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Tropical Storm Norma Discussion Number   3
NWS National Hurricane Center Miami FL       EP172017
900 PM MDT Thu Sep 14 2017

The large circulation of Norma was well-defined in late afternoon
visible satellite images, however, the colder convective cloud tops
remain fragmented in infrared satellite imagery.  Since the
convective banding has not significantly improved this evening, the
Dvorak data T-numbers have remained about the same as this
afternoon, and the initial intensity of 40 kt is maintained for
this advisory.

Recent satellite fixes show that Norma is moving slowly northward
or 360/5 kt.  A large high pressure area over northern Mexico and
an associated ridge that extends southwestward across the Baja
California peninsula should keep Norma on a slow northward track
during the next couple of days.  After that time, the model spread
significantly increases with the ECMWF and UKMET models turning
Norma west-northwestward, taking it west or southwest of Baja
California.  The GFS and HWRF weaken the ridge and move Norma
northward across Baja California Sur ahead of a mid-latitude trough
that deepens southwest of southern California.  The NHC forecast
continues to follow the latter scenario since it is favored by the
HFIP corrected consensus model and is also fairly close to the
TVCN consensus aid.  The updated track forecast is similar to the
previous advisory, but the confidence in the track forecast remains
quite low due to the usually large model disagreement.

Norma is forecast to remain within a favorable environment of low
vertical wind shear and over warm water, however, the large and
sprawling structure of the tropical cyclone may temper the
intensification rate in the short-term.  As a result, the updated
NHC intensity forecast is slightly lower than the previous advisory
through 72 hours, and it is between the ICON intensity consensus
and the slightly more aggressive HCCA consensus. Land interaction
and increasing southwesterly shear is likely to lead to weakening
later in the period.


FORECAST POSITIONS AND MAX WINDS

INIT  15/0300Z 18.3N 109.3W   40 KT  45 MPH
 12H  15/1200Z 18.7N 109.3W   50 KT  60 MPH
 24H  16/0000Z 19.2N 109.3W   60 KT  70 MPH
 36H  16/1200Z 19.7N 109.4W   70 KT  80 MPH
 48H  17/0000Z 20.6N 109.6W   75 KT  85 MPH
 72H  18/0000Z 22.7N 110.3W   70 KT  80 MPH
 96H  19/0000Z 25.1N 110.6W   55 KT  65 MPH
120H  20/0000Z 27.5N 109.0W   35 KT  40 MPH...INLAND

$$
Forecaster Brown

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