An astute reader pointed out that there's a strange storm system sitting off the eastern coast of Florida that strongly resembles a hurricane on radar imagery. Even though it looks just like a hurricane, it's not even close to being one. Why is that? Let's take a look.
The storm looks really impressive on imagery from the Melbourne radar site. The following loop runs from around 1:30 PM to 2:30 PM Eastern, showing a very tight circulation at the surface complete with heavy rain bands spiraling away from the center:
Here's the base velocity image from the same time, showing winds within the system. Red shows wind moving northeast, while green shows wind moving southwest. The area where the red and green are tightly wound together is the center of the low.
On both radar and satellite imagery, this feature certainly looks like a tropical system, but that's about where the similarities end.
Right now, the storm is a strengthening extratropical cyclone—the type of low pressure systems that we see on a regular basis in the United States. The biggest difference between an extratropical cyclone and a tropical cyclone is that the former involves different air masses—they produce cold and warm fronts—while the latter stays warm and muggy the whole storm through.
A quick look at model forecast dew points confirms that there are different air masses rotating around this low pressure system, making this an extratropical cyclone. Cold dew points indicate dry air, while warm dew points indicate moist air. One of the runs of the HRRR weather model this afternoon shows dry air wrapping around the northwestern side of the system, while a warmer, moister air mass exists to its southeast.
Tropical cyclones have a tropical air mass through and through—the entire storm is warm and humid from side to side, top to bottom. The Weather Prediction Center's latest surface analysis shows that there's a stationary front sitting through the center of the low, indicating one air mass to its west and one to its east.
If the storm were able to shed its fronts, there's a strong argument to be made that this is on the brink of becoming Subtropical Storm Isaias—radar estimates show winds between 45 and 50 MPH around the center 5,000 feet above the ocean— but as long as those fronts are there, it is not a tropical (or subtropical) cyclone of any sort. It's just a cool feature bringing rain to the Sunshine State.
While it's unlikely that we'll see any tropical systems form again until next year, coastal residents shouldn't let their guards down yet: hurricane season doesn't officially end until November 30.