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| #935440 (Received by flhurricane at: 4:35 AM 08.Aug.2018) TCDAT4 Tropical Storm Debby Discussion Number 4 NWS National Hurricane Center Miami FL AL042018 500 AM AST Wed Aug 08 2018 Over the past several hours, deep convection with cloud tops of -55 to -60 deg C has developed in the southeastern semicircle, with some of the convective tops covering the previously exposed low-level circulation center. In addition, outer banding features have dissipated, and an elongated upper-level anticyclone has developed over the cyclone. These convective- and synoptic-scale features indicate that Debby has made the transition from a subtropical to a tropical cyclone. The initial intensity of 40 kt is based on a blend of UW-CIMSS ADT and SATCON intensity estimates of 39 kt and 42 kt, respectively. Furthermore, the 34-kt wind radii and radius of maximum winds (RMW) were decreased significantly on this advisory based on ASCAT wind data. The initial motion estimate is now 015/08 kt. Debby has made the forecast turn toward the north-northeast, and a further turn toward the northeast is expected later today as the cyclone moves around the northwestern periphery of a deep-layer ridge and ahead of an approaching mid-level trough. The latest model guidance remains in good agreement on this developing track scenario, and the new NHC forecast track is similar to the previous advisory, and lies close to a blend of the track consensus models HCCA and FSSE. Debby will be moving along a tight sea-surface temperature (SST) gradient for the next 24-36 hours or so, with the northwestern half the circulation being over sub-25C SSTs and the southeastern semicircle being over warmer waters where convection could continue to develop. Given the relatively low vertical wind shear regime that the cyclone will be moving through, along with the possibility of deep convection persisting near the center, the intensity forecast calls for little change in strength today, followed by only slight weakening tonight and early Thursday. By Thursday night or early Friday, SSTs beneath the cyclone are expected to decrease to near 20 deg C and the shear is forecast to increase to more than 20 kt, an unfavorable combination that should result in Debby dissipating over the far north Atlantic by 48 hours. FORECAST POSITIONS AND MAX WINDS INIT 08/0900Z 40.8N 48.8W 40 KT 45 MPH 12H 08/1800Z 41.8N 48.2W 40 KT 45 MPH 24H 09/0600Z 43.2N 46.1W 35 KT 40 MPH 36H 09/1800Z 44.5N 43.4W 30 KT 35 MPH 48H 10/0600Z...DISSIPATED $$ Forecaster Stewart |
