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Tropical Depression Eighteen-E Forecast Discussion

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WTPZ43 KNHC 232042

Tropical Depression Eighteen-E Discussion Number   1
NWS National Hurricane Center Miami FL       EP182017
400 PM CDT Sat Sep 23 2017

ASCAT-A/-B scatterometer data from around 16-17Z indicated that the
low pressure system located just offshore of the southwestern coast
of Mexico that the NHC has been tracking the past several days has
become much better defined, and it also possessed surface winds of
30-33 kt. As a result, the system has been upgraded to a tropical
depression, the eighteenth of the eastern Pacific hurricane season.

The initial motion estimate is 335/04, based primarily on microwave
satellite fixes. The latest NHC model guidance is in fairly good
agreement on the cyclone moving slowly in a general
north-northwestward direction around the western periphery of a
deep-layer ridge for the next 5 days. Some of the models like the
GFS, Canadian, and HCCA take the system just inland near Cabo
Corrientes in about 24 hours, whereas the remainder of the guidance,
especially the UKMET and ECMWF, keep the cyclone just offshore of
the southwestern coast of Mexico. The forecast motion for the next
few days is expected to be 5 kt or less, an indication that steering
currents will be weak. Since there is no strong forcing that would
want to drive the depression inland over the mountainous terrain of
southwestern Mexico, the official forecast calls for the center of
the cyclone to remain just offshore of the coast throughout the
forecast period, similar to the ECMWF and UKMET solutions.

Given the well-defined circulation noted in the aforementioned
scatterometer data, along with vertical wind shear decreasing to
less than 10 kt by 24 hours, steady strengthening is forecast for
the next 24-36 hours. Afterwards, southeasterly to southerly shear
is expected to gradually increase to 25 kt by 72 hours and more
than 30 kt in the 96-120 hour period, which should induce steady to
possible rapid weakening. The NHC intensity forecast is above most
of the guidance through 48 hours, and then is a little lower than
the guidance after that.

Due to the depression being forecast to become a tropical storm by
tonight, along with its proximity to the coast of Mexico, a tropical
storm warning has been issued from Manzanillo northward to El
Roblito, including the Islas Marias. Heavy rainfall causing flash
floods and mudslides will also be possible within the warning area
and extending well inland.


INIT  23/2100Z 18.4N 105.3W   30 KT  35 MPH
 12H  24/0600Z 18.8N 105.5W   40 KT  45 MPH
 24H  24/1800Z 19.4N 105.7W   50 KT  60 MPH
 36H  25/0600Z 20.2N 106.0W   50 KT  60 MPH
 48H  25/1800Z 21.2N 106.2W   45 KT  50 MPH
 72H  26/1800Z 22.7N 107.0W   40 KT  45 MPH
 96H  27/1800Z 24.0N 108.4W   35 KT  40 MPH...POST-TROPICAL
120H  28/1800Z 26.0N 110.0W   30 KT  35 MPH...POST-TROP/REMNT LOW

Forecaster Stewart