000
WTPZ42 KNHC 150859
TCDEP2

Tropical Storm Norma Discussion Number   4
NWS National Hurricane Center Miami FL       EP172017
300 AM MDT Fri Sep 15 2017

The cloud pattern continues to be well organized and convection has
been gradually increasing, primarily in a band to the east of the
center. Dvorak T-numbers and an ASCAT pass over the cyclone indicate
that the initial intensity is 45 kt. With low shear prevailing and
Norma moving over warm waters, gradual strengthening is forecast.
Norma is expected to become a hurricane before it approaches the
Baja California peninsula.

Norma appears to be moving very slowly toward the north at about 2
kt.  Steering currents are weak and are provided by a weak ridge of
high pressure over Mexico.  This flow is not expected to change
much, so Norma should continue to move very slowly.  There was a
change in track guidance tonight, and most of the models shifted a
little bit westward, primarily beyond 3 days.  On this basis, the
NHC forecast was also adjusted westward.  The latter portion of the
forecast, however, is highly uncertain since the track models
diverge significantly. The NHC track forecast follows the HFIP
corrected consensus, HCCA, and the multi-model simple consensus as
well.

A Hurricane Watch will likely be necessary for portions of the
southern Baja California peninsula later today.


FORECAST POSITIONS AND MAX WINDS

INIT  15/0900Z 18.6N 109.7W   45 KT  50 MPH
 12H  15/1800Z 19.0N 109.8W   50 KT  60 MPH
 24H  16/0600Z 19.4N 110.0W   60 KT  70 MPH
 36H  16/1800Z 20.0N 110.1W   70 KT  80 MPH
 48H  17/0600Z 20.6N 110.3W   70 KT  80 MPH
 72H  18/0600Z 22.8N 111.4W   70 KT  80 MPH
 96H  19/0600Z 24.5N 112.0W   50 KT  60 MPH
120H  20/0600Z 26.3N 110.7W   35 KT  40 MPH

$$
Forecaster Avila