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TROPICAL STORM IDA DISCUSSION NUMBER  10
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102015
500 PM AST SUN SEP 20 2015

Deep convection associated with Ida has decreased in coverage and
become somewhat less organized today.  The tropical cyclone remains
sheared with a small band of convection located well east of the
exposed center.  Although Dvorak satellite T-numbers have decreased
this afternoon, the initial intensity remains 40 kt, which is based
on the earlier ASCAT data.

Ida is moving northwestward or 305 degrees at 15 kt, a little faster
than this morning.  Despite the recent increase in forward speed,
Ida is expected to slow down during the next 24 hours, and become
nearly stationary by Tuesday when a mid-level ridge builds to the
north of the system.  Ida is then forecast to meander for a couple
of days, before the ridge slides eastward and allows the tropical
cyclone to resume a northwestward motion.  The track guidance is in
good agreement during the first few days of the forecast, but again
becomes quite divergent by the end of the forecast period.  The
ECMWF, which moves Ida more eastward during the next few days, is
along the eastern edge of the guidance envelope.  Meanwhile the
UKMET and GFS show less of an eastern motion while the steering
currents are weak, and take Ida more westward by day 5.  The updated
NHC track is between these scenarios and remains close to the
multi-model consensus at 96 and 120 h.

The westerly shear is still expected to decrease during the next
day or so, but given the current organization of the system, Ida
may not be able to take full advantage of the potentially more
favorable upper-air pattern.  Therefore, only a slight increase in
intensity is predicted.  After 48 hours, upper-level westerly winds
associated with a trough are forecast to cause an increase in shear
over the cyclone, which would likely cause weakening.  The global
models, however, generally deepen the cyclone between 72 and 120 h
when Ida interacts with an upper-level trough.  The official
forecast shows little change in strength late in the period and
lies between these differing possibilities.

FORECAST POSITIONS AND MAX WINDS

INIT  20/2100Z 18.4N  46.3W   40 KT  45 MPH
 12H  21/0600Z 19.6N  47.5W   40 KT  45 MPH
 24H  21/1800Z 20.9N  48.5W   45 KT  50 MPH
 36H  22/0600Z 21.5N  48.7W   45 KT  50 MPH
 48H  22/1800Z 21.5N  48.5W   45 KT  50 MPH
 72H  23/1800Z 21.0N  48.2W   45 KT  50 MPH
 96H  24/1800Z 20.9N  48.5W   45 KT  50 MPH
120H  25/1800Z 22.5N  49.5W   45 KT  50 MPH

$$
Forecaster Brown

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