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Tropical Storm Norma Discussion Number  11
NWS National Hurricane Center Miami FL       EP172017
900 PM MDT Sat Sep 16 2017

Norma's structure hasn't changed over the past 6 hours.  A sprawling
band of deep convection extends well out from the center, but the
tropical storm still appears to lack an inner core.  The intensity
has been held at 55 kt based on the Dvorak current intensity
estimate from TAFB, but that could be generous given that the
earlier reconnaissance flight struggled to find winds that high, and
the the Final-T number from TAFB was a little lower.

Norma has continued moving slowly north, but a turn toward the
north-northwest is expected overnight.  The global models agree for
the next 2 days that a deep-layer ridge extending westward over
central Mexico will keep Norma on a north-northwest or northwest
heading.  Beyond that time, there is considerable spread in the
guidance.  The ECMWF forecasts that the ridge will weaken
sufficiently to cause Norma to stall off the coast of the Baja
California Peninsula.  On the other hand, the GFS maintains the
ridge long enough to force Norma farther north, and into a fairly
dry and stable environment. The official forecast splits these
scenarios, and is based mostly on the TVCX consensus aid.

Recent total precipitable water imagery suggests that some dry air
may be wrapping into the circulation, and this will likely
contribute to further weakening.  The reliable intensity models all
indicate that steady weakening will occur throughout the forecast
period.  By day 5, SSTs below 26 C and very dry air should cause the
tropical storm to become a remnant low.  If Norma takes the faster
and farther north track of the GFS, it is likely that it will
become a remnant low sooner than indicated.

FORECAST POSITIONS AND MAX WINDS

INIT  17/0300Z 20.2N 110.1W   55 KT  65 MPH
 12H  17/1200Z 20.9N 110.4W   50 KT  60 MPH
 24H  18/0000Z 21.8N 110.8W   45 KT  50 MPH
 36H  18/1200Z 22.4N 111.5W   45 KT  50 MPH
 48H  19/0000Z 22.7N 112.1W   40 KT  45 MPH
 72H  20/0000Z 23.0N 112.7W   35 KT  40 MPH
 96H  21/0000Z 23.5N 113.2W   30 KT  35 MPH
120H  22/0000Z 24.5N 113.0W   20 KT  25 MPH...POST-TROP/REMNT LOW

$$
Forecaster Zelinsky

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