Monday, August 07, 2017

Tropical Storm Franklin and the Atlantic Blobette: August 7, Update A

So much for putting the Caribbean Blob to one side... that’s the last time I throw that phrase out. Clearly Mother Nature took that as a personal challenge! Today was a travel day for me. I am now writing from the Caribbean. Of course.

The NHC have just named our former Caribbean Blob as Tropical Storm Franklin.

Tropical Storm Franklin
Tropical Storm Franklin is a very weak storm with winds of 40mph (TS range: 39-73mph), central pressure 1006mb. He is officially currently centered at 16.4N, 83W, heading WNW at 13mph.

The infrared satellite image shows some pretty robust convection in the Caribbean:

The convection does look like it is east of the official center of circulation (16.4N, 83W):

Just looking at the convection here, it would be easy to say that the center is somewhere around 16N, 82W. So, let’s look at the vorticity (circulation) maps to see what is going on. Here are the maps at the lowest level (850mb) and the middle of the troposphere (500mb):

From the 850mb, you can see the center of the red splodge in the ‘Caribbean’ is actually west of the convection, close to the coast of Honduras and Nicaragua. But the 500mb map shows the 'deepest' red a little offset from the 850mb - a bit more to the east. This offset, along with the convection tells us that:
a)    This is a Tropical Storm, because there is a signal in the entire lower half of the troposphere.
b)    The signal is not very round – it really is a splodge (very technical term – hold onto your hats! ;-)) and is even connected to some circulation across central America – which means that it is not well developed and is not a very strong storm. I would say it is a little stronger than 40mph though – maybe closer to 50 or 55 mph if I had to give an estimate, but not much more than that at the moment.
c)     Because the lower level and mid-level are not over each other, there is some wind shear which is impacting the storm, which is a second clue that it is not a very strong Tropical Storm at this point.

In addition to wind shear, there are two more competing factors that are impacting his strength.  On the one hand, he is fairly close to land, which means that he will be a little inhibited from getting stronger. However, he is over some VERY warm water which will feed his convection.

The sea surface is 28-29 degrees C. To keep a storm fed and happy, we need water temperatures warmer than 26.5 deg C. But beyond that, the side of the storm with a lot of convection is over water where the upper ~125 meters is warmer than 26 deg C. This means that as the water gets churned, he is still pulling up very warm water and getting energy from that.

The pressure field show that he will continue generally westward, and I think he may move possibly slightly WSW, so to me it looks like he is heading to Nicaragua and Honduras. The NHC have a different track in mind: 

One difference between what I am thinking and their track is that we may have different centers of circulation in mind. I think his is a little south of theirs. But as I've said over the last couple of years, they are now better than me at the 1 day track forecast so I would defer to them on this. If he is on the track that they have him, then he will strengthen some more as the water gets warmer and the warm water gets even deeper - so he may even be a mid-to-strong TS level (60-70pmh) if he takes that track.

On the other hand, if he does remain W or even WSW and heads to Honduras and Nicaragua, there isn’t much time for him to develop much more, in which case he would be more of a rain event, which may cause some flooding. We'll find out soon enough. 

Atlantic Blobette
Meanwhile, the Atlantic Blobette that had a 50% chance of formation a couple of days ago now has a 10% chance (aww). From the vorticity maps (circulation), we see that there is a little circulation around 12N, 40W, but it is weak and is not well developed. But it really is that dry air that is inhibiting its convection:
You can see it trying though so it is worth keeping an eye on for now. 

It's night time here... 

Hasta manana Amigos! 

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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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