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TROPICAL DEPRESSION TWELVE DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122016
500 AM AST THU SEP 15 2016

AMSR2 microwave data from a few hours ago indicated that the
depression's center was about a degree west of the deep convection
due to 20 kt of westerly shear.  Since the cyclone's structure has
not improved, and subjective Dvorak estimates have not changed from
six hours ago, the initial intensity remains 30 kt.  The westerly
shear is expected to increase during the next 24 hours, and with a
relatively drier air mass surrounding the cyclone, any strengthening
during the next couple of days now appears less likely.  After 48
hours, the shear is forecast to relax to some degree, but the global
models disagree on exactly how much.  Nonetheless, if the depression
can survive the next couple of days, the environment should improve
enough to allow for some strengthening by the end of the forecast
period.  The new NHC intensity forecast now holds the cyclone as a
30-kt depression for the next 3 days, followed by slight
strengthening on days 4 and 5.  This forecast is in best agreement
with the statistical-dynamical SHIPS and LGEM models.

The microwave data indicate that the depression has been moving a
little faster toward the west at 275/14 kt.  The Bermuda-Azores
high is expected to steer the depression generally westward across
the tropical Atlantic during the entire 5-day forecast period.  A
south-of-due-west motion is even possible on days 2 and 3,
especially if the cyclone remains weak.  With the exception of the
GFDL, which shows more strengthening and is an outlier way to the
north of the other models, the guidance envelope is relatively
tight.  To account for the apparent north bias of the GFDL, the
updated NHC track forecast is a bit south of the TVCN multi-model
consensus.  The new forecast is also a little faster than and south
of the previous forecast, especially on days 4 and 5.

FORECAST POSITIONS AND MAX WINDS

INIT  15/0900Z 17.6N  29.3W   30 KT  35 MPH
 12H  15/1800Z 17.8N  31.2W   30 KT  35 MPH
 24H  16/0600Z 18.0N  33.7W   30 KT  35 MPH
 36H  16/1800Z 17.9N  36.4W   30 KT  35 MPH
 48H  17/0600Z 17.6N  39.0W   30 KT  35 MPH
 72H  18/0600Z 16.7N  43.8W   30 KT  35 MPH
 96H  19/0600Z 16.5N  49.0W   35 KT  40 MPH
120H  20/0600Z 17.0N  53.5W   40 KT  45 MPH

$$
Forecaster Berg

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