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Tropical Storm Don Discussion Number   2
NWS National Hurricane Center Miami FL       AL052017
1100 PM AST Mon Jul 17 2017

Before completing its mission in Don, the Air Force Reserve
reconnaissance aircraft twice measured reliable surface winds around
40 kt via the SFMR instrument, and that is set as the initial
intensity for this advisory.  A burst of deep convection which began
just after 2000 UTC has persisted for the past several hours, and
small clusters of lightning have periodically occurred near the
estimated center.

Don is embedded within a narrow east-west zone of low shear, and
that shear is expected to remain low for another 24 hours or so.
Since the cyclone has a compact circulation, significant changes in
intensity--both up and down--become more likely.  Therefore, given
that the environment looks favorable for the next 24 hours, the new
NHC intensity forecast shows strengthening up to 50 kt as Don
approaches the Windward Islands.  After 24 hours, increasing
westerly shear should induce weakening while Don moves through the
southeastern Caribbean Sea, and the cyclone is expected to
degenerate into an open wave by 72 hours, if not sooner.  Since
model guidance varies widely between only showing an open wave (most
of the global models) to Don reaching hurricane strength (HWRF and
some of the statistical models), it cannot be stressed enough that
confidence in the intensity forecast is very low at this time.

Don's initial motion is 280/16 kt.  There is also a bit of
uncertainty in the track forecast, especially in Don's future
forward speed, since most of the global models don't show a strong
enough vortex to provide a tracker.  However, the general trend
is for Don to move just north of due west and speed up by 24 hours.
The updated NHC track forecast is a little faster than the previous
forecast after 24 hours, mainly based on the latest GFS and ECMWF
global model fields.

FORECAST POSITIONS AND MAX WINDS

INIT  18/0300Z 11.3N  54.6W   40 KT  45 MPH
 12H  18/1200Z 11.6N  57.0W   45 KT  50 MPH
 24H  19/0000Z 12.0N  61.2W   50 KT  60 MPH
 36H  19/1200Z 12.3N  65.4W   45 KT  50 MPH
 48H  20/0000Z 12.5N  69.6W   40 KT  45 MPH
 72H  21/0000Z...DISSIPATED

$$
Forecaster Berg

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