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TROPICAL STORM IDA DISCUSSION NUMBER  24
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102015
500 AM AST THU SEP 24 2015

Ida's low-level center is exposed to the west of a relatively small
cluster of deep convection due to 25 kt of west-northwesterly shear.
By themselves, satellite intensity estimates would no longer
support Ida's status as a tropical storm, but the partial ASCAT
passes from last evening had just enough wind that I'm hesitant to
downgrade the system to a tropical depression just yet.  Vertical
shear is expected to decrease only slightly during the next couple
of days, and along with abundant mid-level dry air, the environment
will not be particularly favorable for strengthening.  The GFDL,
which brings Ida to hurricane strength in 4 days, remains an outlier
and is discounted as a reasonable solution.  Conversely, the ECMWF
and GFS global models show Ida weakening as a result of the
unfavorable environment, with the GFS even showing the cyclone
degenerating to a trough by day 4 or 5.  Because of these more
believable scenarios, the NHC intensity forecast is a little lower
than the previous one at the end of the forecast period, and
overall is very close to the Florida State Superensemble.

Ida had been moving east-southeastward during the past 12 hours,
but the current motion estimate is 090/5 kt.  Ida remains located
within the base of a mid-tropospheric trough, but it should become
detached from the trough during the next 24 hours and turn
northward as low- to mid-level ridging develops over the eastern
Atlantic.  Now that the global models have been trending toward a
weaker system by days 3 through 5, they show Ida becoming blocked
by a surface high over the north Atlantic and moving westward at a
faster forward speed by the end of the forecast period.  The
updated NHC track forecast is a little faster than and southwest of
the previous forecast at days 4 and 5 to account for the model
trends, but it is otherwise very similar before those times.

FORECAST POSITIONS AND MAX WINDS

INIT  24/0900Z 19.6N  45.5W   35 KT  40 MPH
 12H  24/1800Z 20.2N  45.4W   35 KT  40 MPH
 24H  25/0600Z 20.8N  45.4W   35 KT  40 MPH
 36H  25/1800Z 21.6N  45.8W   35 KT  40 MPH
 48H  26/0600Z 22.5N  46.5W   35 KT  40 MPH
 72H  27/0600Z 23.8N  48.2W   40 KT  45 MPH
 96H  28/0600Z 23.5N  50.0W   40 KT  45 MPH
120H  29/0600Z 23.0N  52.5W   40 KT  45 MPH

$$
Forecaster Berg

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