Monday, August 29, 2016

Hurricane Gaston and Tropical Depressions 8 and 9: August 28, Update A

Mother Nature is having a ball out there, isn't she? So much to talk about today that I think I'll just have to jump right in! Lucky you! ;-)

Hurricane Gaston
Hurricane Gaston is now the first major hurricane of the season with winds currently at 120mph, central pressure at 957mb. A major hurricane is a cat 3 or higher intensity storm. With winds of 120mph, Gaston is currently a mid-sized cat 3 storm - the wind speed range for cat 3 storms is 111-129mph. I would agree with this intensity. He has a very well developed eye which has persisted for a number of hours today:

Isn't he a good looking storm? (what can you expect when Disney had already written a song about him to say pretty much the same thing? Their marketing is truly amazing! ;-))

He is currently hovering at around 30.6N, 55.2W, heading absolutely no-where fast - yes, he is stationary - I expect he's probably doing a bit of mid-Atlantic fishing. He hasn't gone very far since yesterday so he didn't move into that area of higher wind shear, which is one of the reasons he continued to grow so nicely today. He also continued to grow because he is still over very warm water, with sea surface temperatures warmer than 29 deg C and the upper 50m of the water column being warmer than 26.5 deg C. 

<Science Alert!> (yay!) Why would a storm stall? You have heard of atmospheric high pressure and low pressure, right? Imagine them as hills and valleys in the atmosphere, except made of air. Now imagine that the storm is a ball on this undulating pressure terrain. If the path ahead of the storm is low pressure, it's like a downhill gradient and the ball will happily keep on rolling forward. If the path in front is high pressure, it is an uphill gradient and the ball (storm) will slow down, or maybe move in a different direction until it finds an easier path downhill (the path of least resistance - because, after all, resistance is futile ;-)). But if the ball gets stuck in a 'dip', it is surrounded by high pressure on all sides, and it becomes difficult for the storm to move in any direction, so it will stop where it is until the pressure fields around it change (which they eventually will do). Of course, because the pressure fields are continually changing, the longer the storm is stuck or slow, the trickier the longer range track forecast becomes. So not only to forecasters need to predict the track of the storm, they also need to predict the entire surrounding pressure field and how it will change before the storm moves again, and that's where computer models really kick in. <End Science Alert!>

This is why I was grumbling about not being able to access the pressure fields over the past few days. But I have a tentative work-around now, so all is right with the world again. :-) Looking at the pressure maps, it doesn't look like he is going to move too far in the next day because he is still surrounded by high pressure. 

We can also see that he is a well formed hurricane by looking at the vorticity maps throughout the troposphere. Here are the vorticity (circulation) maps (thanks to the University of Wisconsin - CIMSS). This is the lowest level of the troposphere (850mb): 

Here's the one from the mid-troposphere (500mb):

And here's the one from the upper troposphere (200mb):

You can see that Gaston (marked by the hurricane symbol) now has a pretty good signal throughout the troposphere - in the upper troposphere it is very well defined - not a big splodge like some of the other blobs and blobettes out there. 

What does tomorrow hold in store for him? Wind shear continues to look quite light, so that will only allow him to grow. He also will remain over warm water, but the longer he stays in the same spot, the cooler he makes the water under him so it is possible that he may make it cool enough that he will begin to weaken. So really, we are waiting to see when he starts to move. The current forecast is that he will move in a northward direction once he starts up again. 

Tropical Depression 8
Formerly known as the Atlantic Blob. This is the pretty weak system that is between Bermuda and the eastern US - around 32.3N, 72.2 W, heading generally NW. 

You can see how weak this blob is from the satellite images:
The center is pretty easy to see because there is so little convection! What there is is heading to North Carolina, along with the blob. As it crosses the Gulf Stream (where sea surface waters are warmer than 30 deg C and the upper 75 m is warmer than 26.5 deg C) I expect it will pick up more convection and there is a chance that it will improve in structure, but it won't be very strong. Be ready for a few spots of rain and, of course, surfs up! 

Another indication that this is a relatively weak blob is in the vorticity (circulation) maps. The 'L' marks the spot. You can see a good strong signal in the lower troposphere, but it's pretty weak by the time it reaches the middle level of the troposphere. It is possible that the mid-tropospheric vorticity will improve as it crosses the Gulf Stream - but it is also heading into an area of higher wind shear. So whether it develops further is really a battle between the opposing influences of the ocean and atmosphere. 

If it does become a tropical storm as it crosses the Gulf Stream the next name is Hermine... although that's a blobette name and TD 9 is also vying for that. The one after that would be Ian. 

Tropical Depression 9
Formerly known as the Atlantic Blobette - this is the one that has been visiting Cuba. She still hasn't quite woken up and is still looking like a bit of a morning mess:
There is a lot of convection - mostly over Cuba. She is such a mess that it is difficult to tell where her approximate center of circulation actually is - so until that has been identified, the models are not going to be very good. If we look at the satellite images (and I usually look at the visible satellite image for this, not the infrared one), it really is difficult to figure out the center:

But if we look at the vorticity maps above, we see that her low-level circulation is right over the western tip of Cuba (actually, it looks like she may be slightly south of Cuba), and her mid-level circulation (although very weak) is a little to the southeast of that - this is because of wind shear, which is continuing to die down. It's the same as yesterday - wind shear continues to weaken, so really the main thing that is keeping her in check is the land mass of Cuba because the surrounding water is quite toasty. 

From the pressure fields, it looks like she may emerge from Cuba in the next day or 1.5 days and, at the moment, she will continue to move generally westward and into the southern Gulf staying south of 25N for the next day at least.

Other Blobs and Blobettes
If you are still awake by the time you read this... you may have noticed a couple of other blobs and blobettes on the vorticity maps. One is just on the coast of Texas - it has circulation in the lower half of the troposphere, but higher up it is connected to an elongated area - more like a front. If we look at the satellite imagery, you'll see that there is very little convection associated with this area:
The NHC have currently given it a 10% chance of development. I would agree - it is already interacting with land, so it seems unlikely that it will go anywhere. 

The other blobette you may have noticed is the area of circulation just north of Puerto Rico. Although there is some convection (clouds really, hardly any rain), this has vorticity in the upper and mid-troposphere, but hardly anything in the lower troposphere. So the structure isn't quite there to consider this a tropical storm - you need low-level vorticity for that. 

I just wanted to mention these two so you can see what the vorticity looks like for storms (and non-storms) of different intensities. It's pretty interesting stuff! (for a geek like me anyway :-)). 

I think that's it for now. Clearly I'll be back tomorrow with beautiful Gaston out there, a fight between the ocean and atmosphere over TD 8, and the possible emergence of TD 9! It's a thrill a minute! ;-) 

Teleporting out for now!

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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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