000
WTNT45 KNHC 181432
TCDAT5

TROPICAL DEPRESSION TEN DISCUSSION NUMBER   1
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102015
1100 AM AST FRI SEP 18 2015

Satellite data indicate that the area of low pressure over the
tropical eastern Atlantic has acquired sufficient organization to be
classified as a tropical depression.  The low-level center appears
embedded beneath a relatively small cluster of deep convection, but
convective banding is also increasing to the west and north.  The
initial intensity is set at 25 kt based on Dvorak estimates from
TAFB and SAB and recent ASCAT data.

The depression is located near the southwestern edge of a mid-level
anticyclone, giving the cyclone an initial motion of 285/8 kt.  The
anticyclone is expected to move westward in tandem with the
depression over the next couple of days, keeping it on a general
west-northwestward track for much of the forecast period.  By days 4
and 5, however, there is significant uncertainty in the track
forecast.  The GFS and GFDL show the cyclone feeling the influence
of an amplifying deep-layer trough over the eastern Atlantic, which
turns the depression northward.  The ECMWF and UKMET, on the other
hand, keep the ridge as the dominant influence and continue a
west-northwestward motion.  Due to the large spread in the guidance,
the official forecast shows the cyclone slowing down considerably by
days 4 and 5, and is a little west of the model consensus aids at
those times.

The environment looks conducive for at least gradual strengthening
during the forecast period.  Sea surface temperatures are warm,
vertical shear should remain generally low, and environmental
moisture patterns appear favorable for at least the next two days.
Beyond 48 hours, most of the global models show the cyclone
threading the needle beneath a narrow upper-level ridge, with
mid-level dry air possibly getting into the western part of the
cyclone's circulation.  This pattern makes the intensity forecast
complicated because hostile upper-level winds won't be too far away,
and the ultimate strength of the cyclone will depend heavily on its
eventual track.  For now, the NHC official forecast shows only
gradual strengthening through day 4, more or less in line with the
SHIPS and LGEM guidance.

FORECAST POSITIONS AND MAX WINDS

INIT  18/1500Z 13.1N  36.7W   25 KT  30 MPH
 12H  19/0000Z 13.4N  37.6W   30 KT  35 MPH
 24H  19/1200Z 14.2N  39.0W   30 KT  35 MPH
 36H  20/0000Z 15.2N  40.8W   35 KT  40 MPH
 48H  20/1200Z 16.4N  43.0W   40 KT  45 MPH
 72H  21/1200Z 18.3N  46.4W   45 KT  50 MPH
 96H  22/1200Z 19.5N  48.0W   50 KT  60 MPH
120H  23/1200Z 20.0N  49.0W   50 KT  60 MPH

$$
Forecaster Berg

000
WTNT45 KNHC 190837
TCDAT5

TROPICAL STORM IDA DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102015
500 AM AST SAT SEP 19 2015

The overall organization of Ida's cloud pattern continues to
increase.  Banding features are becoming more prominent in the
southern part of the circulation, with a new band forming on the
west side.  In addition, a burst of convection has been ongoing
during the past few hours near the apparent center.  Satellite
classifications still support an intensity of 35 kt, and this value
is used as the initial wind speed.

Satellite fixes suggest that the northwestward motion of Ida has
sped up to 9 kt this morning.  A mid-level ridge is expected to
remain the dominant steering mechanism for the next couple of days,
causing the storm to move to the west-northwest or northwest during
that time.  Models are in good agreement and only a slight
northward adjustment was made to the official forecast then.  Beyond
2 days, there is considerable uncertainty on how an amplifying
trough over the east-central Atlantic will affect Ida.  The cyclone
seems likely to slow down due to steering currents collapsing, but
there is poor agreement on whether the trough will pick up Ida,
sending the storm east-northeastward, or leave the storm behind. For
example, the normally reliable ECMWF and GFS models are going in
opposite directions at day 5, with the ECMWF favoring a stronger
trough solution while the GFS has Ida missing the trough and
drifting westward.  With so much uncertainty, I have elected to show
a stationary Ida after day 3, preferring not to commit to
either solution at this time. This forecast blends the previous
forecast with the model consensus, although leans more heavily on
the former.

While the shear near Ida is low for now, most of the global models
show an increase during the next day or so.  This is expected to
somewhat temper future strengthening, although since most of the
other environmental factors are still favorable, a gradual
intensification is forecast. The long-range intensity prediction,
however, is very complex due to the potential interaction of the
trough, which yields very different shear profiles, along with dry
air aloft and possible upwelling of cooler water as Ida moves
slowly.  With the track more uncertain than average and these
challenging factors, it seems prudent to remain conservative with
the wind speed forecast. The official intensity forecast is close to
the previous one, which ends up on the low side of the guidance at
days 4/5.  It almost goes without saying that these conditions are
leading me to have a rather low confidence in the intensity forecast
at 72 hours and beyond.



FORECAST POSITIONS AND MAX WINDS

INIT  19/0900Z 14.4N  38.5W   35 KT  40 MPH
 12H  19/1800Z 15.3N  39.9W   40 KT  45 MPH
 24H  20/0600Z 16.5N  42.2W   45 KT  50 MPH
 36H  20/1800Z 17.6N  44.0W   50 KT  60 MPH
 48H  21/0600Z 18.9N  45.6W   55 KT  65 MPH
 72H  22/0600Z 20.5N  47.5W   60 KT  70 MPH
 96H  23/0600Z 20.5N  47.5W   55 KT  65 MPH
120H  24/0600Z 20.5N  47.5W   55 KT  65 MPH

$$
Forecaster Blake