WTPA43 PHFO 211436

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.

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