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TROPICAL STORM IGNACIO DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP122015
200 PM PDT TUE AUG 25 2015

The organization of the cyclone's cloud pattern has increased
significantly since yesterday.  The center of circulation appears to
be near the eastern edge or barely underneath a small mass of deep
convection instead of being exposed to the east. Satellite pictures
also show increased banding, and low-cloud lines suggest a more
vigorous circulation. Dvorak intensity estimates of T2.5/35 kt from
TAFB and SAB reflect the increase in organization, and the initial
intensity is raised to 35 kt.

The initial motion estimate is 265/05.  A deep longwave trough over
the eastern Pacific has temporarily weakened the mid-level
subtropical ridge west of 130W, which has left the cyclone in a weak
easterly steering flow.  This synoptic pattern is expected to
persist for the next day or so, which should should allow the slow
westward motion to continue.  After 36 to 48 hours, the longwave
trough over the eastern Pacific should lift out and the subtropical
ridge is forecast to rebuild westward into the central Pacific and
cause Ignacio to move on a west-northwestward course. The track
guidance has shifted significantly to the left and has been
shifting toward the left during the past few cycles in response to
the better handling of the strength of the subtropical ridge.  The
new track forecast is adjusted significantly to the left but not as
far south as the multi-model consensus or the ECMWF and GFS ensemble
means.  The official track forecast places much less weight on the
ECMWF solution that spuriously splits the mid-level vortex in 3 days
time, which results in a northward jump in the track.

On the large scale, global models show decreasing shear and enhanced
lower to middle tropospheric moisture along the cyclone's path
during the next 2 to 3 days while it moves over anomalously warm
SSTs of 28 to 29 deg C.  These favorable factors should lead to
steady if not quick intensification, with the one limiting factor
likely to be how quickly the cyclone can establish an inner core.
The intensity forecast late in the period depends very much on
where Ignacio is, and the divergence in the track guidance makes
that intensity forecast after about 3 days of low confidence.  A
more southern track, as is now being shown in some of the guidance,
could result in an even stronger cyclone than indicated.  The
official intensity forecast is increased again over the previous
one and is close to, but below, the dynamical model guidance and the
FSU Superensemble output that has consistently been higher than the
statistical models.


FORECAST POSITIONS AND MAX WINDS

INIT  25/2100Z 13.0N 133.1W   35 KT  40 MPH
 12H  26/0600Z 12.9N 134.1W   45 KT  50 MPH
 24H  26/1800Z 12.9N 135.8W   55 KT  65 MPH
 36H  27/0600Z 13.2N 137.6W   65 KT  75 MPH
 48H  27/1800Z 13.9N 139.4W   75 KT  85 MPH
 72H  28/1800Z 15.4N 142.7W   85 KT 100 MPH
 96H  29/1800Z 16.7N 146.1W   75 KT  85 MPH
120H  30/1800Z 18.1N 149.7W   70 KT  80 MPH

$$
Forecaster Kimberlain

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