[Tropical] Tropical Cyclone Discussion

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WTNT41 KNHC 051447
TCDAT1

Tropical Storm Nate Discussion Number   5
NWS National Hurricane Center Miami FL       AL162017
1100 AM EDT Thu Oct 05 2017

The cloud pattern associated with Tropical Depression Sixteen
increased in organization after the last advisory, with the
formation of a ragged central convective feature and outer banding
in the northeastern semicircle.  In addition, data from the
Colombian radar at San Andres showed a partial eyewall, and surface
observations from Puerto Cabezas, Nicaragua, included a pressure of
1001 mb outside of the center.  Based on these data, the initial
intensity has been increased to 35 kt, and the depression has been
upgraded to Tropical Storm Nate.

The center of Nate is now inland over northeastern Nicaragua, and
little change in strength is expected until the center moves over
the northwestern Caribbean Sea.  After that, a combination of warm
sea surface temperatures and light shear should allow for at least
steady strengthening.  However, the guidance is producing mixed
signals despite a favorable-looking environment.  The Rapid
Intensification Index of the SHIPS model is showing high chances of
rapid intensification, with better than a 50 percent chance of 25
kt of strengthening in the next 24 h and nearly a 50 percent chance
of 65 kt of strengthening in 72 h.  On the other side, the GFS and
Canadian models show only modest development and keep the cyclone
as a tropical storm until it reaches the northern Gulf coast.
Given the environment, the intensity forecast leans towards the high
end of the guidance envelope and calls for Nate to become a
hurricane in about 48 h and reach the northern Gulf Coast as a
hurricane.

The initial motion is 325/8.  A combination of a large cyclonic gyre
over Central America, a trough of low pressure moving westward
across the Gulf of Mexico, and a building subtropical ridge over
the western Atlantic should steer Nate generally north-northwestward
with an increase in forward speed during the next 72 h.  While the
guidance is in better agreement on the direction that Nate should
move, there remains disagreement on the speed despite an overall
trend toward a faster motion.  The new forecast track is similar to
the direction of the previous track, but shows a faster forward
speed that has the center near the northeastern Yucatan Peninsula
in about 36 h and near the northern Gulf Coast in about 72 h.
After the Gulf Coast landfall, Nate or its remnants are expected to
recurve northeastward upon encountering the mid-latitude
westerlies.


KEY MESSAGES:

1. Heavy rainfall is the main threat from Nate in portions of
Central America, with life-threatening flash flooding and mud slides
possible in portions of Nicaragua, Honduras, Costa Rica, Panama, and
Belize through Friday night.

2. Nate is forecast to be near hurricane intensity when it
approaches the Yucatan Peninsula late Friday, bringing direct
impacts from wind, storm surge, and heavy rainfall. A tropical storm
warning and a hurricane watch are in effect for a portion of this
area and life-threatening flash flooding is also possible.

3. Nate is forecast to reach the northern Gulf Coast this weekend as
a hurricane, and the threat of direct impacts from wind, storm
surge, and heavy rainfall is increasing.  However, it is too early
to specify the exact timing, location, or magnitude of these
impacts.  Residents along the Gulf Coast from Louisiana through the
Florida Panhandle should monitor the progress of Nate and heed any
advice given by local officials.


FORECAST POSITIONS AND MAX WINDS

INIT  05/1500Z 14.3N  83.7W   35 KT  40 MPH...INLAND
 12H  06/0000Z 15.6N  84.3W   35 KT  40 MPH...INLAND
 24H  06/1200Z 18.1N  85.4W   45 KT  50 MPH...OVER WATER
 36H  07/0000Z 20.8N  86.8W   55 KT  65 MPH
 48H  07/1200Z 23.7N  88.1W   65 KT  75 MPH
 72H  08/1200Z 29.5N  89.5W   70 KT  80 MPH
 96H  09/1200Z 36.0N  85.0W   30 KT  35 MPH...INLAND
120H  10/1200Z 41.5N  76.5W   20 KT  25 MPH...POST-TROP/INLAND

$$
Forecaster Beven
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