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TROPICAL STORM IDA DISCUSSION NUMBER   7
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102015
1100 PM AST SAT SEP 19 2015

Ida is badly sheared tropical cyclone.  The center, a well-defined
swirl of low clouds, remains well removed from the nearest deep
convection to east.  This cloud pattern is consistent with UW-CIMSS
shear analyses indicating about 20 kt of westerly vertical shear
over the cyclone.  A partial ASCAT overpass at 0016 UTC showed
believable 37-kt wind well east of the center, and the initial
intensity estimate is kept at 35 kt.

Global models show strong westerly shear persisting over Ida for
the next day or so.  With so much shear likely, little to no
intensification is expected to occur.  A respite in the shear is
forecast by 36 hours when a piece of an upper-level trough pinches
off near the Greater Antilles and drifts westward, which may allow
Ida an opportunity to intensify some.  The lower-shear environment
should not last long, however, since a large mid- to upper-level
trough is forecast to dig southwestward into the central Atlantic on
top of Ida in 2 to 3 days, with the ECMWF model showing the
potential for a complex trough interaction to take place. Assuming
that Ida survives, the environment looks less than ideal, with
confluent and strong northwesterly winds aloft and the possibility
of enhanced oceanic upwelling underneath the cyclone. Weakening is
therefore considered the most likely scenario, and it would not be
surprising to see Ida become a remnant low during this time frame.
The new intensity forecast is lower than the previous one, closest
to the HWRF model.  An alternate but less likely possibility is that
Ida could restrengthen due to baroclinic forcing as shown in the
ECMWF solution.

Ida has been moving faster and more westerly because of its more
shallow nature, and the initial motion estimate is 290/13.  A
subtropical ridge migrating westward with the cyclone should keep it
on a general west-northwestward track for another day or so.
Steering currents are expected to collapse in a couple of days in
response to the large mid- to upper-level trough amplifying
southwestward in the vicinity of Ida. The lack of steering should
cause Ida to come to a halt in about 3 days, with the track
forecast after that dependent on the depth of the storm. A deeper
cyclone would move faster toward the north or northeast as shown in
the ECMWF, while a weaker and shallower cyclone would move westward
ad depicted in the GFS.  The official forecast is between these
two extremes but leans more toward the GFS, given the current
intensity forecast, and shows a slow northerly motion consistent
with the previous forecast.


FORECAST POSITIONS AND MAX WINDS

INIT  20/0300Z 15.8N  42.4W   35 KT  40 MPH
 12H  20/1200Z 16.7N  44.1W   35 KT  40 MPH
 24H  21/0000Z 17.8N  46.1W   40 KT  45 MPH
 36H  21/1200Z 18.9N  47.6W   45 KT  50 MPH
 48H  22/0000Z 19.8N  48.4W   50 KT  60 MPH
 72H  23/0000Z 19.9N  48.5W   45 KT  50 MPH
 96H  24/0000Z 19.9N  48.5W   40 KT  45 MPH
120H  25/0000Z 20.3N  48.4W   40 KT  45 MPH

$$
Forecaster Kimberlain

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