Tuesday, August 23, 2016

Tropical Depression Fiona, Tropical Storm Gaston, and the Atlantic Blobette: August 22, Update A

"Rather a busy day today Phipps... distressingly little time for sloth or idleness." - For Lord Goring, myself, and Mother Nature apparently! She has 3 storms (possible storms) in the Atlantic and another 5 storms (or possible storms) in other basins.

Tropical Depression Fiona
She is really struggling now. Although her wind speed is 35mph, central pressure 1010mb, and her convective activity keeps trying to gain ground (or sea to be more accurate), the circulation (vorticity) continues to deteriorate. She only has a signal in the lower level of the troposphere, with nothing in the middle levels (as you can see in the maps below). She is currently around 25.5N, 62.3W, heading WNW at 15mph. There may be some swells on the eastern US seaboard and Bermuda, but not too much more. This will be my last update on Fiona unless that circulation picks back up. 

Tropical Storm Gaston
Ah, Gaston... not quite a beauty, not quite a beast (yet)! :-) Gaston is currently quite a weak storm with winds of 40mph, central pressure of 1005mb. He is at 12.6N, 30.7W, heading WNW at 18mph. He has just left the general Cape Verde Islands region in eastern Atlantic, so he has a long way to travel and therefore a lot of potential to become a hurricane. His convection is still developing, although you can see quite a nice structure beginning to form:

Looking at the vorticity gives us a good idea of how strong he is. Here is the vorticity (circulation) map (from the U. Wisconsin website) in the lower troposphere (850mb): 

This is great because you can see the difference between three storms at three different intensities! First, you can see the low level circulation that is Tropical Depression Fiona (conveniently marked by the 'L'). Around 30W, you can see a big red splodge which is Tropical Storm Gaston, marked by the sign for a storm. And you can also see another blobette on here (in the Atlantic, east of the Caribbean) which I'll talk about next. 

Now if we look at the comparable map showing how much circulation or vorticity each of these systems have in the middle troposphere (500mb), we can see how strong they really are:

The only one that now jumps out at us is Tropical Storm Gaston! Fiona and the other Atlantic Blobette have pretty much vanished. Based on this, I would actually say that Gaston is stronger than the weak storm the NHC have him as - 40mph is barely a Tropical Storm (TS range: 39-73mph). I actually think he is a mid-to-strong intensity Tropical Storm with winds of around 60 mph-70 mph because he already has a vorticity signal in the upper troposphere, which means he is approaching hurricane strength. The only reason I don't think he is a hurricane yet is because his convection hasn't quite caught up - and that is partly because of that lovely Saharan Air Layer which is inhibiting his development:
He is certainly over waters warm enough to feed him (temperatures are above 27 deg C, with the upper 50m warmer than 26.5 deg C) and there is very little wind shear. 

I'll have a look at possible tracks tomorrow. 

Atlantic Blobette
There's a small blobette at around 16.5N, 55W, heading WNW at 15-20mph, but as you can see from the vorticity maps above, she is quite weak at the moment. However, she is heading into an area of low wind shear, she is moving away from that dry and dusty SAL, and water temperatures are warm (and getting warmer) ahead of her. All signs that she will most likely get a bit stronger. The NHC currently think she will be a Tropical Depression by Friday, as she approaches Puerto Rico and Hispaniola. 
I forecast a bit of rain on some of those islands so dust off the spiderwebs from your wellington boots. 

The next name on the list is Hermine. Not Hermione. Yes, I was hoping too. 

More on Gaston and the Atlantic Blobette tomorrow. 

Night from California, where our warnings are like this ... 

:-) (thanks to Trent F. for sharing that one). 


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DISCLAIMER: These remarks are just what I think/see regarding tropical storms - not the opinion of any organization I represent. If you are making an evacuation decision, please heed your local emergency management and the National Hurricane Center's official forecast and the National Weather Service announcements. This is not an official forecast. If I "run away, run away" (Monty Python), I'll let you know.

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