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Tropical Storm Norma Discussion Number  12
NWS National Hurricane Center Miami FL       EP172017
300 AM MDT Sun Sep 17 2017

A pair of ASCAT passes between 0400 and 0500 UTC showed that Norma
is weaker than previously estimated.  The maximum winds in both
passes were around 40 kt, so the initial intensity has been
conservatively lowered to 45 kt.  The new NHC intensity forecast is
a little lower than the previous one because of the lower initial
intensity, but otherwise the reasoning behind the forecast is
unchanged.  Norma is already embedded within a fairly dry
environment, and will be moving over progressively cooler SSTs over
the coming days.  Steady weakening is likely, and Norma is now
forecast to become a remnant low by 96 h.

There has been a significant shift in the track guidance.  The GFS
and various GFS-derived models have come into better agreement with
the ECMWF, and now depict a track farther away from the Baja
California peninsula.  Beyond day 3, there is still a fair amount of
disagreement on the extent to which a ridge to the east will turn
the tropical storm or its remnants back toward the north.  The ECMWF
continues to insist that the cyclone will come to a near halt, while
the GFS shows the remnant low continuing well to the north.  The new
NHC forecast remains close to the various consensus aids, but hedges
toward the ECMWF.

FORECAST POSITIONS AND MAX WINDS

INIT  17/0900Z 20.5N 110.4W   45 KT  50 MPH
 12H  17/1800Z 21.1N 110.7W   40 KT  45 MPH
 24H  18/0600Z 21.7N 111.3W   40 KT  45 MPH
 36H  18/1800Z 22.1N 112.1W   35 KT  40 MPH
 48H  19/0600Z 22.4N 113.0W   35 KT  40 MPH
 72H  20/0600Z 22.9N 114.3W   30 KT  35 MPH
 96H  21/0600Z 23.5N 114.5W   25 KT  30 MPH...POST-TROP/REMNT LOW
120H  22/0600Z 24.0N 114.0W   25 KT  30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Zelinsky

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