ZCZC HFOTCDCP3 ALL TTAA00 PHFO DDHHMM TROPICAL STORM DARBY DISCUSSION NUMBER 47 NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI EP052016 1100 PM HST FRI JUL 22 2016 Darby's satellite presentation continues to be messy, with deep convection now on the decline and beginning to wrap around the south and southwest quadrants. Cloud tops have warmed over the past six hours and the best outflow continues to be toward the northeast, consistent with southwesterly shear near 10 kt depicted by SHIPS. The 53rd Weather Reconnaissance Squadron's WC-130J aircraft did a fine job this evening finding the center of this system, so we are confident we know where Darby is and how it is moving. Based on aircraft SFMR data from the first pass through the center, we will keep an initial intensity of 50 kt for this advisory. Like before, this is higher than the objective Dvorak satellite intensity estimates from the three satellite analysis centers (HFO, JTWC and SAB). These range from 30 kt to 45 kt. Interestingly, an 0630 UTC ASCAT pass only found 35 kt within the northeast quadrant. The next aircraft reconnaissance mission into Darby will be Saturday morning. Initial motion is 280/09 kt as Darby slowly gains latitude along the southern flank of a subtropical ridge. Low pressure farther to the north is expected to dig southward, weakening the ridge. This will simultaneously slow Darby's forward motion and deflect it to a more northwesterly track. Vertical wind shear will also increase, especially after 18 hours according to SHIPS, likely leading to gradual weakening from 24 hours and beyond. Track guidance has changed very little over the past 6 to 12 hours, depicting a gently curving path across the main Hawaiian Island chain. The forecast for this advisory track has changed very little and remains well within the tightening guidance envelope and close to TVCN consensus. This track takes Darby directly across the Big Island of Hawaii between 12 and 24 hours. The intensity forecast is based on the gradual toll that increasing vertical shear will take on Darby, overtaking any sustaining effects from marginal sea surface temperatures. However, this shear is not expected to become strong until day 2. The forecast calls for slow weakening with Darby maintaining tropical storm strength through the weekend. This is consistent with the previous package and represents a compromise between SHIPS, which weakens DARBY quickly, and GHMI, which keeps Darby as a strong tropical storm through day 5. It is important to note that weakening due to land interaction has so far been ignored for that portion of the track beyond the Big Island. Interests outside of the watch and warning areas in the Hawaiian Islands should continue to monitor the progress of Darby. 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 23/0900Z 18.8N 153.0W 50 KT 60 MPH 12H 23/1800Z 19.1N 154.4W 50 KT 60 MPH 24H 24/0600Z 19.8N 155.7W 45 KT 50 MPH 36H 24/1800Z 20.6N 156.7W 40 KT 45 MPH 48H 25/0600Z 21.7N 158.0W 40 KT 45 MPH 72H 26/0600Z 24.3N 160.8W 40 KT 45 MPH 96H 27/0600Z 27.6N 163.7W 35 KT 40 MPH 120H 28/0600Z 31.9N 166.3W 30 KT 35 MPH $$ Forecaster Powell NNNN