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Hurricane Walaka Discussion Number   6
NWS Central Pacific Hurricane Center Honolulu HI   CP012018
500 PM HST Sun Sep 30 2018

Walaka's well defined deep convective wrapping pattern has morphed 
into a cloudy eye pattern in visible imagery this afternoon. 
Satellite loop shows system outflow is good across all quadrants
and overall system symmetry and organization continue to improve. 
Subjective Dvorak current intensity (CI) estimates range from
4.0/65 kt from JTWC and PHFO to 4.5/77 kt from SAB. ADT from
UW-CIMSS is 68 kt. Given the presence of a cloudy eye and higher
overall CI estimates from the analysis centers, we will set the
initial intensity at 65 kt for this advisory. Walaka is now a
hurricane. 

The overall forecast philosophy for Walaka is unchanged. This
system continues moving slightly north of due west along the
southern flank of the subtropical ridge. Global models show a deep
upper trough will dig southward, eroding the western edge of the
subtropical ridge to the north of Walaka on Monday. This will steer
Walaka northwestward, then northward on Tuesday. Track guidance
remains very tightly clustered through Friday, greatly increasing
confidence in the timing of the curve northward. Our forecast
closely follows the previous one, with another tap to the west from
12 to 36 hours needed to stay within the very tight guidance
envelope. A small tap to the east at 72 hours was needed for the
same reason. With the official forecast track bringing the center of
Walaka very near Johnston Atoll on Tuesday, a Hurricane Warning is
now in effect there.

Walaka remains within ideal conditions for strengthening, with high 
SSTs, low shear, high ocean heat content and plenty of deep 
moisture. SHIPS shows vertical shear will remain below 20 kt
through 48 hours, ramping up to near 30 kt by day 3 and beyond. SSTs
stay above 28 degrees C until day 3 even as Walaka tracks northward.
Our forecast calls for rapid intensification through day 2, with
Walaka reaching major hurricane status on Monday. After leveling
off between days 2 and 3, increasing vertical shear and decreasing
SSTs should begin to take their toll on this system. Steady
weakening is forecast for day 3 and beyond. Our intensity forecast
trends are close to those in the previous advisory, following HWRF.
Some of the intensity guidance, like SHIPS, strengthens Walaka into
the 130 to 140 kt range by day 2, with the probability for rapid 
intensification exceeding 90%. While we cap this system at 120 kt 
for this advisory, subsequent advisory packages may begin to
reflect a stronger maximum intensity.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  01/0300Z 11.9N 166.4W   65 KT  75 MPH
 12H  01/1200Z 12.3N 167.9W   85 KT 100 MPH
 24H  02/0000Z 13.3N 169.4W  105 KT 120 MPH
 36H  02/1200Z 14.8N 170.3W  115 KT 130 MPH
 48H  03/0000Z 17.2N 170.3W  120 KT 140 MPH
 72H  04/0000Z 22.7N 168.8W  110 KT 125 MPH
 96H  05/0000Z 28.3N 168.3W   80 KT  90 MPH
120H  06/0000Z 32.1N 167.3W   60 KT  70 MPH
 
$$
Forecaster Powell
 
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