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Tropical Depression Sixteen Discussion Number   3
NWS National Hurricane Center Miami FL       AL162017
1100 PM EDT Wed Oct 04 2017

There have been some structural changes to the depression during
the past few hours.  Inner-core convection began developing just
after the issuance of the previous advisory, and Colombian radar
images from San Andres are now showing a well-defined convective
band to the east and southeast of the center.  Despite these
changes, Dvorak satellite estimates remain T2.0 from TAFB and SAB,
so the initial intensity is held at 30 kt.

The depression probably only has another 12 hours or so before its
center moves inland over northeastern Nicaragua, but it is still
expected to reach tropical storm strength before that happens.
Once the center re-emerges over the waters of the northwestern
Caribbean Sea a little after 24 hours from now, high oceanic heat
content and low shear should contribute to strengthening.  Despite
these favorable conditions, the amount of strengthening will be
unclear until we know how well the inner core survives crossing over
Nicaragua and Honduras.  Strengthening is likely to continue
through at least day 3 up until the time the cyclone reaches the
central Gulf of Mexico.  After day 3, there are some indications
that higher shear and/or cooler shelf waters over the northern Gulf
of Mexico could lead to some weakening, but that scenario is by no
means a definite one at this time.  Needless to say, there continues
to be greater-than-normal uncertainty in the intensity forecast.
The updated NHC intensity forecast has been adjusted downward just
a bit through day 3 to follow an overall shift in the guidance,
although it should be noted that the official forecast still lies
above the normally skillful HCCA model.

If the intensity forecast is complex, the track forecast is not
much easier.  For the first 48 hours, the models appear split on
how the depression will interact with a disturbance currently
located near the Straits of Florida.  For example, the ECMWF model
shows some interaction with the disturbance's low-level vorticity,
which swings the depression more to the east on the right side of
the guidance envelope.  The GFS, on the other hand, shows no such
interaction and has the cyclone on the western side of the guidance
envelope.  This setup has significant downstream effects after 48
hours because it keeps the ECMWF on an eastern route and the GFS on
a western route as the cyclone heads toward the U.S. Gulf coast.
The new NHC track forecast has been shifted slightly westward,
although it is still not as far west as the consensus aids or the
HCCA model.  Interestingly, although the ECMWF ensemble mean is
close to the operational run on the eastern side of the guidance
envelope, there is a high density of members to the left close to
the consensus aids, which lends additional support for the westward
adjustment.

A G-IV mission and Florida special soundings will begin tomorrow to
better determine the synoptic steering flow around the cyclone.


KEY MESSAGES:

1. The depression is forecast to strengthen and bring tropical storm
conditions to portions of Nicaragua and Honduras through early
Friday.  Heavy rainfall could produce life-threatening flash
flooding and mud slides in portions of Nicaragua, Honduras, Costa
Rica, and Panama through Friday night.

2. The system could be near hurricane intensity when it approaches
the Yucatan Peninsula late Friday or Saturday, bringing direct
impacts from wind, storm surge, and heavy rainfall, and a hurricane
watch has been issued for a portion of this area.

3. The system is forecast to continue strengthening over the Gulf of
Mexico and could affect portions of the northern Gulf Coast as a
hurricane this weekend, with direct impacts from wind, storm surge,
and heavy rainfall.  However, it is too early to specify the timing,
location, or magnitude of these impacts.  Residents along the Gulf
Coast from Louisiana to Florida should monitor the progress of this
system for the next several days and heed any advice given by local
officials.


FORECAST POSITIONS AND MAX WINDS

INIT  05/0300Z 12.8N  82.7W   30 KT  35 MPH
 12H  05/1200Z 13.5N  83.2W   35 KT  40 MPH
 24H  06/0000Z 15.1N  83.7W   35 KT  40 MPH...INLAND
 36H  06/1200Z 17.6N  84.7W   45 KT  50 MPH
 48H  07/0000Z 20.2N  86.3W   55 KT  65 MPH
 72H  08/0000Z 25.5N  88.6W   70 KT  80 MPH
 96H  09/0000Z 30.5N  87.0W   65 KT  75 MPH...INLAND
120H  10/0000Z 36.5N  82.5W   30 KT  35 MPH...POST-TROP/INLAND

$$
Forecaster Berg

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