What's the Difference Between Subtropical and Tropical Storms?

Alberto developed over water temperatures of over 80 degrees, but this system had help in its formation by the upper levels of the atmosphere, but not simply from the warm, humid air near the water.

If you look at the graphic below, you'll see these upper-level winds moving away from Alberto's eastern and northeastern sides. These winds are fanning air, moisture and clouds away from Alberto's center. In meteorological terms, this is called divergence. 

Winds at the level where planes fly – roughly 30,000-35,000 feet – are strong and have been strong since early this week across the Gulf of Mexico. 

If these winds are too strong, Alberto's surface low would just blow away. If these winds are weak, then we are more likely to see a more symmetrical system. 

Right now, though, Alberto is in a bit of a Goldilocks zone where these winds are helping it develop. Those winds are removing air from the tops of Alberto's circulation. That air has to be replaced at the bottom of Alberto's circulation. If this replacement of air can keep up with the air being taken off the top, it can and so far, has strengthened Alberto. 

Can subtropical systems become tropical in nature? 

When subtropical systems are above water temperatures over 80 degrees, it is common for upper-level winds to relax. This is what is expected over the next few days. 

The warmer water temperatures of the Gulf of Mexico will also tend to warm the core of Alberto's low-pressure circulation through this weekend which should, when combined with lessening upper-level winds, allow Alberto to become a conventional tropical storm.

Difference Between Tropical and Subtropical Storms

When an area of low pressure forms over waters with sea-surface temperatures of at least 70 degrees, a subtropical low can form. This is due to the core of the storm becoming warm, deriving some of its energy from latent heat, or energy released when water vapor that evaporated from the warm water is condensed into liquid.

A subtropical storm exhibits features of both tropical and non-tropical systems. This includes no cold or warm fronts, a broad wind field and thunderstorms removed some distance from the center of circulation.

Mature subtropical systems also often have a large, cloud-free center and a less symmetric wind field. Maximum sustained winds are also much farther from the center, while the strongest winds in a tropical storm are close to the center.

(MORE: Water, Not Wind, the Deadliest Factor in U.S. Hurricanes and Tropical Storms)

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.

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

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.

As a result, the strongest winds and rain become closer to the center and, with time, further intensification becomes possible.