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TROPICAL DEPRESSION FOUR DISCUSSION NUMBER   1
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL042015
1100 AM AST TUE AUG 18 2015

The low pressure system that has been moving westward across the
eastern tropical Atlantic the past few days has finally acquired
enough organized deep convection to be classified as a tropical
cyclone. A 1232 UTC ASCAT-B overpass indicated the system has a
well-defined circulation, and there was also a large field of 30-kt
and greater wind vectors in the eastern semicircle. Dvorak satellite
intensity estimates are a consensus T2.0/30 kt from TAFB and SAB, so
the initial intensity is set at 30 kt for this advisory. Upper-level
outflow is good to the south and fair to the north.

The initial motion estimate is 280/11 kt. The global and regional
models are in good agreement on the tropical cyclone moving
west-northwestward along the southern periphery of a deep-layer
subtropical ridge for the next 48-72 hours, accompanied by a
decrease in forward speed as the system approaches a weakness in
this ridge. After that time, the ridge is expected to build back in
as a trough to the north lifts out, forcing the cyclone to turn more
westward and accelerate through the remainder of the forecast
period. The official forecast track lies close to but a little
faster than the multi-model consensus TVCN due to the much slower
GFS model creating a slow bias in the consensus.

The overall atmospheric and oceanic environments surrounding the
cyclone appear conducive for slow but steady strengthening
throughout the forecast period. The only inhibiting factor will be
occasional brief intrusions of dry mid-level air associated with the
Saharan Air Layer that lies just to the north of the depression.
However, given the very low vertical wind shear of less than 5 kt,
the convective structure is expected to steadily increase in
organization, allowing the dry air intrusions to be mixed out. The
official intensity forecast closely follows the intensity consensus
model IVCN through 72 hours, and then leans closer to a blend of the
Decay-SHIPS and LGEM models at 96 and 120 hours.

FORECAST POSITIONS AND MAX WINDS

INIT  18/1500Z 10.6N  36.5W   30 KT  35 MPH
 12H  19/0000Z 11.0N  37.9W   35 KT  40 MPH
 24H  19/1200Z 11.3N  39.5W   40 KT  45 MPH
 36H  20/0000Z 11.6N  40.9W   50 KT  60 MPH
 48H  20/1200Z 12.1N  41.9W   60 KT  70 MPH
 72H  21/1200Z 13.2N  44.2W   70 KT  80 MPH
 96H  22/1200Z 13.7N  47.8W   80 KT  90 MPH
120H  23/1200Z 14.0N  52.4W   85 KT 100 MPH

$$
Forecaster Stewart

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