Wednesday, July 29, 2020

'Potential Tropical Cyclone' Nine (The Atlantic Blob): July 28, Update A

Oh what?! No time for dilly-dallying today! What is this made up 'Potential Tropical Cyclone' Nine business?? Either it is a Tropical Depression (in which case, feel free to give it a number) OR it's a Tropical Storm with a name! This 'Potential' is a bit of a wishy-washy designation that's neither one nor t'other. 
Here's the story so far... this Atlantic Blob does NOT have a well-developed closed circulation as we expect to see in Tropical Storms, but  it clearly has some areas of strong convection which are here in the satellite imagery: 

The Lesser Antilles are currently getting quite a drenching with some thundery weather, but not much sustained winds and there isn't a clearly defined center of circulation. This is confirmed if we look at the vorticity map to see what the circulation actually the lowest levels of the troposphere (850mb) it does not have the signature of a tropical storm because it is far too broad and not very circular (it's more like a lava lamp blob actually):

And the vorticity in the middle of the troposphere (at 500mb) shows there are two conjoined areas of circulation - also not a tropical storm signal: 
And there isn't any vorticity in the upper troposphere. So, all the vorticity data indicates that this is NOT a Tropical Storm or a Tropical Depression (because there isn't a single center of circulation). 

It has clearly been trying to develop since yesterday, but the SAL has done an excellent job of keeping it otherwise occupied. I think the reason it is still around and trying to become something is because the dry and dusty air is not near the 'center' - there is a really large band of humid area and clouds around wherever the center should be:
And it's that outer band of clouds to the north and west that are 'battling' the dusty SAL air. 

There is also some wind shear that is at play here which is also helping the SAL to keep it in check. We can see that in the satellite imagery (and the SAL image) with the clouds trailing to the northeast. But it is still producing heavy convection (rain, thunder etc) because it is over water that is 28-29 deg C at the surface of the ocean, with the upper ~100m being warmer than 26 deg C - that is enough to keep a storm well-fed and happy. 

I think by calling it a 'Potential Tropical Cyclone' they can issue warnings and a track, in case it makes a sudden move - which of course means they aren't sure how much it will develop. It is currently officially centered at 14.6N, 59.4W (even though they inferred a center), heading WNW at 25 mph (which, by the way, is super-fast for a Tropical Storm - usually less than 20mph). 
Winds are officially estimated to be 40mph (set just above the magic 39mph threshold which means it can be moved directly to a named Tropical Storm once it has a closed center of circulation), and central pressure is estimated to be 1007mb (which is about right for something that's a depression).

This is a tricky one with all of these complicated factors at play. But why stop here, let's add one more shall we?  The path it will take may take it directly over islands - which are a bit hilly. If the 'center' goes over land and it's a weak system it will get knocked down a notch. 

The story going forward... I think it may slow down very soon (they think it'll slow down in about 48 hours). If it slows down it will most likely intensify, given that the dusty air is quite far from the center and the water is rather toasty warm. However, I think they have put the center too far north - I think it may be closer to 12.5-13N, not 14.6N. So I am not 100% convinced of that track yet either. 

I'd get ready for a bit of a downpour in the VIs, Puerto Rico, and all along that track, but at the moment there is a lot of uncertainty in both the track and the intensity on this one.  

I'll be back tomorrow - maybe twice if I can. This is a very interesting one indeed!

Toodle pip!

Twitter: jyovianstorm
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 local weather service announcements. This is not an official forecast. If I "run away, run away" (Monty Python), I'll let you know. 

No comments: