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TROPICAL STORM IDA DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102015
500 AM AST SAT SEP 19 2015

The overall organization of Ida's cloud pattern continues to
increase.  Banding features are becoming more prominent in the
southern part of the circulation, with a new band forming on the
west side.  In addition, a burst of convection has been ongoing
during the past few hours near the apparent center.  Satellite
classifications still support an intensity of 35 kt, and this value
is used as the initial wind speed.

Satellite fixes suggest that the northwestward motion of Ida has
sped up to 9 kt this morning.  A mid-level ridge is expected to
remain the dominant steering mechanism for the next couple of days,
causing the storm to move to the west-northwest or northwest during
that time.  Models are in good agreement and only a slight
northward adjustment was made to the official forecast then.  Beyond
2 days, there is considerable uncertainty on how an amplifying
trough over the east-central Atlantic will affect Ida.  The cyclone
seems likely to slow down due to steering currents collapsing, but
there is poor agreement on whether the trough will pick up Ida,
sending the storm east-northeastward, or leave the storm behind. For
example, the normally reliable ECMWF and GFS models are going in
opposite directions at day 5, with the ECMWF favoring a stronger
trough solution while the GFS has Ida missing the trough and
drifting westward.  With so much uncertainty, I have elected to show
a stationary Ida after day 3, preferring not to commit to
either solution at this time. This forecast blends the previous
forecast with the model consensus, although leans more heavily on
the former.

While the shear near Ida is low for now, most of the global models
show an increase during the next day or so.  This is expected to
somewhat temper future strengthening, although since most of the
other environmental factors are still favorable, a gradual
intensification is forecast. The long-range intensity prediction,
however, is very complex due to the potential interaction of the
trough, which yields very different shear profiles, along with dry
air aloft and possible upwelling of cooler water as Ida moves
slowly.  With the track more uncertain than average and these
challenging factors, it seems prudent to remain conservative with
the wind speed forecast. The official intensity forecast is close to
the previous one, which ends up on the low side of the guidance at
days 4/5.  It almost goes without saying that these conditions are
leading me to have a rather low confidence in the intensity forecast
at 72 hours and beyond.



FORECAST POSITIONS AND MAX WINDS

INIT  19/0900Z 14.4N  38.5W   35 KT  40 MPH
 12H  19/1800Z 15.3N  39.9W   40 KT  45 MPH
 24H  20/0600Z 16.5N  42.2W   45 KT  50 MPH
 36H  20/1800Z 17.6N  44.0W   50 KT  60 MPH
 48H  21/0600Z 18.9N  45.6W   55 KT  65 MPH
 72H  22/0600Z 20.5N  47.5W   60 KT  70 MPH
 96H  23/0600Z 20.5N  47.5W   55 KT  65 MPH
120H  24/0600Z 20.5N  47.5W   55 KT  65 MPH

$$
Forecaster Blake

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