ZCZC HFOTCDCP1 ALL TTAA00 PHFO DDHHMM 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 NNNN