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Tropical Depression Seventeen-E Discussion Number   4
NWS National Hurricane Center Miami FL       EP172018
300 PM MDT Sat Sep 01 2018

ASCAT scatterometer overpasses at 1625Z and 1704Z indicated that
the depression's inner-core wind field remains elongated
northeast-to-southwest, with a second fully exposed low-level swirl
located near the northeastern end of the wind field. There was also
a fairly large field of 27-29 kt surface wind vectors located in the
southern quadrant. Various Dvorak satellite intensity estimates have
remained unchanged since the previous advisory, so the initial
intensity is being held at 30 kt for this advisory.

The initial motion estimate is an uncertain 285/10 kt. Most of the
global and regional models take the cyclone west-northwestward for
the next 24 h or so, and then move it westward thereafter. The
exception is the new GFS model run, which takes the system northward
for 36 h before turning it back westward. The GFS appears to have
keyed in on the aforementioned exposed low-level swirl noted in the
ASCAT and recent visible satellite data, and makes it the primary
circulation center. The official forecast track calls for the main
low-level center to remain/develop closer to the stronger deep
convection and strongest low-level winds and associated vorticity.
The more northern track of the GFS model is being discounted, but it
has strongly skewed the various consensus tracks farther to the
north.  As a result, the new official forecast track lies south of
the consensus model HCCA, and is close to the previous advisory
track and a blend of the more westerly HWRF and ECMWF models.

Due to the GFS model's more northerly track, the GFS-based shear
computations have a high bias. In contrast, the more southerly ECMWF
model has continued to forecast much weaker shear of 10-15 kt for
the next 72 h, followed by decreasing shear after that. These more
modest shear conditions, combined with very warm SSTs greater than
28.5 deg C, should allow for at least steady strengthening to occur.
The only hindering factor early on remains when the inner-core wind
field contracts down and becomes better defined, which would result
in sooner and more significant intensification to occur than is
indicated by the official forecast. For now, the intensity forecast
remains above the consensus models HCCA and IVCN due to the very low
bias induced by the GFS-based SHIPS and LGEM intensity models.

FORECAST POSITIONS AND MAX WINDS

INIT  01/2100Z 14.9N 111.1W   30 KT  35 MPH
 12H  02/0600Z 15.3N 111.6W   35 KT  40 MPH
 24H  02/1800Z 15.7N 112.5W   40 KT  45 MPH
 36H  03/0600Z 16.0N 113.7W   45 KT  50 MPH
 48H  03/1800Z 16.2N 115.1W   55 KT  65 MPH
 72H  04/1800Z 16.3N 118.3W   75 KT  85 MPH
 96H  05/1800Z 16.9N 122.4W   85 KT 100 MPH
120H  06/1800Z 18.3N 127.3W   95 KT 110 MPH

$$
Forecaster Stewart

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