Tuesday, September 08, 2009

TS Fred: September 8 Update B

TS Fred is still a Tropical Storm, but barely. His winds are now 70 mph
(TS range: 39-73 mph), with central pressure estimated to be 990mb. Also
as expected, he's continued on a westward track all day, with the forecast
having shifted west a bit. He's currently moving at 14mph and is centered
somewhere around 12.1N, 29.8W. Although wind shear is low and sea surface
temperatures are high, and his circulation (vorticity) covers most of the
troposphere now (essentially indicating a hurricane), he has been taking
in some dry air - part of the Saharan Air Layer I wrote about a while ago.
This has kept convection down a bit, but it won't be enough to stop him
from being a hurricane.

He'll keep going westish... He does have some room to move WNW, but it'll
only be a little. Then it looks like he will slow down as there is high
pressure ahead of him at the moment. In this I agree with the NHC.

<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 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 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. That
is why I talk of 'pressure fields' when I talk about the track - I refer
to the large scale atmospheric pressure fields. <end of science alert>

I hope I didn't confuse anyone (well, not too much anyway :)) but of
course, if you have any questions don't hesitate to ask! I'll do my
bestest to explain what I know ... and can always make up what I don't. ;)

The answer to the Famous Fred quote earlier today was, indeed, Fred
Astaire. Another one tomorrow.

That's all for now folks.

Disclaimer: no need to worry about this storm...insert disclaimer from
previous entries here.

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