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Hurricane Norma Discussion Number   7
NWS National Hurricane Center Miami FL       EP172017
900 PM MDT Fri Sep 15 2017

Infrared satellite imagery indicates that banding associated with
Norma has increased during the past few hours.  The convective cloud
tops have cooled and the band surrounding the center has become a
little more solid with the formation of a ragged banding eye. Dvorak
data T-numbers from both TAFB and SAB are 4.0, and with the recent
increase in organization the initial wind speed is increased to 65
kt. Norma becomes the eighth hurricane in the eastern Pacific this
season.

Norma should remain over warm water and within a low shear
environment during the next 24 to 36 h.  These conditions favor
strengthening, but the large size of Norma is likely to keep the
intensification rate in check.  Increasing vertical shear and cooler
waters are expected to impart gradual weakening after 48 hours.  The
latest NHC intensity forecast is similar to the previous advisory
and is close to the SHIPS guidance.

Norma has been moving slowly north-northwestward, with an initial
motion estimate of 335/2 kt.  The hurricane is forecast to move
slowly northward during the next couple of days, while it remains
along the western side of a mid-level ridge.  After that time, the
ECMWF develops a narrow ridge to the north of Norma which causes the
hurricane to turn northwestward.  Meanwhile, the GFS keeps Norma on
a north track over the Baja California peninsula. The trend of the
track guidance has been westward during the past several cycles, so
the NHC forecast lies slightly west of the consensus, and closer to
the ECMWF that has been more consistent over the past few runs.
Given the large spread in the guidance later in the period, the
confidence in the track forecast after 48 hours remains low.

The 48-hour forecast track and wind radii are very close to the
southern tip of the Baja California peninsula.  However, with the
persistent westward shifts in the guidance, a tropical storm watch
does not appear to be needed at this time.  We will assess future
model trends for the possibility of a watch issuance later tonight
or tomorrow.


FORECAST POSITIONS AND MAX WINDS

INIT  16/0300Z 19.0N 110.2W   65 KT  75 MPH
 12H  16/1200Z 19.3N 110.4W   70 KT  80 MPH
 24H  17/0000Z 20.0N 110.5W   75 KT  85 MPH
 36H  17/1200Z 21.0N 110.9W   75 KT  85 MPH
 48H  18/0000Z 22.0N 111.3W   70 KT  80 MPH
 72H  19/0000Z 23.7N 112.3W   60 KT  70 MPH
 96H  20/0000Z 24.6N 112.6W   45 KT  50 MPH
120H  21/0000Z 25.5N 111.5W   30 KT  35 MPH...INLAND

$$
Forecaster Brown

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