ZCZC HFOTCDCP3 ALL TTAA00 PHFO DDHHMM TROPICAL STORM DARBY DISCUSSION NUMBER 43 NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI EP052016 1100 PM HST THU JUL 21 2016 Cloud tops associated with Darby have warmed significantly over the past six hours, while overall organization has decreased. Outflow remains best to the north and northeast, but has become hindered to the southeast. Objective Dvorak satellite intensity estimates remain 3.0/45 knots from all three analysis centers (HFO, JTWC, SAB), but an 0324 UTC WINDSAT pass showed a swath of 55 kt winds across the northeast semicircle close to the LLCC, prompting a continued initial intensity of 55 kt for this advisory. Initial reconnaissance into Darby by the 53rd Weather Reconnaissance Squadron is scheduled for Friday morning, and their data will give us a much better idea as to Darby's intensity and size. Initial motion is 265/09 kt, representing a bit of slowing since the last advisory. WINDSAT and SSMIS from earlier this evening hinted that Darby was tilted slightly to the northeast with height, consistent with SHIPS showing southwesterly shear between 10 and 15 knots. The initial motion slightly south of due west may be due to earlier advisory positions placed slightly too far north. At any rate, Darby continues to move along the southern flank of a ridge centered north of Hawaii and will continue to do so through 36 hours. Afterwards, a deep low will dig south and weaken the ridge, slowing this system down and allowing Darby to curve gently northwestward from 36 to 72 hours, then northward beyond 72 hours. Track guidance shifted slightly to the right, placing the current forecast firmly within a tightening envelope. The forecast track was altered slightly to the left through 12 hours for initial motion, then to the right beyond 72 hours to stay close to TVCN consensus. The rest of the track, including that portion closest to the Hawaiian Islands, is unchanged. The intensity forecast is close to the previous one, with shear initially weakening through 36 hours, then strengthening rapidly to 30 knots from the southwest at 72 hours. The effects of shear and slightly warmer SSTS are expected to essentially cancel each other out through 48 hours, with the weakening effects of shear becoming dominant afterwards as Darby gains latitude. Any weakening effects that land interaction would have on Darby intensity have been ignored so far. Interests outside of the watch and warning areas in the Hawaiian islands should monitor the progress of Darby, as it could eventually have impacts on all islands through early next week. Remember, 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 22/0900Z 18.8N 148.1W 55 KT 65 MPH 12H 22/1800Z 18.7N 149.5W 55 KT 65 MPH 24H 23/0600Z 18.9N 151.5W 55 KT 65 MPH 36H 23/1800Z 19.4N 153.3W 55 KT 65 MPH 48H 24/0600Z 20.1N 154.7W 55 KT 65 MPH 72H 25/0600Z 22.1N 157.4W 45 KT 50 MPH 96H 26/0600Z 24.4N 159.5W 40 KT 45 MPH 120H 27/0600Z 28.1N 160.9W 35 KT 40 MPH $$ Forecaster Powell NNNN