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Tropical Storm Don Discussion Number   3
NWS National Hurricane Center Miami FL       AL052017
500 AM AST Tue Jul 18 2017

Don's convective pattern has continued to improve overnight and
early this morning, including the development of a pronounced
central dense overcast (CDO) feature. There have also been periodic
bursts of deep convection very near the center, accompanied by
significant clusters of lightning activity, which is indicative of
strong updrafts in or near the radius of maximum winds. The last
recon data around 0000Z indicated peak SFMR surface winds of 41 kt.
Given the marked increase in convective organization since that
time, the initial intensity has been increased to 45 kt.

The initial motion estimate remains 280/16 kt. There is no change
to the previous track forecast or reasoning. Although the models
continue to differ some on Don's forward speed, there is very little
cross-track difference. The models are in good agreement that the
broad Bermuda-Azores ridge to north of the cyclone will remain
strong and move little for the next several days, which should act
to keep Don moving briskly westward until dissipation occurs at
around 72 hours. The new NHC forecast track is essentially just an
extension of the previous advisory track, and is slightly north of
the multi-model consensus TVCN.

Don is expected to remain embedded within a narrow east-west zone of
low vertical wind shear for another 24-36 hours, which should allow
for some additional strengthening. Since Don is a compact tropical
cyclone, significant changes in intensity -- both up and down --
can occur due to small fluctuations in wind shear and/or interaction
with the mountainous Windward Islands. By 36 hours, increasing
westerly shear is expected to cause Don to weaken while it moves
across the southeastern Caribbean Sea, and degeneration into an open
wave is forecast to occur by 72 hours. The NHC intensity forecast
is similar to the previous advisory and remains below the consensus
models ICON and IVCN. However, the intensity guidance continues to
vary widely between only showing an open wave (most of the global
models) to Don achieving hurricane strength (HWRF, ECMWF, and some
of the statistical models). As a result, confidence in the intensity
forecast remains low.

FORECAST POSITIONS AND MAX WINDS

INIT  18/0900Z 11.5N  56.2W   45 KT  50 MPH
 12H  18/1800Z 11.8N  59.1W   50 KT  60 MPH
 24H  19/0600Z 12.1N  63.2W   50 KT  60 MPH
 36H  19/1800Z 12.4N  67.4W   50 KT  60 MPH
 48H  20/0600Z 12.7N  71.6W   40 KT  45 MPH
 72H  21/0600Z...DISSIPATED

$$
Forecaster Stewart

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