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Tropical Storm NORBERT Forecast Discussion

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WTPZ44 KNHC 030239

800 PM PDT TUE SEP 02 2014

Norbert has continued to slowly become better organized over the
past few hours. Deep convection has increase near the center in a
CDO-like feature, and a curved convective band extends into the
western semicircle. The initial intensity of 40 kt is based on a
blend of the latest subjective and objective Dvorak estimates, and
this could be a bit conservative. The SHIPS model shows some
moderate northeasterly to easterly shear over the cyclone for the
next 2 to 3 days, but otherwise conditions appear favorable for
strengthening. There is a fair bit of spread in the intensity
guidance, in part due to variability in the track forecast. The
HWRF model now shows the cyclone moving over the Baja peninsula, and
has trended a bit weaker. On the other hand, the SHIPS model now
shows Norbert becoming a hurricane in 36 to 48 hours. The new NHC
track takes Norbert over warmer SSTs closer to the Baja peninsula in
2 to 3 days and the NHC intensity forecast has been adjusted upward,
showing Norbert becoming a hurricane in 48 hours. This forecast is
close to the SHIPS model and the FSU Superensemble. Gradual
weakening is forecast at days 3 through 5 as the cyclone moves over
cooler waters.

Norbert has turned toward the left during the past few hours, with
an initial motion estimate of 330/07. The dominant steering
mechanism through the period will be a mid-level ridge centered over
the southern United States, which should generally steer Norbert
northwestward during the next 72 hours and then west-northwestward.
Much of the track model guidance has shifted well to the right, or
north, for this forecast cycle, especially at 48 hours and beyond.
The GFS, GEFS ensemble mean, HWRF, and GFDL are now north of the
ECMWF, which was previously along the right side of the guidance
envelope. This shift appears to be due to a deeper representation of
Norbert and more erosion of the subtropical ridge to the north.
The NHC track has been shifted to the right this cycle, and now
lies south of the multi-model consensus aid TVCE. The NHC track is
close to a blend of the GFS and ECMWF through 36 hours and is a
little south of that model blend afterward.

Given the large shift in the guidance and the large spread,
confidence in the track forecast is lower than normal and additional
adjustments may be needed in future forecast cycles. The rightward
shift in the track increases the threat to the southern part of the
Baja California peninsula. A tropical storm or hurricane warning
could be needed for this area tonight or early Wednesday.


INIT  03/0300Z 19.2N 106.9W   40 KT  45 MPH
 12H  03/1200Z 19.8N 107.8W   50 KT  60 MPH
 24H  04/0000Z 20.4N 108.9W   55 KT  65 MPH
 36H  04/1200Z 21.1N 109.8W   60 KT  70 MPH
 48H  05/0000Z 21.9N 110.7W   65 KT  75 MPH
 72H  06/0000Z 23.5N 112.8W   60 KT  70 MPH
 96H  07/0000Z 24.5N 115.0W   50 KT  60 MPH
120H  08/0000Z 25.3N 117.0W   40 KT  45 MPH

Forecaster Brennan

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Page last modified: Wednesday, 03-Sep-2014 02:39:51 UTC