000
WTNT45 KNHC 231446
TCDAT5

TROPICAL STORM IDA DISCUSSION NUMBER  21
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102015
1100 AM AST WED SEP 23 2015

Ida's cloud pattern has deteriorated since yesterday, and the
upper-level outflow is very disrupted due to shear. The center of
the cyclone has been placed in the middle of a couple of low cloud
swirls, and to the west of a small area of deep convection. Based on
continuity, the initial intensity is kept at 35 kt, however, a
recent partial ASCAT suggests that the winds could be lower.

Both ECMWF and GFS SHIPS intensity models indicate that the shear
has peaked, and a gradual relaxation should soon begin. However, it
will take 36 to 48 hours for the shear to be low enough to favor
re-strengthening. On this basis, only a small increase in intensity
is forecast beyond that time. This is consistent with the previous
forecast and the intensity consensus model.

The cyclone, as anticipated, has become well embedded in the base
of an upper-level trough and is drifting eastward or 090 degrees at
4 kt. Most of the global models lift the trough out and keep the
cyclone moving very slowly within weak steering currents for the
next day or two. After that time, the subtropical ridge is forecast
to build over the Atlantic, and this flow pattern will force the
cyclone to move toward the north and then to the north-northwest.
For the next 3 days or so, the guidance is in good agreement showing
a northward turn. The model spread increases after four days, but
the general trend is to keep the cyclone moving slowly while is
trapped south of the subtropical ridge.  The NHC forecast is in the
middle of the track envelope and is very similar to the previous
one.


FORECAST POSITIONS AND MAX WINDS

INIT  23/1500Z 20.0N  47.0W   35 KT  40 MPH
 12H  24/0000Z 20.0N  46.4W   35 KT  40 MPH
 24H  24/1200Z 20.5N  46.0W   35 KT  40 MPH
 36H  25/0000Z 21.0N  46.0W   35 KT  40 MPH
 48H  25/1200Z 22.0N  46.5W   35 KT  40 MPH
 72H  26/1200Z 23.0N  47.5W   40 KT  45 MPH
 96H  27/1200Z 25.0N  47.5W   45 KT  50 MPH
120H  28/1200Z 25.5N  48.0W   50 KT  60 MPH

$$
Forecaster Avila