WTPA43 PHFO 210912

1100 PM HST WED JUL 20 2016
Since this afternoon, the satellite cloud pattern around Darby has
become a bit better organized and the area of cold tops has
increased. A series of microwave passes between 03 and 04 UTC
also showed an increase in organization. The latest subjective
Dvorak intensity estimates were 2.5/35 kt from SAB, 3.0/45 kt from
JTWC and 3.5/55 kt from HFO. The initial intensity for this
advisory has been raised to 55 kt.

The center of Darby is not easy to locate on GOES 15 imagery, but
the microwave passes from a few hours ago show that the storm has
continued to track south of west. The track forecast has 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 within the next 12 hours.

Darby will be moving over slightly warmer water the next couple of
days, while vertical wind shear is forecast to increase slightly.
Darby is expected to remain surrounded by rather dry air. Little
change in intensity is expected for the next few days with gradual
weakening beyond that as Darby turns northwest over cooler water
and vertical wind shear increases.  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  21/0900Z 19.3N 143.8W   55 KT  65 MPH
 12H  21/1800Z 18.9N 145.7W   55 KT  65 MPH
 24H  22/0600Z 18.6N 147.9W   55 KT  65 MPH
 36H  22/1800Z 18.5N 150.0W   55 KT  65 MPH
 48H  23/0600Z 18.7N 151.6W   55 KT  65 MPH
 72H  24/0600Z 19.8N 153.8W   55 KT  65 MPH
 96H  25/0600Z 22.3N 155.3W   50 KT  60 MPH
120H  26/0600Z 25.5N 157.5W   40 KT  45 MPH
Forecaster Donaldson