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Tropical Depression Fifteen-E Discussion Number   7
NWS National Hurricane Center Miami FL       EP152017
200 AM PDT Wed Sep 13 2017

The overall convective pattern of the depression has changed little
since the previous advisory and remains sheared to the west of the
exposed surface circulation.  Displaced intermittent bursts of deep
convection with cloud tops colder than -70C yield an initial shear
pattern intensity estimate of 30 kt, and this is further supported
by a 0540 UTC ASCAT-B scatterometer pass which suggests that the
depression is just below tropical storm strength.  The Decay-SHIPS
intensity model and the large-scale deterministic guidance show the
shear gradually diminishing through the entire forecast period.
Accordingly, the official forecast reflects slow strengthening
through 3 days, then little change thereafter, which is just below
the HCCA, DSHP, and the better performing IVCN multi-model
consensus.

A 0201 UTC WindSat image and a 0247 UTC AMSU 89 GHZ composite
overpass were helpful in estimating the position and the initial
motion which is westward, or 270/8 kt.  There is no significant
change to the previous track forecast or reasoning.  A weakening
low- to mid-level subtropical ridge to the north should steer the
depression generally westward through 48 hours, but with a slight
reduction in speed with time.  Beyond 48 hours, the cyclone becomes
trapped in a much weaker steering flow induced by a mid-level ridge
between the depression, a developing system to the east, and a
building high from the northwest.  Subsequently, the official
forecast indicates very little motion, or a meander through day 5.
The NHC track forecast is adjusted to the right of the previous
package beyond 24 hours out of respect for the HFIP Corrected
Consensus technique and the ECMWF.

FORECAST POSITIONS AND MAX WINDS

INIT  13/0900Z 14.9N 119.4W   30 KT  35 MPH
 12H  13/1800Z 14.9N 120.5W   35 KT  40 MPH
 24H  14/0600Z 15.1N 122.0W   35 KT  40 MPH
 36H  14/1800Z 15.4N 123.3W   35 KT  40 MPH
 48H  15/0600Z 15.5N 124.2W   40 KT  45 MPH
 72H  16/0600Z 15.2N 125.2W   45 KT  50 MPH
 96H  17/0600Z 15.2N 125.9W   45 KT  50 MPH
120H  18/0600Z 15.5N 126.4W   45 KT  50 MPH

$$
Forecaster Roberts

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