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Tropical Storm WALAKA

Tropical Storm Walaka Discussion Number   5
NWS Central Pacific Hurricane Center Honolulu HI   CP012018
1100 AM HST Sun Sep 30 2018
Deep convection around Walaka has assumed a good wrapping pattern 
this morning, increasing position confidence with development of a 
relative dry slot along the southeast quadrant of the circulation. 
The northern portion of this slot, which is also quite warm, may
soon develop into an eye. Convective tops remain quite cold, in the
-80 to -90 degree C range. Subjective Dvorak current intensity
estimates ranged from 3.0/45 kt from JTWC to 4.0/65 kt from SAB.
PHFO provided 3.5/55 kt while ADT from UW-CIMSS was 58 kt. The
initial intensity is set at 55 kt based on a mix of these

Walaka is tracking 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 
the tropical cyclone northwestward, then northward on Tuesday. The 
system should curve back north-northwestward on Thursday as it 
begins to interact with the upper trough. Track guidance is very 
tightly clustered through Friday, greatly increasing confidence in 
the timing of the curve northward. Our forecast closely follows the 
previous one, with a slight tap to the west beyond 36 hours needed 
to stay within the very tight guidance envelope.  FSSE and CTCI
mark the right and left envelope limits, with little difference
between them through day 3. With the official forecast track
bringing the center of Walaka very near Johnston Atoll on Tuesday, a
Hurricane Watch remains 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 days 4 and 5. Our intensity forecast
trends are close to those in the previous advisory, following LGEM
and IVRI as Walaka strengthens, then following FSSE and CTCI as it
INIT  30/2100Z 11.9N 165.2W   55 KT  65 MPH
 12H  01/0600Z 12.2N 167.1W   75 KT  85 MPH
 24H  01/1800Z 13.0N 168.8W   95 KT 110 MPH
 36H  02/0600Z 14.3N 169.9W  115 KT 130 MPH
 48H  02/1800Z 16.4N 170.5W  120 KT 140 MPH
 72H  03/1800Z 21.8N 169.6W  120 KT 140 MPH
 96H  04/1800Z 27.7N 168.5W   85 KT 100 MPH
120H  05/1800Z 31.5N 167.9W   65 KT  75 MPH
Forecaster Powell