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TROPICAL STORM GUILLERMO DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP092015
200 PM PDT THU JUL 30 2015

Guillermo is quickly becoming better organized, and both microwave
and visible imagery suggest that the cyclone is developing a ring
of inner core convection.  Dvorak classifications at 1800 UTC were
T3.0/45 kt from both TAFB and SAB, but given the quickly improving
convective structure, the advisory intensity is set at 50 kt.  This
is close to the latest UW-CIMSS ADT estimate of T3.4/53 kt.

The upper-level outflow of Guillermo continues to expand, and the
storm is over very warm waters of around 29 degrees Celsius.  Global
model guidance indicates that Guillermo should remain in a
light-shear environment for at least another 3 days, while also
remaining over warm water and in a moisture-laden atmosphere.
Therefore, continued strengthening is likely for the next 48 hours,
and Guillermo could become a hurricane within the next 12-24 hours.
Gradual weakening is forecast from day 3 through 5 due to stronger
upper-level westerly winds which could affect the cyclone.  The
statistical-dynamical guidance continues to show only modest
strengthening--barely taking Guillermo to hurricane status--but
this scenario seems low given the seemingly favorable environment.
The NHC official intensity forecast remains closer to the higher
dynamical guidance and is largely unchanged from the previous
forecast.

The subtropical ridge is steering Guillermo west-northwestward, or
295/12 kt.  The ridge is forecast to strengthen slightly, causing
Guillermo to accelerate during the next 36 hours.  After that time,
a mid- to upper-level low well west of California is expected to
produce a break in the ridge, which should cause the cyclone to
slow down and turn northwestward by day 5.  The latest track
guidance ended up lying to the west of the previous official
forecast track, and the updated NHC track has therefore been
shifted a bit to the left, especially after 48 hours.

FORECAST POSITIONS AND MAX WINDS

INIT  30/2100Z  9.8N 128.4W   50 KT  60 MPH
 12H  31/0600Z 10.6N 130.6W   60 KT  70 MPH
 24H  31/1800Z 11.5N 133.7W   70 KT  80 MPH
 36H  01/0600Z 12.4N 136.8W   75 KT  85 MPH
 48H  01/1800Z 13.2N 139.4W   80 KT  90 MPH
 72H  02/1800Z 14.9N 143.3W   75 KT  85 MPH
 96H  03/1800Z 16.5N 146.0W   65 KT  75 MPH
120H  04/1800Z 18.0N 148.5W   55 KT  65 MPH

$$
Forecaster Berg

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