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Tropical Depression Four Forecast Discussion Number 2
2017-07-06 10:34:49| National Hurricane Center (Atlantic)
Issued at 500 AM AST Thu Jul 06 2017 000 WTNT44 KNHC 060834 TCDAT4 Tropical Depression Four Discussion Number 2 NWS National Hurricane Center Miami FL AL042017 500 AM AST Thu Jul 06 2017 The cloud pattern of the tropical cyclone consists mainly of a circularly shaped mass of deep convection that has persisted overnight. Since there has been little overall change in the satellite appearance of the system, the current intensity estimate remains 25 kt, which is also in agreement with a Dvorak estimate from SAB. Dry and dusty air related to the Saharan Air Layer, to the east of the tropical cyclone, is beginning to wrap around the northern part of the depression's circulation. Dynamical models indicate that this air mass will be partially entrained into the system over the next couple of days. This, combined with increasing vertical shear, should prevent significant strengthening of the system. Although the statistical-dynamical guidance, SHIPS and LGEM, forecast some modest intensification of the tropical cyclone, these models have been known to have a high bias at times. The GFS, UKMET, and ECMWF global models all show the system quickly degenerating to a wave. As a compromise between the global and the statistical-dynamical guidance, the official forecast more or less maintains the cyclone's intensity for a couple of days followed by weakening to a remnant low in 72 hours and dissipation after 96 hours. The center is not very easy to locate, but based on continuity with earlier data it is believed to be near the eastern edge of the convective mass. There has apparently been some acceleration and the motion is now estimated to be 290/15 kt. The flow on the southern side of a subtropical ridge should continue to steer the tropical cyclone, or its remnants, west-northwestward over the next few days. The official track forecast follows a trajectory very similar to the previous one, but is somewhat faster. This is close to the latest model consensus. FORECAST POSITIONS AND MAX WINDS INIT 06/0900Z 13.2N 40.0W 25 KT 30 MPH 12H 06/1800Z 14.0N 42.5W 30 KT 35 MPH 24H 07/0600Z 15.0N 46.2W 30 KT 35 MPH 36H 07/1800Z 16.3N 50.0W 30 KT 35 MPH 48H 08/0600Z 17.5N 53.5W 25 KT 30 MPH 72H 09/0600Z 20.5N 59.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 10/0600Z 23.0N 65.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 11/0600Z...DISSIPATED $$ Forecaster Pasch
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