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TROPICAL STORM NORA DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP182015
200 AM PDT SAT OCT 10 2015

Nora has changed little in organization during the past several
hours.  The low-level center continues to be near the southeastern
edge of the convection, and the strongest convection is currently
in outer bands well northwest of the center. This is likely due to
the effects of light southeasterly vertical wind shear.  In
addition, recent ASCAT data suggests the circulation is elongated
north to south.  Satellite intensity estimates are 35 kt from both
TAFB and SAB, and the ASCAT data also shows winds near 35 kt.  Based
on this, the advisory intensity remains 35 kt.

Nora is now moving 285/14, with the cyclone continuing to be steered
by a deep-layer subtropical ridge to the north.  The dynamical
models forecast the ridge to weaken during the next 36 hours, with
Nora moving generally westward with a decrease in forward speed.
After that time, the tropical cyclone is likely to turn northward
and probably recurve as a deep-layer trough over the north Pacific
breaks the subtropical ridge.  The track guidance is tightly
clustered for the first 36 hours, but subsequently there is an
increased spread on where and how fast Nora will recurve. The ECMWF
shows a weaker Nora moving farther west and eventually turning
northward along 149W by 120 hours.  The UKMET shows a stronger Nora
making an earlier recurvature with the storm near 19N 138W by 120
hours.  The GFS lies between these models and is near the center of
the guidance envelope.  The new track forecast is a little north of
the previous forecast for the first 48 hours, and then it is a
little west of the previous forecast after that time. The new track
is near the center of the guidance envelope and lies a little west
of the GFS after 72 hours.

Nora is expected to remain in an environment of light shear and
warm sea surface temperatures for the next 2-3 days, which should
allow strengthening.  After that, the cyclone is expected to
encounter unfavorable conditions of strong southwesterly shear and
decreasing sea surface temperatures.  Based on this, the new
intensity forecast follows the trend of the previous forecast in
calling to Nora to peak as a hurricane in about 72 hours and
subsequently weaken to a tropical storm by 120 hours.  The new
forecast is in best agreement with the Florida State Superensemble.


FORECAST POSITIONS AND MAX WINDS

INIT  10/0900Z 11.6N 136.5W   35 KT  40 MPH
 12H  10/1800Z 11.9N 138.3W   45 KT  50 MPH
 24H  11/0600Z 12.1N 140.5W   50 KT  60 MPH
 36H  11/1800Z 12.4N 142.3W   60 KT  70 MPH
 48H  12/0600Z 12.7N 143.7W   65 KT  75 MPH
 72H  13/0600Z 13.5N 145.5W   75 KT  85 MPH
 96H  14/0600Z 16.0N 145.0W   70 KT  80 MPH
120H  15/0600Z 18.5N 142.5W   60 KT  70 MPH

$$
Forecaster Beven

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