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Tropical Depression SIX


1100 AM AST WED AUG 17 2016

The depression's overall cloud pattern and low-level wind field have
continued to improve, although cloud tops have warmed considerably
near the center since the previous advisory. An 1139Z ASCAT-B
overpass showed a well-defined low-level circulation center with a
tight radius of maximum winds of only 10-15 nmi. Although there were
a few vectors near tropical storm force, and satellite estimates at
1200Z were T2.5/35 kt from TAFB and SAB, the intensity is being
maintained at 30 kt for this advisory due to the sharp decrease in
deep convection near the center during the past few hours.

The modest northeasterly vertical wind shear across the cyclone is
forecast to weaken and veer to the southeast during the next 12
hours, and remain less than 10 kt for the next 36-48 hours, which
typically favors strengthening. Sea-surface temperatures just below
27C are marginal for intensification and the small cyclone will be
moving through dry mid-level air with humidity values dropping below
50 percent by 48 hours and beyond, conditions that are generally not
conducive for significant strengthening. But given the tight
inner-core wind field noted in recent ASCAT data, the cyclone is
expected to be able to mix out any dry air intrusions and slowly
strengthen for about the next 48 hours. By 72 hours and beyond,
however, increasing southwesterly to westerly wind shear is expected
to induce gradual weakening. The official intensity forecast is
similar to the previous advisory and closely follows the intensity
consensus model IVCN.

The initial motion estimate is 300 /13 kt, based primarily on
microwave and scatterometer fixes. The NHC model guidance is in
good agreement on the cyclone moving west-northwestward to
northwestward toward a weakness in the Bermuda-Azores ridge for the
next 48 hours or so. After that, however, there is significant
divergence in the models with the GFDL, GFS, and GFS-ensemble mean
models taking a stronger and more vertically deep cyclone more
toward the northwest, whereas the HWRF, UKMET, NAVGEM, and ECMWF
models show a weaker and shallower cyclone turning more westward
and moving along the southern periphery of the low-level ridge. As
a result, the forecast track depends heavily on the strength and
vertical structure of the cyclone. The new NHC forecast track is a
little to the south or left of the previous advisory, but not nearly
as far south as the ECMWF model due to expectations that the cyclone
will not weaken nearly as much as that model is indicating.

The forecast wind radii were decreased somewhat based on the smaller
wind field depicted in recent ASCAT data.


INIT  17/1500Z 14.0N  36.4W   30 KT  35 MPH
 12H  18/0000Z 14.9N  37.7W   35 KT  40 MPH
 24H  18/1200Z 16.0N  39.3W   40 KT  45 MPH
 36H  19/0000Z 17.0N  40.7W   45 KT  50 MPH
 48H  19/1200Z 17.9N  42.3W   50 KT  60 MPH
 72H  20/1200Z 19.6N  45.9W   45 KT  50 MPH
 96H  21/1200Z 21.8N  49.8W   40 KT  45 MPH
120H  22/1200Z 23.9N  53.4W   35 KT  40 MPH

Forecaster Stewart