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Tropical Storm Norma Discussion Number   5
NWS National Hurricane Center Miami FL       EP172017
900 AM MDT Fri Sep 15 2017

Norma has an elongated convective band that curls about three
quarters of the way around the low-level center.  A well-defined
low-level ring was visible in recent 37-GHz SSMIS imagery,
indicating that the cyclone's structure continues to improve.  The
initial intensity is raised to 55 kt based on a blend of Dvorak
estimates which range from 50 kt (UW-CIMSS ADT) to 65 kt (TAFB).

Norma is essentially drifting toward the north-northwest with an
initial motion of 340/2 kt.  There's not a whole lot more clarity
on Norma's eventual track compared to yesterday.  The most that can
be said is that Norma will continue to drift northwestward or
northward for the next 24-36 hours.  After that time, there remains
considerable spread in the track guidance.  The ECMWF and UKMET
models, which maintain a stronger ridge over northwestern Mexico,
push Norma northwestward to the west of the Baja California
peninsula through day 5.  The GFS and HWRF models, as well as the
consensus aids, remain closer to the Baja California peninsula and
show a northeastward turn by day 5.  The NHC track forecast remains
close to the TVCN consensus and is a little slower than, but on top
of, the previous NHC forecast.

Low shear and warm ocean temperatures should foster continued
strengthening, with Norma forecast to become a hurricane by this
evening.  Norma is then expected to continue strengthening through
36 or 48 hours.  Some southwesterly shear could begin to affect the
cyclone by day 3 as it approaches stronger mid-latitude westerly
flow, and some weakening is forecast as Norma approaches the Baja
California peninsula.  The new NHC intensity forecast mirrors the
trend shown in the previous advisory, and is just a little higher
due to the updated initial intensity.  This forecast is generally
close to HCCA and the ICON intensity consensus.

A hurricane or tropical storm watch will likely be necessary for
portions of the southern Baja California peninsula later today.

FORECAST POSITIONS AND MAX WINDS

INIT  15/1500Z 18.7N 109.7W   55 KT  65 MPH
 12H  16/0000Z 18.9N 109.9W   65 KT  75 MPH
 24H  16/1200Z 19.2N 110.0W   70 KT  80 MPH
 36H  17/0000Z 19.9N 110.1W   75 KT  85 MPH
 48H  17/1200Z 20.8N 110.3W   75 KT  85 MPH
 72H  18/1200Z 23.0N 111.5W   65 KT  75 MPH
 96H  19/1200Z 24.5N 112.0W   50 KT  60 MPH
120H  20/1200Z 26.5N 110.5W   35 KT  40 MPH

$$
Forecaster Berg

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