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TROPICAL STORM IDA DISCUSSION NUMBER   6
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102015
500 PM AST SAT SEP 19 2015

Ida's low-level center has been exposed northwest of the deep
convection all day due to about 20 kt of westerly shear, and the
initial intensity remains 35 kt based on Dvorak estimates from TAFB
and SAB.  Vertical shear is expected to be steady or increase
slightly during the next 12-24 hours, so little, if any,
strengthening is anticipated in the short term.  The shear could
then decrease between 36-72 hours as Ida moves beneath a narrow
upper-level ridge, but there is high uncertainty as to how the
upper-level environment will evolve during the next few days.  The
intensity models continue to disagree on the future intensity.
While the GFDL and HWRF bring Ida to hurricane strength in 2 to 3
days, the SHIPS and LGEM models have actually backed off from this
morning's runs and show Ida getting no stronger than about 45 kt
through day 5.  Since the upper-level environment only seems
marginally conducive for strengthening, the NHC intensity forecast
remains closer to the statistical models and lower than the
intensity consensus.

Ida has sped up a bit, and the initial motion is 300/12 kt.  Low-
to mid-level ridging will continue to steer the storm west-
northwestward for the next 24 hours.  After that time, the ridge is
expected to weaken, the steering currents will collapse, and Ida
will meander between days 3 through 5.  The spread in the track
models increases considerably beyond 48 hours, with the ECMWF, GFDL,
and HWRF showing a northeastward turn by day 5, while the GFS and
UKMET continue to show a westward motion.  Given the large spread
in the models, very little motion is indicated in the official
forecast at the end of the forecast period.  This scenario is the
same as that shown in previous forecasts.

FORECAST POSITIONS AND MAX WINDS

INIT  19/2100Z 15.6N  40.8W   35 KT  40 MPH
 12H  20/0600Z 16.4N  42.5W   35 KT  40 MPH
 24H  20/1800Z 17.6N  44.5W   40 KT  45 MPH
 36H  21/0600Z 18.9N  46.1W   45 KT  50 MPH
 48H  21/1800Z 19.9N  47.2W   50 KT  60 MPH
 72H  22/1800Z 20.5N  48.0W   55 KT  65 MPH
 96H  23/1800Z 20.0N  48.0W   55 KT  65 MPH
120H  24/1800Z 20.5N  47.5W   50 KT  60 MPH

$$
Forecaster Berg

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