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TROPICAL STORM IDA DISCUSSION NUMBER   9
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102015
1100 AM AST SUN SEP 20 2015

The low-level center of Ida remains exposed to the west of the deep
convection, due to moderate westerly shear that has been affecting
the tropical cyclone during the past couple of days. A recent ASCAT
overpass revealed winds of 35 to 40 kt over the eastern portion of
the circulation so the initial intensity has been increased to 40
kt.  The ASCAT data also indicate that the area of
tropical-storm-force winds is larger than previous estimated, and
the wind radii have been adjusted accordingly.

Ida continues to move west-northwestward or 300 degrees at 13 kt.
The forward motion of the tropical cyclone is forecast to slow down
during the next day or two, as a blocking ridge strengthens to the
north of the system over the central Atlantic.  All of the
dynamical models show Ida becoming stationary or meandering over
the central Atlantic between 48 and 96 hours.  The NHC forecast
follows this scenario and shows the tropical cyclone stationary for
a couple of days.  Late in the forecast period, the ridge is
expected to weaken, which should allow a northwestward or northward
motion to begin, however, the track guidance is quite divergent at
that time.  Given the large spread in the guidance late in the
period, the NHC forecast remains near the multi-model consensus at
days 4 and 5.

The westerly shear that has been affecting Ida is expected to
decrease during the next day or so.  This should allow for some
strengthening, and the NHC forecast is near the IVCN consensus
model and the previous advisory.  After 48 hours, the global models
are suggesting that an upper-level trough to the north of Ida may
cause an increase in westerly shear.  As a result, the updated
official forecast shows no change in intensity late in the period.


FORECAST POSITIONS AND MAX WINDS

INIT  20/1500Z 17.3N  44.8W   40 KT  45 MPH
 12H  21/0000Z 18.2N  46.3W   40 KT  45 MPH
 24H  21/1200Z 19.5N  47.7W   45 KT  50 MPH
 36H  22/0000Z 20.4N  48.5W   50 KT  60 MPH
 48H  22/1200Z 20.8N  48.7W   50 KT  60 MPH
 72H  23/1200Z 20.8N  48.7W   50 KT  60 MPH
 96H  24/1200Z 20.8N  48.7W   50 KT  60 MPH
120H  25/1200Z 21.2N  49.0W   50 KT  60 MPH

$$
Forecaster Brown

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