Some of you may be wondering what exactly subtropical storm Andrea was and how it was different from a typical tropical storm. A subtropical storm is a hybrid between a tropical and extratropical storm that exhibits characteristics of each.
Both tropical and subtropical storms have low pressure at the surface. What differs is how the pressure field changes with increasing height in the atmosphere. Extratropical storms, commonly referred to as mid-latitude cyclones, represent low pressure at all levels of the atmosphere from the surface to the tropopause. The pressure field is not vertical and the upper level low pressure is often tilted to the west of the surface low. The temperature field has cold air at the center of circulation, which leads to increasing intensity of the circulation with height (strongest winds are in the upper atmosphere). The influx of cold air aloft decreases the pressure field and maintains the trough in the upper atmosphere. These systems are referred to as cold-core lows, and are fueled by advection of warm-moist air and cold-dry air along frontal boundaries.
Tropical systems are warm-core lows which means there is warm air at the center of the circulation. The fuel source for these storms is convection of warm-moist air from the tropics (>80F air temp). The release of latent heat warms the atmosphere aloft which increases the pressure field and leads to a ridge of high pressure directly over the surface low. The intensity of the circulation decreases with height (strongest winds are near the surface).
Subtropical storms typically form when an extratropical storm (often a cut-off low) drifts over the tropics. In the case of Andrea, the eastern upper level circulation of the omega block drifted over the Gulf Stream just of the coast of Florida. Since an omega block represents a stagnant flow in the atmosphere, the upper level circulation began to transition into a tropical system as it sat over the warm water. It began to exhibit tropical characteristics (organized convection around the circulation, surface winds >40 mph) but the relatively cool water temps in the area (around 75F) kept Andrea from transitioning into a true warm-core low.
The National Hurricane Center only began naming subtropical lows in 2002. Perhaps the most famous subtropical storm was the “Perfect Storm” of October 1991.