Caribbean Blob and Atlantic Blobette: August 13, update A

It looks like St. Thomas did get a bit of rain this morning. From Tom: “Rain came in short bursts...but enough to keep plants watered for the day”. So all is well there. Here is a picture of what I imagine the plants may look like with a spot of rain (it’s just a garden gnome. Really. ;-)):

The NHC have upped the ante since yesterday, and now both the Caribbean Blob and the Atlantic Blobette have a 20% chance of forming in the next 48 hours. Here is the latest satellite image of water vapour over the Gulf/Caribbean/tropical Atlantic:

The brown areas are dry air, everything else has water vapour and clouds of some description. The Atlantic Blobette is at around 23N, 47W and the Caribbean Blob is at around 14N, 78W. If the image of the SAL from a few days ago wasn’t enough, this shows that the Atlantic Blobette continues to be embedded in some very dry air. There is very little circulation (vorticity) in the lower (or any) part of the troposphere – and it has actually got weaker since yesterday. Anyway, no more on this one unless it does something interesting (unlikely).

The Caribbean Blob lost a lot of convection since yesterday partly because of the stronger wind shear he did battle with, partly because he was interacting with the South America landmass, and partly because of the dry air to his west. Although this weakened his circulation earlier today, it has started to improve again. The NHC have a note to say that an Air Force plane will go into the system to investigate tomorrow afternoon if necessary. Although the circulation has improved because wind shear has decreased and he is over the nourishingly warm waters of the western Caribbean, he does have some dry air ahead of him which will inhibit his development a bit. Also, as he is heading for central America he won’t have time to develop into much more than a Tropical Storm, if that! (the NHC recognize this too). He is moving westward at about 20mph.

This nicely leads me to a question from ‘Hurricaneagle’ (cool handle! J): “I was wondering why hurricanes coming off the Atlantic seem to move so much faster than hurricanes that form in the Gulf.“ Jolly good question. So good in fact that the answer comes in two parts, along with some lovely free Ikea assembly instructions. ;-) I don’t think I’ve gone over the basics of storm tracks during this season yet, but if you remember it (or if you remember it from last year), then this would be a nice time to go and make a cup of tea and get a slice of cake. For everyone else… oooh… <Science Alert>! (are you all tingling?) ;-)

<Science Alert Part One>

The basic answer is because the pressure fields that storms meet as they cross the Atlantic are generally much ‘smoother’ than those that are encountered in the Caribbean or the Gulf. These pressure fields are what determine a storm’s track and speed. So the first part of this <science alert> is the connection between the pressure fields and storm tracks.

In the northern hemisphere 'things' (technical jargon ;-) ) tend to move clockwise around high pressure systems, and counter-clockwise around low pressure systems. For example, a tropical storm has low pressure in the center so winds move counter-clockwise (or anticlockwise if you prefer) around a storm. Similarly, tropical storms also move around larger pressure systems.

There is generally a region of high pressure that likes to hang out over the Atlantic, sometimes called the Bermuda High or the Bermuda-Azores High. You can imagine it as a big clock face over the Atlantic. Here is an image which shows the tracks of all the  tropical storms in the Atlantic in August from 1851-2005 and this imaginary ‘clock’ (image credit: Moi J):

As storms cross the ocean, they move westward along the six o'clock region. As they turn WNW and NW they are moving from 6 to 9. Then they move N and NE, from 9 to 12. Of course in the real world this imaginary clock face isn't nice and round, nor does it stay in the same place (otherwise forecasting the track would be easy peasy :-)). It's more like a Dali clock, with wiggly bits (technical jargon ;-)) and jiggly bits (more technical jargon ;-)) that are always moving. This is why I will sometimes talk of 'pressure fields' when I talk about the track - I am referring to the large scale atmospheric pressure fields.

The second part of this <science alert> is the connection between pressure fields and speed but, in classic soap-opera cliff-hanging endings, (and somewhat ironically) my internet speed just sllowweed dowwwnnn. So I think I will cover the ‘speed’ part tomorrow. <end Science Alert part one!>  

Tune in again tomorrow for <Science Alert Part Deux, the exciting conclusion>.

Time for a nap in this corner of the planet.
Night!
J.

Blogs archived at http://jyotikastorms.blogspot.com/
Twitter @JyovianStorm

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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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