Tropical Cyclone Discussion
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WTPZ44 KNHC 010258

800 PM PDT FRI JUL 31 2015

Both microwave and conventional satellite imagery indicate that
Guillermo has not become any better organized during the past
several hours. In fact, the satellite presentation appears to have
deteriorated since earlier today with microwave data showing the
northwestern portion of the eyewall eroding.  Satellite intensity
estimates from TAFB and SAB and the latest objective intensity
estimate from UW-CIMSS ADT suggest that initial intensity remains 90

Guillermo continues to move rapidly to the west-northwest, with an
initial motion estimate of 285/18 kt. Little change to this motion
is expected for the next 24 hours, with the cyclone remaining to the
south of a mid-level subtropical ridge. Beyond 24 hours to around
72 hours, a deepening mid- to upper-level trough over the central
Pacific is forecast to create a break in the subtropical ridge,
which should cause the tropical cyclone to slow down and turn more
toward the northwest. The spread in the guidance suite increases
beyond 72 hours, but most of the models maintain a general
west-northwestward motion.  The new forecast track is basically an
update of the previous track through 72 hours, and is then nudged
slightly to the right or north of the previous track toward the
multi-model consensus thereafter.

The intensity forecast has become somewhat problematic. The reason
for the recent arrested development is unclear, although it might be
related to dry air entrainment into the northwestern quadrant of the
cyclone. However, there is still the opportunity for Guillermo to
strengthen a bit more during the next 12 to 24 hours, while the
system remains within a low-shear environment and over warm sea
surface temperatures. The new intensity forecast is adjusted
slightly downward to 95 kt at the 12- and 24-hour forecast times.
Thereafter, the cyclone is forecast to move into an upper-level
deformation zone between subtropical ridges to the east and west,
which is a less favorable environment.  Also, sea surface
temperatures gradually cool along the forecast track. We have
followed the previous forecast trend of gradual weakening, which is
in good agreement with the latest SHIPS/LGEM and IVCN consensus
guidance. When Guillermo approaches the Hawaiian Islands, it is
expected to encounter increasing westerly shear, and the new
intensity forecast follows the previous forecast in weakening
Guillermo to tropical storm strength.


INIT  01/0300Z 13.2N 136.1W   90 KT 105 MPH
 12H  01/1200Z 13.7N 138.5W   95 KT 110 MPH
 24H  02/0000Z 14.3N 141.1W   95 KT 110 MPH
 36H  02/1200Z 15.0N 143.2W   90 KT 105 MPH
 48H  03/0000Z 15.8N 145.0W   80 KT  90 MPH
 72H  04/0000Z 17.0N 147.5W   65 KT  75 MPH
 96H  05/0000Z 19.0N 150.5W   60 KT  70 MPH
120H  06/0000Z 20.5N 153.5W   50 KT  60 MPH

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