ZCZC HFOTCDCP1 ALL TTAA00 PHFO DDHHMM 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 estimates. 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 weakens. FORECAST POSITIONS AND MAX WINDS 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 NNNN