L'Heureux's Weather: Tropical Storm Alberto

Google Earth image of Tropical Storm Alberto

For those of you who have been in the North Carolina area for a while, a tropical storm forming off the coast during the month of May should not be a surprise. Statistically, one storm forms during the month of May every other year, and those that form off the coast of the Carolinas during this time of the year can form from a backdoor cold front. Before we go any farther, I would like to introduce you to Subtropical Storm Andrea.

Subtropical Storm Andrea visible satellite, source Wikipedia

Subtropical Storm Andrea water vapor, source Wikipedia

This storm and the current tropical storm have a remarkable amount in common. Both formed from a backdoor cold front and both were originally attached to a cold core upper level low. Both went through a process of a burst of convection underneath the upper level low. This is not atypical: cold core upper level lows that either detach from the upper level trough or are still attached to the trough and move towards the tropics steadily lose the cold/dry air contained within it. Convection that occurs nearby the cold core low can actually moisten the environment around the low, making it more conducive for thunderstorms to form closer to the center. Prior to a subtropical storm forming, you will see a scattered band of convection displaced from the center of circulation.

But what is a subtropical storm? A subtropical storm is a storm that has some characteristics of a tropical storm, but may not fully meet the criteria (weak surface low, still attached to upper level low, still transitioning from a cold core to a warm core system). A subtropical storm does need to lose most if not all of its frontal characteristics.

If the surface circulation becomes better identified, has the strongest convection near the center, and is a true warm core low, then it will become a true tropical storm, such as Alberto. This never happened with Andrea; Andrea had too much dry air disrupting thunderstorm formation near its center. The next four images below are from SRRS and show how both Andrea and Alberto formed from backdoor cold front systems.