Wednesday, August 15, 2012

Altantic Blobette and Caribbean Blob: August 14, Update A

The Caribbean Blob did get a little better organized and the convection improved, but then suddenly Central America appeared and now he is almost no more (well not actually ‘suddenly’. Central America was there all along, that ‘suddenly’ part was just for dramatic effect). Nicaragua knocked the circulation out of him and over the last few hours his convection has decreased too. This is my last entry on the Caribbean Blob.

The Atlantic Blobette (yes, the one I wasn’t going to write anything about unless she did something interesting) is still struggling to get to grips with being a teenager. She has been spluttering all day with a bit of convection here, a bit of convection there. Here is a satellite image of her at the moment – not very impressive at all, is she? There is hardly any rain (the yellow parts).

Her circulation improved very slightly today, but she is still in that dry air so she’s not really managed to do much. It looks like she is around 26.5N, 54W, heading NW (towards that little dot in the Atlantic… oh yeah, Bermuda ;-)). The NHC have increased her chances of becoming a Tropical Storm in the next 2 days to 50%. This may happen, but it looks like there is some strong wind shear ahead of her which will inhibit development and she is looking a bit pathetic. Also, it looks like she might get caught up in a low pressure front in the day or so, which may help to strengthen her winds (but this doesn’t mean she’s a tropical storm!!) and will eventually whisk her to the north and northeast. At the moment sea surface temperatures are around 28 deg C, so certainly warm enough to sustain her if she forms. Ok, ok, I suppose I’ll have to carry on about this one tomorrow.

Hey… it’s time for the long awaited <Science Alert Part Deux>! I was asked (by ‘hurricaneagle’) why storms moved so quickly across the Atlantic compared to those that are in the Gulf. In an attempt to answer this, I need to tell you about pressure fields and speed! (umm… I think I may have covered this already this season, so if it sounds familiar, please feel free to go and do the dishes or something.) Everyone else… got your cuppa tea and popcorn ready? ;-)

<Science Alert Part Deux>

Pressure fields have High and Low Pressures. For example, the semi-permanent Bermuda-Azores High I mentioned yesterday. You can imagine the highs as ‘hills’ in the atmosphere, and the lows as the ‘dips’ and ‘valleys’. So there is a topography to the pressure field but unlike the earth these atmospheric pressure areas are always changing. Now imagine the storm as a ball. If you put a ball in a valley, it will not roll up any surrounding hill. When the pressure ahead of a storm is low then it will happily move at a good speed, but if the pressure ahead becomes high (i.e. it would have to climb uphill), then it will either move around the high pressure (i.e. the track changes) and take the easiest path, or slow down until the pressure field changes again. If the pressure is high all around the storm (if the ball is in a dip), then it becomes stationary.

Generally the part of the high pressure field over the Atlantic is fairly clear cut and so a storm is not (usually) hindered by large changes in the pressure field and can move across quickly. This is the 6 o’clock portion of the clock in the diagram from yesterday. We also know this because this pressure field not only impacts storms, but also impacts the general wind field; we have a consistent trade wind that blows from east to west across the tropical Atlantic. But when we get to the curve (the 6-9 o’clock), the storm is trying to move in the north-south direction, not the east-west direction, and the pressure fields are not as steady for a number of reasons. For example, they are impacted by low pressure fronts that move from the west to the east. So storms that are in the Caribbean or Gulf maybe hindered by little areas of high pressure in front of them, which will slow them down.  I will look out for a good example of a storm to show you (assuming we have any more this season J).

<End of Science Alert Part Deux>

Time for that nap thing again. Yay! J More tomorrow!

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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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