So, believe it or not, we have our first named storm of the year, and we still have a few weeks before the official start of tropical season.
Sub tropical storm Andrea was named yesterday (wed. may 9) afternoon, and has since been downgraded to a tropical depression. It is very interesting that they decided to name this subtropical system... Pretty soon, all year will be tropical season...
the difference between extratropical storm systems and tropical storm systems is that extratropical is a cold core system, and tropical storms are warm core.
So, what does this mean:
extratropical cyclones develop from your average frontal storm systems (caused by temperature gradients), whereas tropical storms develop from rising air motion created over warm waters. A subtropical storm is a combination of the two or an in between stage.
Taking a look at sub tropical storm Andrea (the storm that developed this week), the strongest winds and swells were from the frontal system draped off the coast and interaction with high pressure in the northeast. As that system sat offshore a defined center of circulation developed off the southeast coast right over the warm gulf stream waters.