000
WTNT45 KNHC 230252
TCDAT5

TROPICAL STORM IDA DISCUSSION NUMBER  19
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102015
1100 PM AST TUE SEP 22 2015

Last-light visible imagery, microwave imagery, and a timely ASCAT
pass around 00Z showed that the center of Ida is located on the
northeastern side of the larger gyre seen earlier today, just west
of a new burst of deep convection. This structure is consistent with
the 30 kt of westerly shear seen in UW-CIMSS satellite analyses. The
initial intensity is set to 35 kt based on data from the ASCAT pass
and in agreement with the latest Dvorak classification from TAFB.
Little change in intensity is expected during the next 48 hours,
with moderate to strong shear forecast to continue. By 72 hours the
shear decreases a bit, which should allow for a little
strengthening. The new NHC intensity forecast is close to a blend of
the latest SHIPS, LGEM, and HWRF models.

The initial position results in a relocation of the center to the
northeast of the previous advisory with a slow east-southeastward
drift as the initial motion. Ida is currently embedded in the base
of a mid-latitude trough which should result in a continued slow
east-southeastward motion during the next day or so followed by a
gradual turn toward the northeast and then the north by 48 hours as
the trough moves away. A slightly faster north-northwestward motion
is forecast at days 3 through 5 as a mid-level ridge builds to the
north of the tropical cyclone. During the latter portion of the
forecast period much of the guidance has shifted westward, now
showing a weaker Ida remaining south of the mid-latitude westerlies
through day 5. The NHC track during this time has been nudged to
the left of the previous one adjusted for the initial position.
However, out of respect for continuity the NHC forecast remains
close to the ECMWF on the eastern side of the guidance envelope and
is well right of the consensus aids. Given the large shift in the
guidance this cycle, confidence is quite low in the track forecast
late in the period.

FORECAST POSITIONS AND MAX WINDS

INIT  23/0300Z 20.9N  46.8W   35 KT  40 MPH
 12H  23/1200Z 20.7N  46.4W   35 KT  40 MPH
 24H  24/0000Z 20.5N  45.8W   35 KT  40 MPH
 36H  24/1200Z 21.0N  45.3W   35 KT  40 MPH
 48H  25/0000Z 21.6N  45.1W   35 KT  40 MPH
 72H  26/0000Z 23.0N  45.5W   40 KT  45 MPH
 96H  27/0000Z 25.0N  46.5W   45 KT  50 MPH
120H  28/0000Z 26.0N  47.0W   50 KT  60 MPH

$$
Forecaster Brennan