What Is A Subtropical Storm And How Is It Different From A Tropical Storm?

To explain subtropical storms, we need to start by explaining these two different storm types with which subtropical storms share some characteristics.

Tropical storms, or more broadly, tropical cyclones, are low-pressure systems fueled solely by the heat energy released when water vapor evaporated off warm ocean water condenses into liquid. Due to all this heat, the core circulation of a tropical cyclone is warm and its strongest winds are usually found near its center.

With low wind shear and moist air, these tropical storms can intensify into hurricanes. The National Hurricane Center issues forecasts for each of these tropical cyclones once they become at least tropical depressions.

An extratropical or non-tropical low, however, derives its energy from contrasts between cold and warm air.

They always have at least one front – cold, warm or occluded – attached to them. You can see several examples in the current weather map below.

They can occur both over the ocean and over land, and are responsible for most of the precipitation that falls over the land areas in the middle latitudes. They take the shape of winter storms, powerful storms marching in from the Pacific Ocean, coastal storms along the East Coast, or even just a low driving a strong cold front out of Canada with relatively little precipitation.

An extratropical storm's core is cold, and its strongest winds are typically quite far from its center.

Subtropical Storms: A Strange Mix

An interesting thing happens when an extratropical or non-tropical low begins to warm up.

This can happen when the low slows down, tracks over ocean water just warm enough – usually at least 70 degrees Fahrenheit – and thunderstorms begin to percolate closer to the low-pressure center.

If winds aloft aren't strong enough to simply blow away the low-pressure center but instead help ventilate the storm, encouraging rising air and thunderstorms, the core of the low-pressure circulation will warm, first in the lower levels.

At that point, this system becomes a subtropical cyclone, exhibiting features of both tropical and non-tropical systems.

It then derives some of its energy from warmer ocean water, like a tropical storm, but also some energy from the lingering temperature contrast that originally spawned it as an extratropical storm.

Subtropical cyclones typically are associated with upper-level lows and have colder temperatures aloft, whereas tropical cyclones are completely warm-core and upper-level high-pressure systems overhead help facilitate their intensification.

Mature subtropical cyclones often have a large, cloud-free center with thunderstorms and rainbands displaced some distance away.

The NHC issues advisories and forecasts for subtropical depressions and storms. They are assigned a number or name, just like a tropical depression or storm.

Subtropical storms were not officially recognized until the beginning of the satellite era, and they weren't named until 2002.

(MORE: The History of Atlantic Subtropical Storms)

Strictly speaking, subtropical storms cannot directly intensify into hurricanes. But there is a path to doing so.

If the subtropical storm remains over warm water, thunderstorms can build close enough to the center of circulation, and latent heat given off from the thunderstorms can warm the air enough to create a fully tropical storm.

Once that happens, further intensification into a hurricane becomes possible.

Recent Examples

Subtropical storms don't occur every hurricane season.

When they do, it's typically early or late in the season, when colder upper-level lows and former extratropical lows in more southern latitudes are more common.

The first storm to use a Greek letter in the 2020 Atlantic hurricane season, Alpha, was a short-lived subtropical storm that quickly moved into Portugal.

In 2019, there were two short-lived subtropical storms in the central Atlantic: Andrea formed before the official season started in late May, and Rebekah around Halloween.

The 2018 hurricane season produced seven storms that were subtropical at least for part of their lifetime, a record number for any season.

Among these was Alberto, which eventually developed into a tropical storm. Its remnants remained intact as far north as Michigan in late May.

"The conditions over the central North Atlantic, where we had a lot of these subtropical systems, were extremely conducive," Gerry Bell, the lead seasonal hurricane forecaster at NOAA's Climate Prediction Center, told weather.com in 2018.

"We had record-warm ocean temperatures and also record-weak vertical wind shear. That was certainly a player in the overall strength of the hurricane season, as far as numbers go."

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