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TROPICAL STORM IDA DISCUSSION NUMBER   3
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102015
1100 PM AST FRI SEP 18 2015

The depression's cloud pattern has been increasing in organization.
A primarily convective band over the eastern half of the circulation
has been increasing in curvature and the cloud tops are cooling
significantly. The initial wind speed is increased to 35 kt in
agreement with the latest CI numbers from TAFB and SAB and UW-CIMSS
ADT value.

Westerly shear associated with an upper-level trough, extending
from the eastern subtropical Atlantic into the eastern Caribbean,
could increase over Ida during the next day or so and slow the
cyclone's rate of intensification.  Global models agree that a
relative reduction in the shear should occur in 2 to 3 days when a
piece of the upper-level trough cuts off near the Greater Antilles,
which could allow Ida to strengthen, perhaps even more than
forecast.  Late in the forecast period, the uncertainty in the
intensity forecast increases considerably, with the ECMWF calling
for more northwesterly shear and less favorable thermodynamic
variables in the storm's environment than in the GFS.  The new
intensity forecast is only slightly higher than the previous one
through 72 hours.  The forecast after that time is of low confidence
and essentially levels Ida's intensity off in agreement with the
SHIPS model output.

Recent fixes suggest that the cyclone's motion has been more
northwesterly (or 305/06) since earlier today.  Ida is being steered
by east-southeasterly to southeasterly flow underneath a subtropical
ridge migrating westward with the cyclone, and some increase in
forward speed is predicted by the models during the next day or so.
In about 48 hours, a deep-layer trough amplifying southwestward
across the eastern Atlantic will cause steering currents surrounding
Ida to collapse, which should result in the cyclone's forward motion
coming to a halt.  With increasing northerly or northwesterly flow
aloft, Ida should begin to meander in about 4 days and even
drift southward or southeastward by day 5.  The new track forecast
is adjusted to the right some throughout the period, primarily due
to the more northwesterly initial motion but also in the direction
of the multi-model consensus.


FORECAST POSITIONS AND MAX WINDS

INIT  19/0300Z 13.7N  37.5W   35 KT  40 MPH
 12H  19/1200Z 14.7N  38.9W   40 KT  45 MPH
 24H  20/0000Z 15.8N  40.9W   45 KT  50 MPH
 36H  20/1200Z 16.9N  43.2W   50 KT  60 MPH
 48H  21/0000Z 18.1N  45.0W   50 KT  60 MPH
 72H  22/0000Z 20.1N  47.4W   60 KT  70 MPH
 96H  23/0000Z 20.7N  48.0W   55 KT  65 MPH
120H  24/0000Z 20.3N  47.8W   55 KT  65 MPH

$$
Forecaster Kimberlain

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