ZCZC HFOTCDCP3 ALL TTAA00 PHFO DDHHMM TROPICAL STORM DARBY DISCUSSION NUMBER 40 NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI EP052016 500 AM HST THU JUL 21 2016 The satellite cloud pattern around Darby remains well-organized, but the area of cold tops has decreased a bit since last evening. Microwave passes at 1059 and 1156 UTC also showed good banding wrapping from west through south through east through north of the center. Comparison of 36 and 89 GHz imagery shows the center sloping up to the southeast. The latest subjective Dvorak intensity estimates were 3.0/45 kt from SAB, JTWC and HFO. The initial intensity for this advisory has been kept at 55 kt. The center of Darby is not easy to locate precisely with GOES 15 imagery, but the recent microwave passes show that the storm has continued to track south of west. The track forecast has again shifted very slightly southwest from the previous track over the next 72 hours. The mid-level ridge northwest of Darby will steer the storm west southwest with a very gradual turn to the west. Beyond 72 hours, Darby is expected to turn rather sharply toward the northwest as a low aloft in the Gulf of Alaska digs strongly south. This is a rather unusual track for a tropical cyclone in the central Pacific, so there is a greater than normal amount of uncertainty to the forecast. Since Darby continues to track toward the main Hawaiian Islands it will probably be necessary to issue a Tropical Storm Watch later today. Darby will be moving over .5 degree Celsius warmer water the next couple of days, while vertical wind shear is forecast to increase slightly. Darby is expected to remain surrounded by fairly dry air. With no big changes in the environment, little change in intensity is expected for the next few days. As Darby turns northwest in 72 hours it will be moving over cooler water and vertical wind shear will increase. Darby is expected to weaken steadily through day 5. The official forecast lies close to the IVCN consensus. Interests in the Hawaiian islands should monitor the progress of Darby. However, it is important not to focus too closely on the exact track and intensity forecasts because the average track error 72 hours out is near 100 miles, while the average intensity error is about 15 kt. In addition, the hazards of a tropical cyclone can extend over a broad area well away from the center. FORECAST POSITIONS AND MAX WINDS INIT 21/1500Z 18.9N 145.2W 55 KT 65 MPH 12H 22/0000Z 18.6N 147.0W 55 KT 65 MPH 24H 22/1200Z 18.4N 149.2W 55 KT 65 MPH 36H 23/0000Z 18.5N 151.1W 55 KT 65 MPH 48H 23/1200Z 18.7N 152.5W 55 KT 65 MPH 72H 24/1200Z 20.2N 154.4W 55 KT 65 MPH 96H 25/1200Z 23.0N 156.0W 50 KT 60 MPH 120H 26/1200Z 26.5N 158.5W 40 KT 45 MPH $$ Forecaster Donaldson NNNN