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Tropical Depression EIGHTEEN-E Forecast Discussion

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WTPZ43 KNHC 252032

200 PM PDT SUN SEP 25 2016

The overall organization of the depression has not changed much
since this morning.  The center is exposed to the north of the main
area of thunderstorm activity, with some banding noted over the
northeastern quadrant.  Visible imagery shows that there are
several smaller swirls rotating around the mean center.  Dvorak
intensity estimates are T2.5/35 kt from TAFB and T1.5/25 kt from
SAB, so the initial intensity remains 30 kt for this advisory.

The depression is forecast to remain over warm water and to be in a
low-shear environment during the next 12 to 24 hours, but the large
and sprawling nature of the system suggests that any intensification
should be slow to occur.  By late Monday, the cyclone will be
moving into an area of increasing southwesterly shear, and by
Tuesday the system will be moving over cooler waters and into
a more stable air mass.  These conditions should result in weakening
within 48 hours, and the system is expected to become a remnant low
in about 3 days.  The NHC intensity forecast is unchanged from the
previous advisory and is once again in best agreement with the IVCN
consensus model.

The depression is moving northward or 360/6 kt.  The track forecast
reasoning is unchanged from the previous advisory.  The cyclone
should move north-northeastward around the western portion of a
weak ridge that extends southwestward from southern Mexico.
Although most of the track models agree with this scenario, the
GFS and GFDL take a stronger and deeper cyclone more northeastward,
and northward around the aforementioned upper-level low later in the
period.  The NHC track favors the ECMWF and GFS ensemble mean
solutions, which show a weaker and more shallow system turning
northwestward well west of the Baja California peninsula. The new
NHC forecast is essentially an update of the previous advisory.


INIT  25/2100Z 16.1N 119.6W   30 KT  35 MPH
 12H  26/0600Z 16.8N 119.4W   35 KT  40 MPH
 24H  26/1800Z 17.4N 119.1W   40 KT  45 MPH
 36H  27/0600Z 18.2N 118.6W   35 KT  40 MPH
 48H  27/1800Z 19.1N 118.0W   30 KT  35 MPH
 72H  28/1800Z 21.5N 117.5W   25 KT  30 MPH...POST-TROP/REMNT LOW
 96H  29/1800Z 24.0N 118.5W   20 KT  25 MPH...POST-TROP/REMNT LOW
120H  30/1800Z 25.0N 120.0W   20 KT  25 MPH...POST-TROP/REMNT LOW

Forecaster Brown