At 11am on Wednesday, May 9th, the National Hurricane Center put out its first advisory on the storm off of the southeastern coast of the United States. They called it Sub-Tropical Storm Andrea.
(Sub-Tropical Storm Andrea - Source: whptv.com)
So, what is a "Sub-Tropical Storm" anyway? Why don’t we just call it "Tropical Storm Andrea?"
Well, let's figure it out...
The National Hurricane Center defines a Sub-Tropical Storm as “a low pressure system that develops over sub-tropical waters that initially has a non-tropical circulation but in which some elements of tropical cyclone cloud structure are present.”
What the heck does that mean? Well, pretty much, that’s saying that it’s not the normal low-pressure storm system that brings us rain in our day-to-day weather pattern here in the US. But, it doesn’t meet all of the criteria in order for it to be called a Tropical Storm.
So, what does a storm have to have to be called a Tropical Storm? Well, it needs a “warm-core.” This means that it’s feeding off of the warm ocean waters, which help it to maintain it’s strength or even strengthen. Your normal run-of-the-mill storms that pass through Pennsylvania and the US are “cold-core” storms.
So, sub-tropical storms are a hybrid (if you will) storm, with both tropical and non-tropical characteristics.
What’s interesting about these storms, is that more often than not, they form outside of the typical Atlantic Hurricane Season (June to November), and are not as rare as you think…
Sub-Tropical Storm Nicole formed near Bermuda in October 2004. Interestingly, she did not go on to become a Tropical Storm.
(2004 Sub-Tropical Storm Nicole - Source: wikipedia.org)
And, during the very active 2005 Atlantic Hurricane Season, a sub-tropical storm formed off of Africa (near The Azores). This storm never got a name, because it was determined to be a sub-tropical storm during post-season re-analysis of the year’s storms. If it were to have been given a name, it would have been called Sub-Tropical Storm Tammy.
(2005 Un-Named Sub-Tropical Storm - Source: wikipedia.org)
A lot of times, these storms form on the southern tip of weakening frontal systems. This is similar to what happened with Andrea.
So, there you go – your Weather 101 lesson for today.
