Wednesday, May 09, 2007

SUB-Tropical Storm Andrea?!

Say what?? It hasn’t happened very often, in fact, just a handful of times, but we have a “sub-tropical” storm in the Atlantic Ocean. This, as you may suspect, is causing a lot of confusion. Most people know what a tropical storm is, kind of the minor league version of a hurricane. But this hybrid classification comes about because the meteorological dictionary doesn’t quite know what to do with a storm that “looks” like a tropical cyclone with the tell-tale spinning clouds that forms outside the tropical latitude belt.

So how does this come about? Well, at its core, any tropical system is low pressure at the surface, usually fueled by warm water. As you up in the atmosphere, the air remains warm, as forming clouds release heat, which continues to rise, like in this diagram prepared by the Weather Service:

Sub-tropical storm Andrea began as low pressure off the North Carolina coast a few days ago, but then began drifting south of all things, into warmer water. Aloft, there was cold air, not warm air, yet the satellite pictures were taking on the familiar swirl pattern of a normal tropical storm.

So, early Wednesday, the Hurricane Hunter C-130 aircraft probed the storm to find out what was happening and sure enough, they found a center of low pressure and wind fields consistent with a tropical storm. Especially noteworthy were the sustained winds of over 39 mph in a circular core around the center. Even though the system was lacking all of the features of a true tropical storm, it was already battering the South Carolina, Georgia and Florida coasts with higher than normal waves. By elevating the system to some kind of tropical storm status, the Hurricane Center might have just been trying to ensure that the public was aware that something was out there. The spiral rain bands might even bring some sporadic rain to the wildfire ravaged parts of southern Georgia and northern Florida.

Yes, it is unusual that we’re talking about a tropical system this early in the season. After all, Hurricane Season doesn’t start until June 1. So we scoured the history books and found out that there are only a handful of seasons in which storms formed so early. Here’s a breakdown:

Earliest storm: Feb 2, 1952
Earliest year with a storm: March 6, 1908
Earliest sub-tropical storm: Ana - April 20, 2003

In case you’re wondering, a year with an early storm doesn’t necessarily lead to a busy year for storms. For example, 1952 had 7 storms overall and 1908 had 10. However, 2003 did end up with 16 storms after the early start with Ana. An average year has around 11 storms.

One thing is interesting for this season already. Last year, we lacked the large high pressure in the Atlantic that acts as a steering mechanism for storms traveling across the ocean. This year, the high is already setting up. Also, we’re noticing that the tropical Atlantic and eastern Gulf waters are pretty close to 80 degrees.

That temperature is a trigger for storm formation. It’s too early to predict doom and gloom, but so far, some of the signs are pointing to a busy year. Check back with me in August and we’ll see how things are shaping up.

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