ZCZC MIATCDAT5 ALL
TTAA00 KNHC DDHHMM

TROPICAL STORM IDA DISCUSSION NUMBER  11
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102015
1100 PM AST SUN SEP 20 2015

Ida is showing new signs of life.  A new burst of intense convection
has developed over and east of the center, suggesting that westerly
shear over the cyclone could be decreasing.  Although Ida's cloud
pattern has recently become better organized, an ASCAT pass from
2358 UTC indicates that there has been no increase in winds, with
peak uncontaminated winds of 37 kt.  Thus the initial wind speed
is held at 40 kt.

A large upper-level low northeast of the Lesser Antilles is moving
away from Ida, resulting in a gradual relaxation of the westerly
shear over the cyclone while the surrounding flow gradually becomes
more diffluent.  All other factors being equal, this change in the
upper-air pattern should favor some intensification during the
next 24 hours or so, perhaps more than forecast.  After 36 hours, a
mid- to upper-level trough amplifying southwestward from the eastern
Atlantic should induce an inhospitable environment of strong
northwesterly shear, drier mid-tropospheric air and confluence over
Ida.  The storm's slow movement by that time could also cause
enhanced oceanic upwelling and locally cooler waters underneath the
cyclone.  It is hard to imagine that these conditions would not
induce weakening, yet global models show Ida re-strengthening from
days 3 to 5 likely, probably due to a baroclinic forcing arising
from the storm's interaction with the trough.  The new intensity
forecast shows more intensification in the short term, and is above
all of the intensity guidance.  Given the larger than normal
uncertainty in the extended range, the intensity is essentially
flatlined not far from the multi-model consensus.

Recent fixes suggest that Ida's forward speed is decreasing, and the
initial motion estimate is 310/12.  Ida is quickly approaching a
weakness in the low- to mid-level subtropical ridge along 50W,
which should result in the cyclone's forward speed decelerating
during the next 12 to 24 hours.  Steering currents should collapse
in about 36 hours in response to the eastern Atlantic trough
deepening southwestward, and Ida should suddenly come to a halt and
then meander with some eastward component of motion for a few days.
As the effects of the trough diminish by day 4, a majority of the
track guidance shows Ida moving faster toward the north or north-
northwest between two mid-level ridges.  The GFS is left of nearly
all the guidance, however, showing Ida encountering a blocking ridge
that would cause a more westerly track.  The details of the track
forecast between days 3 to 5 are still in greater than normal doubt,
and thus the track is hardly adjusted through 72 hours, but is
shifted toward the north and east after that time toward the
multi-model consensus without the GFS.


FORECAST POSITIONS AND MAX WINDS

INIT  21/0300Z 18.9N  47.0W   40 KT  45 MPH
 12H  21/1200Z 20.2N  48.0W   45 KT  50 MPH
 24H  22/0000Z 21.0N  48.4W   50 KT  60 MPH
 36H  22/1200Z 21.4N  48.3W   50 KT  60 MPH
 48H  23/0000Z 21.2N  48.0W   50 KT  60 MPH
 72H  24/0000Z 20.6N  47.5W   45 KT  50 MPH
 96H  25/0000Z 21.3N  47.4W   45 KT  50 MPH
120H  26/0000Z 23.2N  48.6W   50 KT  60 MPH

$$
Forecaster Kimberlain

NNNN