The National Hurricane Center has designated the area of disturbed weather off our south and east coastline for the potential for further development over the next seven days. However, they evidently note in their discussion surrounding this feature, this is a NON-tropical area of low pressure. Why is that?
Have you ever heard the saying “home-grown season” during the months of June, maybe July, and then again during October and November of the Atlantic hurricane season? There’s a reason for this, and it all has to do with the tilt of the Earth, the transitioning seasons, and cold fronts.
If you’ve never heard that little metaphor before, a storm can be called “home-grown” if the contributor is something that happened to plop off the United States coastline and then marinate over open, warm waters long enough to produce showers and thunderstorms.
Why early and late season only? Why not the peak?
The feature off our east coast, while unlikely to truly organize into anything incredibly menacing, still poses the potential to throw heavy rains and gusty winds to those along the immediate coastline of Central Florida, Georgia, and the Carolinas.
Early and later into the hurricane season, these parts of the year are characterized by a phrase we used to use a lot in Air Force Weather - transition season.
All this means is we’re rotating from a fairly dynamic part of the year into a not-so-active or fierce part of the year in terms of our weather. Think about it.
During the winter and the spring, we’re battling a combination of mid-latitude cyclones; low-pressure systems producing snow storms, freezing rain, and sleet up north, and possible tornado outbreaks/severe weather across the south. The same can be said for spring. We’ve had a handful of cold fronts work their way through Central Florida, and we’ve seen the re-emergence of our sea breeze storms.
What typically begins towards October and November? The cooler air from up north begins to return. We start to see our polar front jet come back with a bit of an attitude, dropping low-pressure systems across the United States that sometimes make it as far south as us in the Sunshine State.
That’s where the home-grown name comes from.
Here’s how home-grown tropical features form
As we prepare to enter the summertime, the waters off the coastline of Florida tend to warm in sometimes dramatic fashion. That is certainly the case now, where waters off both our western and eastern shores are anywhere between 87-90 degrees Fahrenheit.
We also just had an unseasonably late cold front pass through our neighborhood early Saturday, and that same front is now hung up over our state. At the end of a frontal system, you usually have some leftover spin and energy that can occasionally get parked over open water.
The energy and lift provided by a front, combined with water temperatures capable of forming tropical-style thunderstorms, increases upward motions.
When you have upward motion in the atmosphere, you develop low pressure closest to the surface of the Earth. As a result, you start seeing that trademark counter-clockwise spin begin to take shape.
The name “home-grown” comes from the fact these areas of spin that can sometimes take on subtropical or tropical characteristics didn’t originate from a tropical wave across the Atlantic, or from the equatorial regions.
It was a direct result of something having wandered off the coast of the U.S. and staying over open water with favorable conditions above it to support the development of a center of circulation.
June, sometimes into very early July, and especially October and November, are usually seen as this portion of the hurricane season. Then, during peak summer months, when cold fronts are nearly non-existent (at least from an observable weather standpoint), we also see an enormous reduction in vertical wind shear over the majority of the Atlantic Ocean.
Now we’ve set the stage for the long-track portion of the season. Tropical waves find their way off the coastline of Africa into warm waters of the Main Development Region (a video on that will be in the works very soon), and you then tend to see tropical features forming far off to our east.
Most home-grown storms don’t go on to be major systems (Most)
As we close out, I also want to add — these attempts at subtropical or tropical formation off our shorelines don’t usually become as aggressive or intense as your more traditional tropical waves that go on to produce tropical storms or hurricanes.
Of course, there are exceptions to this rule, as with anything in the field of meteorology. But generally, because of the nature with which we end up with an area of rotation, the qualities about these entities aren’t the same as your natural course of building a tropical cyclone.
Forming off of old, decaying cold fronts, or upper-level pockets of cooler air high up in the atmosphere tends to result in broad circulations that aren’t too heavily packed together or tightly spinning like a hurricane you’d see in the Atlantic Ocean. They can be very heavy rain makers, instigate some hefty rip current threats and beach erosion, and maybe go on to drop a few tornadoes if they directly impact your area.
As always, I emphasize that even if they aren’t the scariest weather phenomena you’ve seen before, especially during hurricane season, they still demand a level of respect if one decides it wants to pay you a visit.
