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TROPICAL STORM IDA DISCUSSION NUMBER  15
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102015
1100 PM AST MON SEP 21 2015

Ida's cloud pattern has become increasingly disorganized.  Satellite
data indicate that the large separation between the cyclone's low-
to mid-level centers persists as a result of a strong west-
northwesterly shear.  The nearest deep convection, coincident with
the mid-level center, remains well removed to the southeast.  The
initial wind speed is conservatively lowered to 40 kt and is above
the latest Dvorak intensity estimates.

The tail of a large mid- to upper-level trough over the eastern
Atlantic is forecast to sweep southward over Ida during the next 24
hours.  Even stronger deep-layer northwesterly shear and confluence
aloft associated with this feature should adversely affect the storm
during the next few days, and cause weakening. One could envision
Ida not surviving such a greatly inhospitable environment.  However,
global models depict Ida remaining a coherent feature and eventually
re-strengthening, though Ida's forecast intensification occurs in
different ways.  The ECMWF has consistently shown Ida re-
strengthening, seemingly as a consequence of baroclinic forcing from
the trough interaction.  Such an interaction could potentially
result in Ida's taking on the characteristics of a hybrid cyclone.
The GFS shows Ida becoming disentangled from the trough, and
re-intensifying after the trough lifts out.  Regardless of which
scenario occurs, any re-strengthening of Ida as a tropical cyclone
would likely be tempered by substantially drier air on the backside
of the trough being entrained by the cyclone.  The new intensity
forecast is lower than the previous one in the short term due to the
overall harsh environment affecting the storm.  A modest increase in
strength is shown at the end of the forecast, similar to the
previous forecast.

The definition of Ida's center has been deteriorating, and it has
thus become harder to track.  It appears though the the cyclone's
forward speed has been decreasing, and the initial motion estimate
is an uncertain 340/04.  The deep layer west-northwesterly to
northwesterly flow, associated with the trough overtaking Ida, could
impart a slow eastward or east-southeastward motion for a couple of
days.  After that, the bulk of the track guidance shows Ida caught
between two mid-level ridges, which should induce a north-
northwestward and then northward motion at a gradually increasing
forward speed. Only the GFS shows Ida encountering a blocking ridge
with a track much farther to the west, a solution very different
than its ensemble mean.  The new track has been adjusted somewhat to
the east overall through 72 hours and is a little faster than the
previous one, in good agreement with a model consensus without the
GFS.


FORECAST POSITIONS AND MAX WINDS

INIT  22/0300Z 21.7N  49.5W   40 KT  45 MPH
 12H  22/1200Z 21.9N  49.3W   40 KT  45 MPH
 24H  23/0000Z 21.5N  48.4W   35 KT  40 MPH
 36H  23/1200Z 20.9N  47.5W   35 KT  40 MPH
 48H  24/0000Z 20.9N  46.9W   35 KT  40 MPH
 72H  25/0000Z 22.3N  47.1W   40 KT  45 MPH
 96H  26/0000Z 24.4N  48.2W   45 KT  50 MPH
120H  27/0000Z 26.2N  48.9W   50 KT  60 MPH

$$
Forecaster Kimberlain

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