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Tropical Storm Walaka Discussion Number   3
NWS Central Pacific Hurricane Center Honolulu HI   CP012018
1100 PM HST Sat Sep 29 2018
 
The satellite presentation of Walaka has improved this evening,
with an increase in deep convection around the low level
circulation center. Several microwave passes were helpful in
determining the center location, but also indicated that the
tropical storm lacks good organization at the moment. The latest
satellite intensity estimates from PHFO, SAB and JTWC all came in at
2.5 (35 knots) with the ADT value from UW-CIMSS appearing
unrealistically high at 3.6 (57 knots). As a result, the initial
intensity for this advisory will be held at 35 knots. The initial
motion based on interpolating between microwave passes along with
geostationary satellite animations will be set at 270/14 knots.

Walaka is expected to track nearly due west along the south side of 
a subtropical ridge through Sunday night, with a gradual decrease
in forward speed. A deep north Pacific upper trough will erode the 
subtropical ridge to the north of Walaka on Monday and steer the 
tropical cyclone northwestward, with a turn toward the north and 
northeast along with an increase in forward speed forecast beyond
72 hours. The guidance is fairly tightly clustered through 48 hours 
with increasing model spread from forecast hour 72 onward. The 
official forecast closely follows the GFEX and TVCN consensus
models through 48 hours, and lies nearly on top of the track from
the previous advisory. Beyond 48 hours, the official forecast track
was nudged to the right to better align with the latest consensus 
guidance. Of note, the forecast track currently brings the center
of Walaka very near Johnston Atoll, late Monday night or early
Tuesday, and watches will likely be needed on Sunday.

The environmental conditions surrounding Walaka will remain very 
conducive for intensification over the next several days, with high 
sea surface temperatures of 84 to 86 Fahrenheit, 10 knots or less
of vertical wind shear, high ocean heat content, and plenty of deep 
moisture. As a result, the forecast calls for steady
intensification over the next 24 hours as the system becomes better
organized, with rapid intensification expected between forecast hour
24 and forecast hour 48, with Walaka expected to become a major
hurricane by Monday night. Wind shear should begin to impact the
system as it interacts with a deep upper level trough beyond 72
hours, and the forecast calls for steady weakening by forecast hours
96 and 120. The official intensity forecast is generally a blend
between the statistical and dynamical models, with the dynamical
models showing even a faster rate of intensification than the
official forecast. 
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  30/0900Z 11.6N 162.1W   35 KT  40 MPH
 12H  30/1800Z 11.8N 164.2W   50 KT  60 MPH
 24H  01/0600Z 12.1N 166.6W   65 KT  75 MPH
 36H  01/1800Z 12.8N 168.3W   90 KT 105 MPH
 48H  02/0600Z 14.0N 169.4W  110 KT 125 MPH
 72H  03/0600Z 17.9N 169.7W  110 KT 125 MPH
 96H  04/0600Z 24.5N 167.5W   95 KT 110 MPH
120H  05/0600Z 30.5N 167.5W   75 KT  85 MPH
 
$$
Forecaster Jelsema
 
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