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Hurricane Walaka Discussion Number  10
NWS Central Pacific Hurricane Center Honolulu HI   CP012018
500 PM HST Mon Oct 01 2018

Recent infrared images showed that cloud tops surrounding the eye
have warmed somewhat since early this morning, but still form a
solid ring with temperatures in the -70 to -76C range. Dvorak
intensity estimates came in at 6.5/127kt from PHFO and SAB. UW/CIMSS
ADT data indicated 7.0/140kt at 0000 UTC. The current intensity for
this advisory is 140 kt as a nod to the ADT value, since the
subjective Dvorak estimates have been constrained by fix rules. Of
additional interest, microwave data from the NOAA-18 1949 UTC and
METOP-B at 2108 UTC showed the development of an outer eyewall,
which indicates Hurricane Walaka is going through an eyewall
replacement cycle. An ASCAT pass from 2107 UTC caught the eastern
semicircle of Walaka's circulation, which resulted in the expansion
of the 34 kt radii in the northeast quadrant.

The initial motion of Walaka is 310/6 kt as it rounds the
southwestern flank of a subtropical ridge. A deepening low pressure
system north of Walaka is in the process of altering the steering
current, causing the hurricane to turn toward the north. The
numerical models are in good agreement with the scenario and the
track guidance is tightly packed, especially through 48 hours.
Walaka is expected to accelerate northward under the
influence of the low Wednesday into Thursday. The forecast
track for this advisory is close to the previous forecast and the
HCCA consensus, especially through 36 hours.
 
With sea surface temperatures under Walaka expected to remain near
30C through tonight, high ocean heat content, and low vertical wind
shear, there is still a window for additional strengthening tonight.
However, eyewall replacement cycles will result in intensity
fluctuations, which are difficult to forecast. SHIPS guidance
indicates strengthening to 153 kt while the dynamical models show 
peak intensities below 145 kt. The forecast maintains slight
intensification tonight, followed by a gradual weakening through 48
hours. This is consistent with HCCA. Beyond 48 hours, increasing
vertical wind shear, followed by a track over sub-26.5C waters will
result in more rapid weakening through 120 hours. The forecast
drops Walaka below hurricane strength after 96 hours, which is
a less aggressive weakening than all objective aids.
 
The forecast track will take Walaka near Johnston Island on Tuesday,
so the Hurricane Warning remains in effect for this location. The
forecast track also takes the hurricane across the Papahanaumokuakea
Marine National Monument between French Frigate Shoals and Laysan
Island late Wednesday. Thus, a Hurricane Watch has been issued for
the region from Nihoa to Maro Reef.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  02/0300Z 13.2N 169.8W  140 KT 160 MPH
 12H  02/1200Z 14.4N 170.5W  145 KT 165 MPH
 24H  03/0000Z 16.3N 170.6W  140 KT 160 MPH
 36H  03/1200Z 18.9N 170.0W  135 KT 155 MPH
 48H  04/0000Z 22.2N 168.7W  125 KT 145 MPH
 72H  05/0000Z 28.4N 167.7W   90 KT 105 MPH
 96H  06/0000Z 31.5N 166.4W   65 KT  75 MPH
120H  07/0000Z 37.0N 161.0W   50 KT  60 MPH
 
$$
Forecaster Kodama
 
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