Tuesday, July 09, 2013

Tropical Storm Chantal: July 9, Update A

Today’s entry is brought to you by carrier pigeon and a bowl of yummy ice cream. J

Tropical Storm Chantal is currently at 15.3N, 66.2W heading WNW at an incredibly impressive 28mph (this is unusually fast for a tropical storm but is the case with this one!). She moved according to plan today and tracked generally westward, passing between the windward islands of Martinique and Saint Lucia. Martinique had the worst weather with plenty of thunderstorms as you can see in the infrared satellite image from this morning – there is an island somewhere under that bright red target!

(refresher Technical Alert! on deciphering infrared satellite imagery here.)

Here’s the radar loop from Martinique this morning which quite nicely shows her center of circulation passing by (ooh … moving pictures, go get your popcorn… it’s better viewing than the last Star Trek movie! ;-)).

She slowly intensified as expected and is now a mid-to-strong Tropical Storm with winds of 60mph (TS range: 39-73mph), central pressure is 1009mb. The islands took a little steam out of her (puns always intended, especially the geeky science ones J), but she is now firmly in the Caribbean where the sea surface water is over 30 deg. C (toasty!) and the upper 70-100m is warmer than 26.5 deg. C. Circulation is good in the lower half of the troposphere but it looks like wind shear will begin to increase tonight/tomorrow and, although there isn’t much dust, she is still surrounded by dry air so I don’t think she will intensify too much (I agree with the NHC on this), and may even weaken tomorrow.  

I think she has been tracking a little west of the center of cone track because there is high pressure to her north. This will take her a little south of the current projected path and the forecast track may move to the west. I feel a <Science Alert!> coming up! You have been warned! ;-)

<Science Alert!> 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, like this:

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 face, with wiggly bits (more technical jargon ;-)) that are always moving:

Sometimes this high pressure stays out in the Atlantic and we don’t see many storms making landfall (which was the case for most of 2003), and at other times this high pressure extends across the Caribbean and storms end up in Central America instead of turning north. 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!>

That’s about it for today...my carrier pigeon can’t carry any more! ;-) I’ll be back with more on Chantal tomorrow.

Ciao for now,

Twitter @JyovianStorm
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.

1 comment:

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