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