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Tropical Storm Norma Discussion Number   2
NWS National Hurricane Center Miami FL       EP172017
300 PM MDT Thu Sep 14 2017

Norma has a broad and well-defined circulation with multiple
convective bands, especially to the east and south of the center.
Although the convective bands are a little broken in infrared
imagery, Dvorak intensity estimates have gone up to T3.5 from TAFB
and T2.5 from SAB and CIMSS at the University of Wisconsin.  The
initial intensity has therefore been raised to 40 kt.

The cyclone continues to move slowly northward with an initial
motion of 010/5 kt.  As mentioned in the previous discussion, a
blocking high to the north of Norma should impede its northward
motion for the next 48 hours, with the forward speed staying below
5 kt.  There is still no clarity on the forecast track after 48
hours, with the new 12Z ECMWF and UKMET models remaining on the
western side of the guidance envelope to the west of the Baja
California peninsula, while the remainder of the models generally
show a track over the southern part of the peninsula then turning
into northwestern Mexico.  Since the tracks of the GFS and HWRF
models lie close to the TVCN multi-model consensus and HCCA, the
NHC official forecast continues to favor this set of models.
Still, confidence in the forecast after 48 hours is quite low at
this time.

Since Norma already has a well-structured circulation, warm waters
and low shear should lead to a fairly fast increase in intensity
over the next couple of days.  The updated NHC intensity forecast
is a little higher than the previous one and is generally close to
SHIPS and the ICON intensity consensus.  An important note,
however, is that HCCA is higher than the NHC forecast, and the
rapid intensification indices, while not high, have increased from
6 hours ago.  These trends will be watched, and it is possible that
Norma could strengthen more than shown here.  Weakening is likely
to occur by days 4 and 5 due to land interaction with the Baja
California peninsula and increasing vertical shear.

FORECAST POSITIONS AND MAX WINDS

INIT  14/2100Z 17.7N 109.3W   40 KT  45 MPH
 12H  15/0600Z 18.1N 109.2W   50 KT  60 MPH
 24H  15/1800Z 18.7N 109.2W   65 KT  75 MPH
 36H  16/0600Z 19.2N 109.3W   75 KT  85 MPH
 48H  16/1800Z 19.8N 109.5W   80 KT  90 MPH
 72H  17/1800Z 21.9N 110.1W   75 KT  85 MPH
 96H  18/1800Z 24.5N 110.5W   60 KT  70 MPH
120H  19/1800Z 27.0N 109.5W   45 KT  50 MPH...INLAND

$$
Forecaster Berg

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