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| #796274 (Received by flhurricane at: 10:49 AM 18.Aug.2015) TCDAT4 TROPICAL DEPRESSION FOUR DISCUSSION NUMBER 1 NWS NATIONAL HURRICANE CENTER MIAMI FL AL042015 1100 AM AST TUE AUG 18 2015 The low pressure system that has been moving westward across the eastern tropical Atlantic the past few days has finally acquired enough organized deep convection to be classified as a tropical cyclone. A 1232 UTC ASCAT-B overpass indicated the system has a well-defined circulation, and there was also a large field of 30-kt and greater wind vectors in the eastern semicircle. Dvorak satellite intensity estimates are a consensus T2.0/30 kt from TAFB and SAB, so the initial intensity is set at 30 kt for this advisory. Upper-level outflow is good to the south and fair to the north. The initial motion estimate is 280/11 kt. The global and regional models are in good agreement on the tropical cyclone moving west-northwestward along the southern periphery of a deep-layer subtropical ridge for the next 48-72 hours, accompanied by a decrease in forward speed as the system approaches a weakness in this ridge. After that time, the ridge is expected to build back in as a trough to the north lifts out, forcing the cyclone to turn more westward and accelerate through the remainder of the forecast period. The official forecast track lies close to but a little faster than the multi-model consensus TVCN due to the much slower GFS model creating a slow bias in the consensus. The overall atmospheric and oceanic environments surrounding the cyclone appear conducive for slow but steady strengthening throughout the forecast period. The only inhibiting factor will be occasional brief intrusions of dry mid-level air associated with the Saharan Air Layer that lies just to the north of the depression. However, given the very low vertical wind shear of less than 5 kt, the convective structure is expected to steadily increase in organization, allowing the dry air intrusions to be mixed out. The official intensity forecast closely follows the intensity consensus model IVCN through 72 hours, and then leans closer to a blend of the Decay-SHIPS and LGEM models at 96 and 120 hours. FORECAST POSITIONS AND MAX WINDS INIT 18/1500Z 10.6N 36.5W 30 KT 35 MPH 12H 19/0000Z 11.0N 37.9W 35 KT 40 MPH 24H 19/1200Z 11.3N 39.5W 40 KT 45 MPH 36H 20/0000Z 11.6N 40.9W 50 KT 60 MPH 48H 20/1200Z 12.1N 41.9W 60 KT 70 MPH 72H 21/1200Z 13.2N 44.2W 70 KT 80 MPH 96H 22/1200Z 13.7N 47.8W 80 KT 90 MPH 120H 23/1200Z 14.0N 52.4W 85 KT 100 MPH $$ Forecaster Stewart |
