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TROPICAL STORM HILDA DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP102015
200 PM PDT THU AUG 06 2015

Hilda's cloud pattern has rapidly increased in organization during
the past 12 hours. Conventional satellite imagery reveals a tiny
tropical cyclone with a nearly symmetric dense overcast and a
prominent band over the northern half of the circulation.  A 1503
UTC Windsat pass showed a closed low-level ring of convection,
suggesting that the inner core of the tropical cyclone is already
well established. The initial intensity estimate is raised to 50 kt
in best agreement with an earlier AMSU pass.

The initial motion has been due west or 270/11.  Nothing has
changed regarding the forecast philosophy in the previous advisory.
Hilda should continue moving westward and then turn west-
northwestward in about 36 hours as it nears the western periphery of
a subtropical ridge to its north.  The cyclone should begin to
respond to a weakness in the ridge located to the east of the
Hawaiian Islands and turn northwestward at a significantly reduced
forward speed by 96 hours.  The official track forecast has been
shifted significantly to the left during the first 48 hours but
still lies on the right side of the guidance envelope.  The track
forecast after 48 hours is also left of the previous forecast but
is closer to the multi-model consensus late in the forecast period.

There are no obvious obstacles to additional intensification in the
short term, except for somewhat drier and more stable air mass to
the north and west of the cyclone.  Given Hilda's small size, it is
assumed that the entrainment of this air is not likely to be an
inhibitor.  The SHIPS model output indicates less conducive
thermodynamic variables in 2 to 3 days which should halt any further
intensification.  Once the cyclone gains enough latitude in the
central Pacific after 72 hours, increasing southwesterly shear
associated with a mid- to upper-level trough near the longitude of
Hawaii should result in weakening.  In fact, the shear could be
strong enough late in the period to cause a decoupling of the
cyclone as depicted in global model fields.  The new intensity
forecast is boosted much higher in the short term to account for the
current strengthening trend and is closest to the LGEM.  The
intensity forecast is near or below the multi-model consensus after
72 hours to emphasize the weakening expected at that time.


FORECAST POSITIONS AND MAX WINDS

INIT  06/2100Z 12.7N 133.4W   50 KT  60 MPH
 12H  07/0600Z 12.6N 135.1W   55 KT  65 MPH
 24H  07/1800Z 12.7N 137.4W   65 KT  75 MPH
 36H  08/0600Z 13.1N 139.5W   70 KT  80 MPH
 48H  08/1800Z 13.7N 141.6W   75 KT  85 MPH
 72H  09/1800Z 15.5N 145.5W   70 KT  80 MPH
 96H  10/1800Z 17.2N 147.9W   60 KT  70 MPH
120H  11/1800Z 18.5N 150.0W   45 KT  50 MPH

$$
Forecaster Kimberlain

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