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Tropical Depression Eighteen Discussion Number   4
NWS National Hurricane Center Miami FL       AL182017
1100 AM EDT Sat Oct 28 2017

Data from an Air Force Reserve reconnaissance aircraft indicate
that the low pressure system located over the northwestern
Caribbean Sea has developed a well-defined low-level circulation
center. Satellite imagery also shows more central deep convection
has developed and persisted in the inner-core region since
yesterday.  Based on these data, the disturbance has been upgraded
to Tropical Depression Eighteen.

With the recent redevelopment of the inner-core region this morning,
the initial motion estimate is an uncertain 020/19 kt. The global
models remain in excellent agreement on an approaching frontal
system and deep mid-tropospheric trough accelerating the cyclone
north-northeastward across west-central Cuba today, followed by a
motion toward the northeast tonight and Sunday, with the system
passing about midway between Bermuda and the southeastern United
States in 36-48 hours. The new NHC forecast track is essentially
just an update of the previous advisory due to the good agreement in
the global and consensus models, which lie along or just a tad to
the west of the official forecast track.

The vertical wind shear affecting the cyclone is expected to remain
somewhat favorable for strengthening to occur for the next 24 hours
or so, with the depression expected to become a tropical storm by
tonight.  Thereafter, baroclinic processes associated with the
aforementioned frontal system are forecast to induce some additional
strengthening before the cyclone merges with the frontal system
and becomes an extratropical low by 48 hours. Dissipation is
expected by 72 hours when the system is located over the far North
Atlantic.

KEY MESSAGES:

1. While the forecast track of the center lies about 35 miles
southeast of the Upper Florida Keys and extreme southeast Florida
and most of the winds are expected to remain east of the center,
only a slight deviation to the west of the expected track or an
increase in the size of the wind field could bring tropical-storm-
force winds across these land areas. For that reason, a tropical
storm watch has been issued for these areas.

FORECAST POSITIONS AND MAX WINDS

INIT  28/1500Z 20.8N  82.9W   30 KT  35 MPH
 12H  29/0000Z 23.0N  81.8W   35 KT  40 MPH
 24H  29/1200Z 26.6N  78.5W   40 KT  45 MPH
 36H  30/0000Z 31.5N  74.0W   45 KT  50 MPH
 48H  30/1200Z 37.5N  69.5W   50 KT  60 MPH...POST-TROP/EXTRATROP
 72H  31/1200Z...DISSIPATED

$$
Forecaster Stewart

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