000
WTNT45 KNHC 230845
TCDAT5

TROPICAL STORM IDA DISCUSSION NUMBER  20
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102015
500 AM AST WED SEP 23 2015

Shortwave infrared satellite imagery and a 0413 UTC GCOM microwave
pass suggest that multiple low-level swirls are revolving around
Ida's center, which has been fixed to the west-northwest of an
ongoing burst of deep convection.  This pattern is the consequence
of 30 kt of west-northwesterly shear, as analyzed by UW-CIMSS.
Maximum winds remain 35 kt based on earlier ASCAT data, and it
appears that Ida's intensity probably won't change much during the
next couple of days.  Vertical shear is forecast to gradually
weaken, but it likely won't fall to less than 15 kt for at least
another 48 hours.  Some strengthening is possible beginning on day
3 once the shear has decreased.  With the exception of the GFDL,
the intensity models are in close agreement through day 4, with
some more spread by day 5.  Based on this latest guidance, no
changes were made to the previous NHC intensity forecast for this
advisory.

Although Ida's exact center is difficult to locate, the entire
circulation has seemingly been drifting southward since the previous
advisory.  This is probably for good reason, as Ida is embedded
within northerly flow near the base of a mid-tropospheric trough
which stretches across the eastern Atlantic.  The estimated initial
motion is 180/2 kt, but Ida will likely gain an eastward component
soon while being tugged by the eastern Atlantic trough.  The
subtropical ridge is expected build westward over the eastern
Atlantic by 48 hours, which should impart a slightly faster
northward motion through the end of the forecast period.  The
spread in the 00 UTC track guidance decreased on this forecast
cycle compared to earlier runs, with the UKMET model being the only
obvious outlier.  Given the tight clustering, the NHC official
track forecast was again adjusted westward, mainly on days 3-5,
and lies very close to the various multi-model consensus aids.

FORECAST POSITIONS AND MAX WINDS

INIT  23/0900Z 20.4N  47.2W   35 KT  40 MPH
 12H  23/1800Z 20.4N  46.3W   35 KT  40 MPH
 24H  24/0600Z 20.5N  45.8W   35 KT  40 MPH
 36H  24/1800Z 21.0N  45.7W   35 KT  40 MPH
 48H  25/0600Z 21.7N  45.9W   35 KT  40 MPH
 72H  26/0600Z 23.0N  47.0W   40 KT  45 MPH
 96H  27/0600Z 25.0N  47.5W   45 KT  50 MPH
120H  28/0600Z 26.0N  47.0W   50 KT  60 MPH

$$
Forecaster Berg