WTPA43 PHFO 202047

TCDCP3



TROPICAL STORM DARBY DISCUSSION NUMBER  37

NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI   EP052016

1100 AM HST WED JUL 20 2016



Darby's satellite signature has degraded slightly since the previous

advisory but convective cloud tops as cold as -65C persist near the

center.  However, microwave and conventional satellite imagery

indicate that the low level center is located on the southwest side

of the deep convection, with a 1615Z SSMI/S pass helpful in locating

the center.  Since then, visible imagery shows that the center has

become partially exposed.  Subjective Dvorak intensity estimates

from PHFO/SAB both indicated a current intensity of 3.0/45 kt, and

the initial intensity has been lowered to 45 kt for this advisory.



The initial motion vector is 270/11 kt, with Darby located well

south of a mid-level ridge that is supported by anticyclones

centered far north of the Hawaiian islands and over the central

mainland U.S.  The forecast philosophy with respect to track has

changed little from the previous advisory.  Through the first 48 to

72 hours of the forecast, Darby's motion is expected to be

increasingly influenced by the ridge building southeastward from

north of the Hawaiian Islands.  This is expected to impart a motion

slightly south of due west until early Saturday.  After that time, a

break in the ridge is still expected in response to a southward-

digging low aloft.  This is expected to cause Darby to turn sharply

toward the northwest and north on days 4 and 5 as it becomes

embedded within deep southerly flow between the low aloft and a

ridge extending westward from the eastern Pacific.  The updated

forecast track is close to the previous, especially through 48

hours, and lies between the previous and TVCN thereafter. This

forecast slightly delays the turn toward the northwest, bringing the

center a little closer to the Big Island of Hawaii, influenced by

the latest ECMWF guidance.



The intensity forecast continues to be complicated by competing

environmental factors.  Darby will be moving over slightly warmer

waters during the next couple of days, but this is still expected to

be counteracted by moderate southwesterly shear and a generally dry

environment.  The SHIPS and LGEM models show steady weakening that

briefly levels off in the 60 to 72 hour time frame, while the

dynamical models are showing some restrengthening in the 36 to 60

hour time frame, primarily due to a brief reduction in vertical wind

shear.  Given this, the official forecast shows little change in

intensity during the first 36 hours, with a slight strengthening on

days 2 and 3 before increasing shear at the end of the period leads

to steady weakening.  The official forecast lies close to the IVCN

consensus, which lies between the much stronger HWRF/GHM and the

weaker SHIPS/LGEM.





FORECAST POSITIONS AND MAX WINDS



INIT  20/2100Z 19.9N 141.3W   45 KT  50 MPH

 12H  21/0600Z 19.7N 143.0W   45 KT  50 MPH

 24H  21/1800Z 19.3N 145.5W   45 KT  50 MPH

 36H  22/0600Z 19.0N 147.9W   45 KT  50 MPH

 48H  22/1800Z 18.8N 149.9W   50 KT  60 MPH

 72H  23/1800Z 19.4N 152.8W   50 KT  60 MPH

 96H  24/1800Z 21.3N 154.3W   45 KT  50 MPH

120H  25/1800Z 24.7N 156.0W   40 KT  45 MPH



$$

Forecaster Birchard