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Tropical Storm WALAKA

Tropical Storm Walaka Discussion Number   1
NWS Central Pacific Hurricane Center Honolulu HI   CP012018
1100 AM HST Sat Sep 29 2018

The tropical disturbance passing far south of the main Hawaiian
Islands has developed persistent deep convection near the center
with a banding feature developing within the eastern and southern
quadrants. A series of SSMI and SSMIS passes between 1348 and 1716
UTC showed this structure and provided some indication of the
location of the low level circulation center. Subjective Dvorak
current intensity estimates from JTWC and SAB were constrained to
2.0/30 kt, with data T numbers coming in at 2.5/35 kt, and the
current intensity from HFO was 2.5/35 kt. These inputs support
initiating advisories for Tropical Storm Walaka, the sixth tropical
cyclone of 2018 in the Central Pacific basin and the first to form
in the basin this year.

Walaka is moving toward the west (265 degrees) at 13 kt. A westward
motion will continue for the next 36 hours or so as the system
remains south of a deep ridge. This will take Walaka well southwest
of the main Hawaiian Islands. Late Sunday through Monday night, the
cyclone will make a gradual turn toward the northwest then north as
a deep North Pacific trough digs southward and steadily erodes the
ridge. As a result, Walaka will pass very close to Johnston Island
as early as Monday night and likely sometime Tuesday. The deep
trough will stall roughly along 171W longitude on Tuesday, causing
the tropical cyclone to accelerate northward. On days four and
five, the motion becomes uncertain due to model differences in the
handling of the interaction between Walaka with the deep trough. The
track forecast follows a rather tightly clustered guidance envelope
through Tuesday, then favors the HMON, which is closer to TVCE than
the faster GFS and slower ECMWF. Due to the uncertainty in the long
term motion, interests in the Papahanaumokuakea Marine National
Monument should monitor Walaka.

Strengthening, possibly rapid, is expected through the next three
days, and Walaka is forecast to remain intense through day five.
Walaka will remain under low vertical wind shear and over SSTs of
around 29C through at least Monday, and likely into Tuesday. In this
environment, the SHIPS rapid intensification guidance for Walaka is
well above the climatological mean. The intensity forecast is close
to LGEM and ICON through the initial 48 hours and remains near the
HWRF and ICON, but below LGEM, thereafter. Walaka is forecast to
become a hurricane by Sunday, and along the forecast track,
hurricane conditions could be experienced on Johnston Island as
early as Monday night. Some weakening is indicated beyond Tuesday
due to increasing southwesterly vertical wind shear.
INIT  29/2100Z 11.5N 159.1W   35 KT  40 MPH
 12H  30/0600Z 11.5N 161.5W   45 KT  50 MPH
 24H  30/1800Z 11.8N 164.2W   60 KT  70 MPH
 36H  01/0600Z 12.5N 166.6W   75 KT  85 MPH
 48H  01/1800Z 13.4N 168.4W   90 KT 105 MPH
 72H  02/1800Z 16.5N 169.6W  110 KT 125 MPH
 96H  03/1800Z 21.4N 168.5W  100 KT 115 MPH
120H  04/1800Z 27.0N 168.0W   80 KT  90 MPH
Forecaster Wroe