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Tropical Storm Norma Discussion Number   6
NWS National Hurricane Center Miami FL       EP172017
300 PM MDT Fri Sep 15 2017

Norma's primary convective band continues to elongate and curls
about one and a half times around the circulation.  Objective and
subjective Dvorak estimates are wide ranging, going from 50 kt up
to 77 kt, but since the deep convective cloud tops are not very
cold, the initial intensity is raised conservatively to 60 kt.
This lower intensity is supported by recent ASCAT data, which only
showed maximum winds of 50 kt.

The environment appears conducive for continued gradual
strengthening.  Norma's large size argues against rapid
intensification, but the cyclone should be able to at least
steadily strengthen during the next 36 hours or so.  Vertical
shear begins to increase after that time, however, and Norma is
likely to weaken beginning in about 48 hours.  The HCCA guidance and
the ICON intensity consensus were notably lower on this cycle, and
the updated NHC forecast has begun to follow that trend, especially
on days 3 through 5.

The center has wobbled northwestward during the day, and the
initial motion estimate is 325/2 kt.  Norma should turn slowly
northward during the next day or so, steered along the western
periphery of the subtropical ridge.  The track models continue to
play the same tune, with the ECMWF and UKMET models on the far
western side of the guidance envelope and the HWRF and GFS on the
eastern side.  Since the overall trend in the guidance has been
westward, the new NHC track forecast leans left of the consensus
aids and is a little left of the previous forecast.

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  15/2100Z 18.9N 110.1W   60 KT  70 MPH
 12H  16/0600Z 19.2N 110.3W   65 KT  75 MPH
 24H  16/1800Z 19.6N 110.4W   70 KT  80 MPH
 36H  17/0600Z 20.6N 110.6W   75 KT  85 MPH
 48H  17/1800Z 21.6N 111.0W   70 KT  80 MPH
 72H  18/1800Z 23.5N 112.1W   60 KT  70 MPH
 96H  19/1800Z 25.0N 112.0W   45 KT  50 MPH...INLAND
120H  20/1800Z 26.0N 111.0W   30 KT  35 MPH...OVER WATER

$$
Forecaster Berg

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