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TROPICAL STORM IDA DISCUSSION NUMBER   8
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102015
500 AM AST SUN SEP 20 2015

Ida remains a sheared tropical cyclone, but there are some
indications that the mid-level and deep-layer shear is beginning to
abate somewhat. A burst of deep convection has developed during the
past few hours near the center and in the eastern quadrant, which
so.me cloud tops near -80C. The initial intensity is being held at
35 kt based on consensus Dvorak current-intensity estimates of
T2.5/35 kt from TAFB and SAB, but this could be a little
conservative based on earlier partial ASCAT overpass that contained
slightly higher wind speeds in the eastern quadrant.

The initial motion estimate is 295/13 kt. Ida is expected to move
west-northwestward and gradually slow down over the next several
days, possibly even stalling by days 4 and 5. Steering currents are
forecast to weaken considerably after 72 hours as a strong mid- to
upper-level trough digs southeastward across the central subtropical
Atlantic and significantly erodes the ridge to the north of the
cyclone. The global models are in good agreement on the developing
synoptic flow scenario through day 3, but then diverge significantly
after that with the ECMWF, UKMET, and GFDL models taking a deeper
and stronger tropical cyclone more northward, whereas the GFS and
HWRF models take a weaker and more shallow system westward. Given
the uncertainty in the expected strength and vertical depth of Ida
on days 3-5, the official forecast track lies down the middle of the
guidance envelope, and is close to a blend of the consensus models
TVCA and GFEX.

The moderate to strong westerly vertical wind shear that Ida has
been experiencing the past day or so is forecast by the GFS and
ECMWF models to gradually subside and become less than 10 kt by 24
hours through 72 hours.  With such a robust low-level circulation
and the mid-level humidity values expected to be near 70 percent,
convection should steadily increase and become better organized,
producing at least some slow intensification through the next 48
hours. The leveling off of the intensity on days 3-5 is due to the
uncertainty on whether or Ida moves beneath a small upper-level col
region with weak winds as indicated by the ECMWF model, which would
support more strengthening than what the official forecast is
indicating. The NHC intensity forecast lies essentially midway
between the two intensity extremes of the weaker GFS-HRWF and the
stronger ECMWF-GFDL model solutions, and is supported by the
intensity consensus model IVCN.

FORECAST POSITIONS AND MAX WINDS

INIT  20/0900Z 16.7N  43.6W   35 KT  40 MPH
 12H  20/1800Z 17.5N  45.2W   35 KT  40 MPH
 24H  21/0600Z 18.8N  47.0W   40 KT  45 MPH
 36H  21/1800Z 19.8N  48.1W   45 KT  50 MPH
 48H  22/0600Z 20.5N  48.6W   50 KT  60 MPH
 72H  23/0600Z 20.6N  48.7W   50 KT  60 MPH
 96H  24/0600Z 20.7N  48.7W   50 KT  60 MPH
120H  25/0600Z 21.0N  48.7W   50 KT  60 MPH

$$
Forecaster Stewart

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