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TROPICAL STORM IDA DISCUSSION NUMBER  14
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102015
500 PM AST MON SEP 21 2015

Satellite data indicate that the low- and mid-level centers of Ida
have become separated, with the low-level center exposed well to the
northwest of the deep convection.  This appears to be the result
of some unanticipated mid-level shear that is occurring below the
typical 200 mb outflow layer.  The latest TAFB Dvorak classification
and objective ADT estimates from UW/CIMSS support an initial
intensity of 40-45 kt, and based on the earlier ASCAT data the
advisory wind speed is held at 45 kt, although this could be
generous.  The forecast upper-level wind pattern over the next
couple of days, which features continued mid-level shear along
with an increase in upper-level northwesterly winds, should prevent
strengthening.  In fact, Ida could weaken during the next 2-3 days,
and perhaps not survive as a tropical cyclone.  The NHC forecast
assumes that Ida will maintain tropical cyclone status, and that
upper-level winds will become a little more conducive for
strengthening by late in the period.  The updated NHC intensity
forecast is lower than the previous one, but still shows some
increase in strength at 96 and 120 h.

Ida continues to move north-northwestward, but recent visible
satellite imagery suggest that the forward speed is decreasing.
The track forecast reasoning remains unchanged from the previous
advisory.  The steering currents around the tropical storm are
expected to weaken very soon, and Ida is expected to meander
eastward or east-southeastward over the next couple of days.  After
that time, Ida should begin a northwestward or north-northwestward
motion as a mid- to upper-level trough lifts out to the northeast.
The track guidance has become quite divergent this cycle with very
large spread between a faster and more eastward ECMWF solution, and
a slower and more westward GFS track.  For now, the official
forecast is near the middle of the guidance envelope, closest to
the GFS ensemble mean and FSU Superensemble.

FORECAST POSITIONS AND MAX WINDS

INIT  21/2100Z 21.2N  48.8W   45 KT  50 MPH
 12H  22/0600Z 21.6N  48.7W   45 KT  50 MPH
 24H  22/1800Z 21.6N  48.1W   45 KT  50 MPH
 36H  23/0600Z 21.1N  47.3W   45 KT  50 MPH
 48H  23/1800Z 20.8N  46.7W   45 KT  50 MPH
 72H  24/1800Z 21.5N  46.8W   45 KT  50 MPH
 96H  25/1800Z 23.0N  47.8W   50 KT  60 MPH
120H  26/1800Z 25.0N  48.5W   55 KT  65 MPH

$$
Forecaster Brown

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