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Tropical Storm Walaka Discussion Number   4
NWS Central Pacific Hurricane Center Honolulu HI   CP012018
500 AM HST Sun Sep 30 2018
 
The satellite presentation of Walaka has improved significantly 
overnight, with a large area of deep convection featuring 
-80 to -90C cloud tops over the difficult to locate low level 
circulation center. Due to the lack of any recent microwave imagery 
to assist in confidently determining the center location, the 
initial position was based on extrapolation from earlier microwave 
passes and current geostationary satellite animations. The latest 
satellite intensity estimates from PHFO and SAB were 3.0 (45
knots), and 2.5 (35 knots) from JTWC. Meanwhile, the ADT value from
UW-CIMSS continues to appear unrealistically high at 4.1 (67 knots).
Given the improvement in organization and appearance of Walaka since
the 12Z satellite intensity estimates, the initial intensity for
this advisory has been increased to 50 knots. The initial motion has
been set at 275/14 knots.

Walaka is expected to track nearly due west along the south side of 
a subtropical ridge today and tonight, 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 
expected on Tuesday along with an increase in forward speed. The 
system should then slow down and be pulled back to the north-
northwest on Thursday as it begins to interact with the upper 
trough. The guidance is fairly tightly clustered through entire 
forecast cycle, with a bit more spread noted by forecast hour 120. 
The official forecast closely follows the GFEX, TVCN, and HCCA 
consensus guidance, and is very closely aligned with the forecast 
track from the previous advisory. With the official forecast track 
bringing the center of Walaka very near Johnston Atoll on Tuesday,
a Hurricane Watch has been issued for this location.

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 rapid intensification 
today through Monday night, with Walaka expected to become a major 
hurricane on Monday. The intensity should level off a bit Tuesday 
into Tuesday night following the period of rapid intensification
and accounting for the potential for eyewall replacement cycles. 
Vertical wind shear should then begin to impact the tropical
cyclone beginning late Tuesday night or Wednesday as it interacts
with a deep upper level trough, and as a result, the intensity
forecast calls for steady weakening by forecast hours 96 and 120.
The intensity forecast has been increased slightly from the
previous advisory and is on the high end of most of the intensity
guidance.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  30/1500Z 11.7N 163.9W   50 KT  60 MPH
 12H  01/0000Z 11.9N 165.6W   65 KT  75 MPH
 24H  01/1200Z 12.3N 167.5W   85 KT 100 MPH
 36H  02/0000Z 13.3N 168.9W  110 KT 125 MPH
 48H  02/1200Z 14.9N 169.8W  120 KT 140 MPH
 72H  03/1200Z 19.8N 169.2W  120 KT 140 MPH
 96H  04/1200Z 27.0N 167.5W   90 KT 105 MPH
120H  05/1200Z 30.0N 168.5W   65 KT  75 MPH
 
$$
Forecaster Jelsema
 
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