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TROPICAL DEPRESSION NINE DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092015
500 AM AST THU SEP 17 2015

Tropical Depression Nine continues to have a sheared appearance
with an area of strong convection located 75-100 n mi east of the
low-level center.  Satellite intensity estimates from TAFB and SAB
have increased to 30 kt, and reports from NOAA buoy 41041 indicate
that the center pressure has fallen below 1007 mb.  Based on these
data, the initial intensity is increased to 30 kt.

The depression is located between a developing upper-level low to
its northwest and a tropical disturbance to the southeast.  The
global models are in poor agreement on how the depression will
interact with these systems, which makes for a low confidence
intensity forecast.  The UKMET and Canadian models merge the
depression with the disturbance as the latter system intensifies.
The NAVGEM shows the depression surviving for several days as the
disturbance weakens.  The GFS weakens both the depression and the
disturbance while developing another low pressure area nearby.
Finally, the ECMWF keeps the depression separate from the other
disturbance and forecasts it to survive for five days.  Even if the
depression and the disturbance do not merge, the upper-level low is
likely to cause 20-25 kt of shear over the depression for at least
3-4 days.  The new intensity forecast follows the no merger scenario
but calls for no additional strengthening due to the shear.  The
depression is expected to degenerate to a remnant low by 48 hours
and dissipate completely after 96 hours.

Assuming that the depression and the disturbance do not merge, a
weak low- to mid-level ridge to the north of the cyclone should
steer it generally northwestward through the forecast period.  The
new forecast track is similar to the previous track, and it lies to
the south of the center of the guidance envelope between the
consensus models and the BAM shallow.  An alternative forecast
scenario is that the depression could move erratically if it and
the disturbance get closer enough to try to merge.


FORECAST POSITIONS AND MAX WINDS

INIT  17/0900Z 16.3N  45.3W   30 KT  35 MPH
 12H  17/1800Z 17.1N  45.6W   30 KT  35 MPH
 24H  18/0600Z 18.1N  46.3W   30 KT  35 MPH
 36H  18/1800Z 19.1N  47.3W   25 KT  30 MPH
 48H  19/0600Z 20.0N  48.5W   25 KT  30 MPH...POST-TROP/REMNT LOW
 72H  20/0600Z 21.0N  51.0W   20 KT  25 MPH...POST-TROP/REMNT LOW
 96H  21/0600Z 22.5N  53.5W   20 KT  25 MPH...POST-TROP/REMNT LOW
120H  22/0600Z...DISSIPATED

$$
Forecaster Beven

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