Sunday, August 21, 2016

Tropical Storm Fiona: August 20, Update A

I see that TS Fiona is still out there. Of course she is, it's the weekend isn't it? ;-) She's a persistent little thing (and has the potential to get a bit bigger - although for now she remains weak). 

She is officially at at 21.7N, 50.1W, officially heading NW at 15mph - although the satellite images show her center very clearly to be currently at 22N, 51W. Winds are officially estimated to be 50mph, central pressure is 1004mb, which still means she is a weak Tropical Storm (TS range: 39-73mph). I think she may now be weaker than this for a couple of reasons... 

First, she has very little convection, and it is all to the east of the center. If we look at her infra-red satellite imagery, we can see the point where the convection got spectacularly removed from the center of circulation: 
This is because she is facing some strong wind shear. However, the wind shear looks like it will die down in the next day or so, which means she has a chance to get a little stronger again. 

Second, there is very little organized circulation (vorticity) in the middle and upper troposphere. Although there is some, it looks like she is part of a front instead of a stand-alone system. 

So why did her convection vanish? The sea surface is warmer than 28 deg C, and the upper 75 m of the water is warmer than 26.5 deg C - both suggest that she has enough to maintain strong convection (rain and thunder and stuff (technical jargon! ;-))), her convection just went poof (yes, it may even have made that sound). I think the reason is because she is encountering a layer of dry and dusty air from the Saharan Desert (goes by the ingenious name of the 'Saharan Air Layer' (SAL) :-)), which you can see as the red areas on this map:

<Forecasting Tool Alert!>This SAL map is from the fantastic University of Wisconsin website:, should you want to have a look for yourselves (and I'm sure you do! :-)). This is the same website that has the vorticity maps I talked about in an earlier post. To find the SAL map, click on the turquoise colour block in the lower map for the North Atlantic and in the drop down menu (amongst a number of things) you will see 'Saharan Air Layer Analysis'.  Easy peasy! <End Forecasting Tool Alert!>

It does look like she will emerge from the SAL soon thought. If she still has some circulation, there is a good possibility that the convection will pick up again. We'll have to see what toll this dry 'spell' has taken.  

As for her track - that is uncertain as far as I can tell because we don't have good data over the ocean and also because she is such a weak storm. There is high pressure to her north, which suggests that she would move generally westward or WNW for now, but I would have to rely on the NHC who are using models at this point. Maybe the pressure field maps will improve tomorrow. I'll keep checking. 

That's it for tonight! 

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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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