Thursday, August 04, 2011

Tropical Storm Emily: August 3, Update A

I’m back! And TS Emily is still out there? Bother, I had set my time machine for next week… clearly a glitch brought me back here. Oh well. ;-) I see that in my absence Emily read my blog and decided to continue on a more westward track after all. It looks like she will clip southern Haiti on her way to Cuba, before curving to the NW and then N. She is currently centered at around 17.1N, 71W, with winds of 50mph and central pressure of 1004mb. She is barely moving – officially she is moving westward at 5mph.

I suppose I should let you in on the “big secret” about why the models are insisting she will make that WNW, then NW and then finally N turn, despite her taking a more westward track than they first expected. This will also explain why she has practically stopped moving. But first some science background on the basics of storm tracks… Oh gosh, how exciting! Got your cup of tea and biscuits (British biscuits = American cookies)? Are you ready? Here we go…

<science alert> 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. 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, 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 like a Dali clock, with wiggly bits (more technical jargon ;-)) that are always moving. I have an image somewhere that I made for a talk… I’ll see if I can dig it out for you all. 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. <end of science alert>

(I’ll be talking about this during the season, so you might want to highlight this section before you laminate the print out of this update to put in the folder you have of all my updates… ;-))

Now why are they predicting Emily to turn to the north? So far she has been moving along the squiggly 6 o’clock section of the clock face along the southern edge of this high pressure system as she crossed the Atlantic – i.e. generally westward. However, this high pressure (from what I can see) extends all the way across the Caribbean towards the southern Gulf. This means I would normally expect her to keep moving westward, which is why I have said repeatedly that she would stay more westward than the track they were forecasting initially and why I thought she would pass south of Haiti or clip it on her way to Cuba, instead of passing over central Hispaniola. However, slowly creeping in from the north is an area of low pressure! <insert Jaws soundtrack> If that low pressure can ‘break through’ the high pressure to her north, it will give her room to speed up and move from the 6 to the 9 position on the clock. The models are predicting this low to move down fairly quickly… I think they thought it would already have broken through by now, so their curve was sooner. My clue to what is going on is in Emily’s forward speed… she has slowed down. Storms slow down when they bump up against high pressure in the direction they are moving. She has high pressure in front of her (both to the north and the west) and until that erodes in one direction or the other, she is a bit stuck. So there are two scenarios I see now…

1. The low pressure from the north breaks through, and she speeds up and heads WNW, NW and then N and around in a fairly sharp curve.
2. The high pressure in front (to the west) of her erodes a bit (the wiggly clock face er… does some more wiggling ;-)), and she carries on moving on a more westward-WNW track at a reasonable pace, in which case she may stay over Cuba longer than expected before moving NW – if that’s the case then she may even dissipate. Again, the intensity depends on the track.

Tomorrow I will be looking for signs in a change in speed (and possibly direction).  

Either way, she is over warm water and warm water with depth, so she has some of what she needs to create lots and lots of rain. To counter this, the island land masses may be able to reduce her intensity and convection, and she is now experiencing some pretty strong wind shear which will also help keep her in check. Unfortunately it is almost inevitable that parts of Haiti will get a bit soaked. At the moment their worst scenario is for her to get stuck for a long time just south of them, over the ocean, raining away. I don’t think she will – but I am not sure which of my two scenarios above will happen first.

Done for today.

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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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