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TROPICAL STORM IDA DISCUSSION NUMBER  17
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102015
1100 AM AST TUE SEP 22 2015

Ida continues to be a sheared cyclone with the low-level center
located to the northwest of an area of very deep convection. The
upper-level outflow in the southern half of the cyclone has become
better defined since yesterday, but it is not existent elsewhere.
Dvorak numbers are decreasing and do not support 40 kt at this
time. However, I would wait for scatterometer or any other data, if
available, to reduce the winds, if necessary.

The cyclone is embedded within an uncommon flow pattern. It is
located at the southern end of a mid-level trough, which is forcing
the cyclone to move east-southeastward at about 7 kt, and also
causing shear. The evolution of this trough will be crucial for the
future of Ida. Unanimously, global models keep the cyclone drifting
generally eastward for the next 2 days or so, while embedded within
the trough. After that time, all models forecast the trough to lift
out, leaving Ida south of an amplifying ridge. This forecast pattern
should result in a decrease of the shear with favorable conditions
for strengthening, and a slow motion of the cyclone toward the
north-northwest. It is interesting to note the current agreement of
all global models with this scenario, and at long range, the GFS and
ECMWF primarily forecast a significant strengthening of Ida.  Given
the uncommon pattern and the currently hostile environment, the NHC
forecast keeps the cyclone with the same intensity for the next day
or two, and allows for some intensification once Ida becomes
detached from the trough. The NHC track forecast is in the middle of
the guidance envelope, and follows the trend of the ECMWF and GFS
consensus.


FORECAST POSITIONS AND MAX WINDS

INIT  22/1500Z 20.5N  47.5W   40 KT  45 MPH
 12H  23/0000Z 20.5N  47.0W   40 KT  45 MPH
 24H  23/1200Z 20.5N  46.3W   40 KT  45 MPH
 36H  24/0000Z 20.0N  45.5W   40 KT  45 MPH
 48H  24/1200Z 20.5N  45.0W   40 KT  45 MPH
 72H  25/1200Z 22.0N  45.5W   45 KT  50 MPH
 96H  26/1200Z 23.5N  46.5W   50 KT  60 MPH
120H  27/1200Z 25.5N  47.0W   55 KT  65 MPH

$$
Forecaster Avila

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