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Tropical Depression FOUR


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.


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