[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 151437
TCDAT2

TROPICAL DEPRESSION TWELVE DISCUSSION NUMBER   5
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122016
1100 AM AST THU SEP 15 2016

Visible satellite images indicate that the depression has a rather
impressive low-level circulation, although all the deep convection
is far removed from the center due to southwesterly shear.  Given
such a well-defined circulation, the depression could have become a
tropical storm overnight.  The initial wind speed, however, remains
30 kt using a blend of the TAFB/SAB estimates with any of the more
reliable ASCAT passes unfortunately missing the center during the
past 18 hours.

An upper-level trough to the northwest of the cyclone is expected to
cause an increase in shear during the next day or so, which will
likely cause the cyclone to weaken.  Beyond 48 hours, most of the
models show the shear relaxing, with an upper-level high taking the
place of the trough, and the cyclone is forecast to move over warmer
waters.  This will probably promote a restrengthening trend,
although guidance is in poor agreement on how much the winds could
increase.  Thus, the official forecast is reduced a little bit for
Friday/Saturday due to the shear, then is the same as the previous
one, showing a slow restrengthening early next week. It is also
possible the cyclone will degenerate into a remnant low for a while
due to the shear, although this isn't the likeliest scenario.

The depression is moving westward at about 11 kt, and this general
motion should continue for a day or so while it moves around the
Atlantic subtropical ridge.  This ridge is forecast by much of the
guidance to strengthen and build westward, which would force the
depression to the west-southwest over the weekend.  Early next
week, the cyclone could begin to gain some latitude as it reaches
the southwestern periphery of the ridge.  Guidance is in good
agreement on this scenario, except for the GFDL which appears to be
a northward outlier.  The official forecast track is very close to a
blend of the other models in the TVCN consensus minus the GFDL, and
is basically an update of the previous forecast.

FORECAST POSITIONS AND MAX WINDS

INIT  15/1500Z 17.7N  30.4W   30 KT  35 MPH
 12H  16/0000Z 17.8N  32.2W   30 KT  35 MPH
 24H  16/1200Z 17.9N  34.8W   30 KT  35 MPH
 36H  17/0000Z 17.7N  37.4W   25 KT  30 MPH
 48H  17/1200Z 17.4N  40.1W   25 KT  30 MPH
 72H  18/1200Z 16.5N  45.2W   30 KT  35 MPH
 96H  19/1200Z 16.7N  50.3W   35 KT  40 MPH
120H  20/1200Z 17.5N  55.0W   40 KT  45 MPH

$$
Forecaster Blake
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