image of Alberto on the morning of May 27, 2018. (Source: College of DuPage/Nexgen Weather Lab)

If a subtropical storms stays over warm water for an extended period of time, thunderstorms can build closer to the center. Eventually the heat given off from these thunderstorms can warm the air enough to transition the subtropical storm into a fully tropical one. This puts the strongest winds and precipitation closest to the center of the storm; when this happens the storm can continue to strengthen.

Tropical cyclones can have winds more than 150 mph (157 mph or greater is a Category 5 hurricane) but maximum sustained winds in a subtropical cyclone typically stay below 74 mph, slower than what you can legally drive on some highways.

Before 2002 these storms were not given names. Gustav in 2002 was the first ever subtropical storm to be named.

Science Behind the Forecast: Subtropical Storms

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