Monday, October 13, 2008

Tropical Depressions Nana and 15: October 13 Update A

TD Nana:

As expected, in the wee small hours of the morning she was downgraded to a
Tropical Depression (by the 5am advisory in fact).

In the good old days, unless there happened to be a ship out there, it's
quite likely that she wouldn't even have been noticed. A question I get
asked quite often is how do we know about old storms before we had
satellites or planes? The official records date back to 1851, and are
based on anecdotal information - if there was a ship that got caught in
one (and survived) or if one made landfall in an inhabited place etc. So
obviously the further back in time we go the less certain we are of the
total number of storms in any given year or their tracks or intensities.
This makes it very tricky when we talk of long term trends in hurricanes -
the data doesn't stretch back reliably far enough.

In 1933 for example (pre satellites and pre-planes), the records show
there were 21 named storms in the Atlantic! I bet they were thinking that
the sky was falling... at least the global economy was solid back then
though... er... hmmm...hang on a second... ;)

Back to Nana... she's not got much convection, circulation is easy to see
in the satellite images because she's relatively cloud free, winds are
around 35 mph, and she's moving WNWish at around 8mph around a High
pressure system in the Atlantic, which is where she'll stay. Wind shear is
still strong.

Unless she decides to pull a fast one, this is my last entry on this system.

TD 15:

The Blob that was in the eastern Caribbean has now officially become TD
15. A plane is being sent into the system to investigate. I applaud the
forecaster for making the leap instead of waiting for reconnaisance plane
information. Bravo.

The center is at around 14.8N, 69.6W, and it's moving NW at a rapid (!!)
2mph. As I can stagger (under the influence of alcohol) faster than that,
I would call that pretty much stationary. Central pressure is 1005mb,
winds near 35mph (same as Nana at this point in time).

The forecast calls for this to be a hurricane in a couple of days, and
heading towards the PR - Virgin Islands area on Weds and then out into the
Atlantic. It looks like there are some strong thunderstorms passing
through that area already. Water temperatures are warm enough to sustain
this storm and there's not much dry air, so the inhibiting factor will be
the wind shear.

There is some wind shear from the south west and the convective activity
is, indeed, east and north-east of the center so this is not a very well
organized system yet. Alas, there seems to be a glitch and the full suite
of wind information has not been posted since yesterday - at least not in
the places I usually look. And a double whammy... I don't have high
resolution pressure fields to assess the track either. Bother. Bother.

I'll try and find out what's going on (maybe they've finally cut me off?
;) ) and send out another update later.

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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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