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TROPICAL STORM IDA DISCUSSION NUMBER  16
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102015
500 AM AST TUE SEP 22 2015

Shortwave infrared satellite imagery indicates that Ida's low-level
circulation center is now moving or developing east-southeastward
closer to the large mass of deep convection that has been persisting
in the southeastern portion of the larger circulation. The initial
intensity is being maintained at 40 kt, especially now that the
low-level center has moved closer to the mid- and upper-level
circulations as seen in microwave and conventional satellite images.
This intensity is supported by a Dvorak current intensity estimate
of T3.0/45 kt from TAFB.

The initial motion estimate is now 105/04 kt. The recent and much
anticipated turn to the east-southeast appears to have occurred
either due to actual storm motion or due to redevelopment of the
low-level center closer to the strong convection. Regardless of the
cause in the Ida's recent motion change, the global and regional
models are in very good agreement on a continued general slow motion
toward the east for the next 48 hours or so as the southern portion
of a large mid- to upper-level trough moves across and captures the
cyclone. By 72 hours, the trough lifts out to the northeast and
releases Ida, allowing the cyclone to move slowly toward the
northwest by day 4 and toward the north on day 5. As would be
expected in such a weak flow regime, the model guidance is widely
divergent after 72 hours with the UKMET, HWRF, and NAVGEM models
taking Ida more toward the west-northwest, whereas the GFS, ECMWF,
GFDL, and Canadian models take Ida more toward the northwest and
north. The one thing that the all of the models do agree on,
however, is that Ida is not expected move very quickly during the
next 5 days. The NHC track forecast is similar to but slower than
the previous advisory track, and lies close to a blend of the
consensus models TCVA and GFEX.

Given the uncertainty in exactly when and where the low-level center
of Ida will move underneath the mid/upper-level circulation due to
fluctuations in the deep-layer vertical shear profiles during the
next 3 days, the intensity forecast calls for no significant
changes in the strength of the cyclone during that time. By days 4
and 5, however, some gradual strengthening is expected as the
vertical shear abates somewhat while Ida is moving over 29C
sea-surface temperatures. The official intensity forecast is similar
to the previous advisory, and closely follows the IVCN intensity
consensus model.

FORECAST POSITIONS AND MAX WINDS

INIT  22/0900Z 21.3N  48.6W   40 KT  45 MPH
 12H  22/1800Z 21.3N  48.1W   40 KT  45 MPH
 24H  23/0600Z 20.9N  47.3W   40 KT  45 MPH
 36H  23/1800Z 20.6N  46.7W   40 KT  45 MPH
 48H  24/0600Z 20.8N  46.4W   40 KT  45 MPH
 72H  25/0600Z 22.0N  46.7W   40 KT  45 MPH
 96H  26/0600Z 23.0N  48.0W   45 KT  50 MPH
120H  27/0600Z 25.2N  48.5W   50 KT  60 MPH

$$
Forecaster Stewart

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