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TROPICAL DEPRESSION THIRTEEN DISCUSSION NUMBER   1
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL132016
500 PM AST MON SEP 19 2016

Satellite imagery, scatterometer data, and ship observations
indicate that the large low pressure system located southwest of the
Cabo Verde Islands has maintained a well-defined circulation and has
developed sufficient organized convection to be considered a
tropical depression. The initial intensity is set at 30 kt based on
scatterometer wind data of near 30 kt along with Dvorak satellite
intensity estimates T2.5/35 kt and T2.0/30 kt from TAFB and SAB,
respectively. The central pressure is based on reports from ship
D5ET2 during is traversal through the center of the cyclone over the
past 12 hours. The depression is a large tropical cyclone with a
wind field that is more than 400 n mi in diameter.

The initial motion estimate is an uncertain 280/10 kt, based
primarily on scatterometer and microwave satellite data.  The
depression is located along the southern periphery of a large
deep-layer ridge that is foreast to steer the cyclone westward for
the next 24 hours or so, followed by west-northwestward motion
through 120 hours. The official forecast track lies close to the
consensus model TVCN through 96 hours, and then leans toward a blend
of the ECMWF, UKMET, and CMC models since the GFS dissipates
the cyclone by 120 hours, a scenario that seems premature given the
current large size of the cyclone.

Vertical wind shear is forecast to remain low at less than 10 kt
over the cyclone for the next 48 hours, which favors steady
intensification. However, the mid-level moisture is only expected to
be marginal during that time, with humidity values around 60 percent
or less. The drier air along with the large size of the cyclone are
expected to temper the development process, and this is reflected by
the slower-than-average intensification rate. The NHC intensity
forecast closely follows the intensity model IVCN. This forecast is
lower than the SHIPS, LGEM, and ECMWF models, which brings the
cyclone to near hurricane strength by 72 hours.

FORECAST POSITIONS AND MAX WINDS

INIT  19/2100Z 13.6N  28.6W   30 KT  35 MPH
 12H  20/0600Z 14.3N  29.8W   35 KT  40 MPH
 24H  20/1800Z 15.6N  31.8W   40 KT  45 MPH
 36H  21/0600Z 16.7N  33.5W   45 KT  50 MPH
 48H  21/1800Z 17.6N  35.4W   50 KT  60 MPH
 72H  22/1800Z 19.5N  38.9W   45 KT  50 MPH
 96H  23/1800Z 21.5N  42.5W   35 KT  40 MPH
120H  24/1800Z 23.5N  44.7W   30 KT  35 MPH

$$
Forecaster Stewart

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