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| #796431 (Received by flhurricane at: 10:47 PM 18.Aug.2015) TCDAT4 TROPICAL STORM DANNY DISCUSSION NUMBER 3 NWS NATIONAL HURRICANE CENTER MIAMI FL AL042015 1100 PM AST TUE AUG 18 2015 Danny has become a little better organized since the last advisory, with a central convective feature and some outer banding in the southeastern semicircle. At 2200 UTC, PIRATA buoy 41026 reported 42-kt sustained winds just north of the center, and an ASCAT-B overpass near 2330 UTC showed many 40-45 kt vectors. Based on these data, the initial intensity is increased to 45 kt. The ASCAT data also showed that the storm was a little larger than previously thought. The initial motion is now 285/12. Danny is on the south side of the subtropical ridge. The dynamical models forecast the ridge to weaken during the next 48-72 hours due to developing mid- to upper-level troughs near Bermuda and over the northeastern Atlantic. This should result in Danny continuing on a west-northwestward track with a decrease in forward speed. After 72 hours, the trough near Bermuda should move northward and allow the subtropical ridge to intensify. In response, Danny is expected to turn westward with an increase in forward speed. The GFS had a better initialization of Danny on the 1800 UTC run, and it now forecasts a faster forward motion. Otherwise, there are no significant changes in the track guidance since the last advisory. Thus, the new forecast track is an update of the previous track and lies near the consensus models and the center of the guidance envelope. Danny is forecast to be in an environment of light vertical wind shear through the forecast period, and earlier Windsat data suggest the storm has tight convective banding near the center. In addition, the Rapid Intensification Index of the SHIPS model has about a 50 percent chance of 25 kt of strengthening during the next 24 hours. However, there is abundant dry air near Danny, particularly to the north of the storm, and the dynamical models forecast Danny to encounter increasingly drier air through the forecast period. The intensity forecast, which follows the overall trend of the intensity guidance, calls for continued strengthening through 96 hours, followed by a slight weakening due to dry air. The new forecast has a higher peak intensity than that of the previous advisory, but is not as intense as the SHIPS and LGEM models. There are two alternative possibilities for this forecast. The first is that rapid intensification occurs in the next 24 hours before dry air entrains into Danny's core. The second is that enough dry air starts entraining during the next 24 hours to cause a slower rate of development than currently forecast. FORECAST POSITIONS AND MAX WINDS INIT 19/0300Z 11.2N 38.8W 45 KT 50 MPH 12H 19/1200Z 11.6N 40.2W 55 KT 65 MPH 24H 20/0000Z 11.9N 41.7W 60 KT 70 MPH 36H 20/1200Z 12.3N 42.9W 70 KT 80 MPH 48H 21/0000Z 12.8N 44.1W 75 KT 85 MPH 72H 22/0000Z 13.5N 47.5W 85 KT 100 MPH 96H 23/0000Z 14.5N 51.5W 90 KT 105 MPH 120H 24/0000Z 15.0N 56.5W 85 KT 100 MPH $$ Forecaster Beven |
