ZCZC HFOTCDCP3 ALL TTAA00 PHFO DDHHMM TROPICAL STORM DARBY DISCUSSION NUMBER 41 NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI EP052016 1100 AM HST THU JUL 21 2016 Darby's satellite signature has changed little since the last advisory, with a compact area of deep convection wrapping around the center, mainly in the northern semicircle. Although subjective Dvorak fixes from HFO/SAB/GTW all indicated a data-T of 3.0/45 kt, the initial intensity for this advisory will be maintained at 55 kt. Reconnaissance aircraft from the 53rd Weather Reconnaissance Squadron are en route to Hawaii, and initial flights into Darby are scheduled for this time tomorrow, allowing for a much better estimate of Darby's actual intensity. The initial motion for this advisory is estimated to be 260/12 kt, with Darby being steered by a retrograding mid-level ridge centered well north of Hawaii. This ridge will be the primary steering mechanism over the next 48 hours or so, after which time a deep layer low digging southward will weaken the ridge, causing a reduction in Darby's forward speed. Between 48 hours and 72 hours, steering currents are expected to be weak, with Darby still expected to make a turn toward the northwest. Latest guidance however has trended toward a less dramatic turn, and shifted closer to Hawaii. The updated track forecast is similar to the previous through 48 hours, after which time it has been shifted closer to Hawaii, in response to a significant westward shift in most of the reliable guidance, namely the TVCN and GFEX. The official forecast lies on the right hand side of the guidance suite, and if model trends continue, the forecast may need to shifted westward with subsequent forecasts. The updated track forecast necessitates a the issuance of a tropical storm watch for the Big Island and Maui county. The intensity forecast is essentially unchanged from the previous, with light to moderate shear inhibiting strengthening while Darby traverses slightly warmer water over the next 48 hours. Gradually cooling water temperatures and increasing shear are expected to lead to a weakening trend on days 4 and 5, and the updated forecast is close to the IVCN consensus. Of note, while the official track forecast indicates Darby tracking over portions of the Hawaiian Islands, the intensity forecast is thus far not indicating significant weakening due to interaction with land. Interests elsewhere in the Hawaiian islands should monitor the progress of Darby, as it could eventually have impacts on all islands. 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 21/2100Z 18.6N 146.6W 55 KT 65 MPH 12H 22/0600Z 18.4N 148.3W 55 KT 65 MPH 24H 22/1800Z 18.3N 150.5W 55 KT 65 MPH 36H 23/0600Z 18.5N 152.1W 55 KT 65 MPH 48H 23/1800Z 18.9N 153.6W 55 KT 65 MPH 72H 24/1800Z 20.3N 155.7W 55 KT 65 MPH 96H 25/1800Z 22.5N 157.9W 50 KT 60 MPH 120H 26/1800Z 25.5N 160.5W 40 KT 45 MPH $$ Forecaster Birchard NNNN