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Tropical Depression Four Forecast Discussion Number 6
2017-07-07 10:42:25| National Hurricane Center (Atlantic)
Issued at 500 AM AST Fri Jul 07 2017 000 WTNT44 KNHC 070842 TCDAT4 Tropical Depression Four Discussion Number 6 NWS National Hurricane Center Miami FL AL042017 500 AM AST Fri Jul 07 2017 The small, tenacious depression has continued to hold its own early this morning with the low-level center now having moved up into the deep convective cloud mass, with the center being north of the previous advisory positions based on earlier scatterometer and buoy wind data. Upper-level outflow is fair in all quadrants, albeit elongated from northwest to southeast, and the vertical wind shear has now shifted from northeasterly to light southwesterly at less than 5 kt. The initial intensity is being maintained at 25 kt based on 0600 UTC satellite classifications of 25 kt from TAFB and SAB, but it wouldn't surprise me if the system isn't a little stronger given the noticeable improvement in the convective cloud pattern since the time of those fixes. The initial motion estimate is 285/19 kt. The new NHC forecast track has been shifted northward of the previous advisory track, but only due to the more northward initial position. Otherwise, there is little change to the previous forecast reasoning. A strong low- to mid-level ridge to the north of the depression should act to steer the small cyclone generally west-northwestward until dissipation occurs in about 96 h. The new forecast track lies down the middle of the tightly clustered model suite, near the consensus model TVCN. The depression is expected to retain that status for another 36 h or so while the vertical wind shear remains relatively low at less than 15 kt. By 36-48 h, westerly vertical wind shear is forecast to increase to around 20 kt, which should induce gradual weakening. The small cyclone is expected to degenerate into a remnant low by 48-72 h and dissipate by 96 h. There is one important caveat to note and that is the UKMET model, which continues to show less weakening and even strengthening in 96 and 120 h when the system is approaching the Bahamas. Although the other global and regional models do not show regeneration at this time, they do however show similar improving upper-level wind conditions east of Florida by 120 h. For now, the official intensity forecast remains similar to the previous advisory and the consensus model IVCN through 48 h, after which the forecast is lower than the intensity consensus. FORECAST POSITIONS AND MAX WINDS INIT 07/0900Z 15.0N 48.8W 25 KT 30 MPH 12H 07/1800Z 15.8N 51.6W 25 KT 30 MPH 24H 08/0600Z 17.1N 55.1W 25 KT 30 MPH 36H 08/1800Z 18.6N 58.7W 25 KT 30 MPH 48H 09/0600Z 20.1N 61.9W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 10/0600Z 22.8N 67.7W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 11/0600Z...DISSIPATED $$ Forecaster Stewart
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