[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT41 KNHC 052045
TCDAT1

Tropical Storm Nate Discussion Number   6
NWS National Hurricane Center Miami FL       AL162017
500 PM EDT Thu Oct 05 2017

The center of Nate is now over eastern Honduras, and the cloud
pattern has lost some organization since the last advisory due to
passage over land.  In addition, recent ASCAT overpasses suggest
that there are no tropical-storm-force winds outside of the coastal
waters of Nicaragua and Honduras.  However, surface observations
indicate that the central pressure remains near 1000 mb, and based
on this the initial intensity remains a possibly generous 35 kt.
An Air Force Reserve Hurricane Hunter aircraft is scheduled to
investigate Nate as it emerges from the northern coast of Honduras
around 0300 UTC.

Analyses from CIMSS at the University of Wisconsin suggest that
Nate is currently experiencing about 20 kt of southwesterly vertical
wind shear, which is more than suggested by the SHIPS model.  The
large-scale models are in good agreement that this shear should
diminish during the next 12-24 h, leaving Nate in an favorable
environment for strengthening.  One change in the models from the
previous advisory is that the GFS now shows more development as Nate
crosses the Gulf of Mexico.  The new intensity forecast shows little
change during the first 12 h due to the shear and land interaction,
then it calls for steady intensification though landfall on the
northern Gulf Coast.  After landfall, Nate should weaken as it
traverses the eastern United States.  It should be noted that while
the forecast shows a peak intensity of 70 kt at 48 h, Nate is
expected to continue to strengthen between 48 h and landfall and
thus is likely to be stronger than 70 kt.  It should also be noted
that SHIPS Rapid Intensification Index values remain quite high, and
any period of rapid intensification would lead to Nate being
stronger than currently forecast.

The initial motion is now 330/9.  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 48 h or so,
followed by a turn toward the north as the storm reaches the
western end of the ridge.  The guidance is in decent agreement on
the direction of motion, but there remains disagreement on the speed
despite a continued trend toward a faster motion.  The new forecast
track is similar to the direction of the previous track, but again
shows a faster forward speed that now has the center near the
northeastern Yucatan Peninsula in 24-36 h and near the northern Gulf
Coast in 60-72 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 from Louisiana through the
Florida Panhandle. A hurricane watch and storm surge watch will
likely be issued for portions of the northern Gulf Coast tonight or
Friday morning, and residents in these areas should monitor the
progress of Nate and heed any advice given by local officials.


FORECAST POSITIONS AND MAX WINDS

INIT  05/2100Z 14.9N  84.3W   35 KT  40 MPH...INLAND
 12H  06/0600Z 16.5N  84.9W   35 KT  40 MPH...OVER WATER
 24H  06/1800Z 19.2N  86.1W   45 KT  50 MPH
 36H  07/0600Z 22.3N  87.9W   55 KT  65 MPH
 48H  07/1800Z 25.6N  89.5W   70 KT  80 MPH
 72H  08/1800Z 31.5N  89.5W   50 KT  60 MPH...INLAND
 96H  09/1800Z 38.0N  81.5W   25 KT  30 MPH...INLAND
120H  10/1800Z 41.5N  72.0W   20 KT  25 MPH...POST-TROP/INLAND

$$
Forecaster Beven
This weather bulletin was brought to you by:
www.nashvilleweather.net