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Tropical Depression THIRTEEN Forecast Discussion Number 1
2016-09-19 22:53:34| National Hurricane Center (Atlantic)
Issued at 500 PM AST MON SEP 19 2016 000 WTNT43 KNHC 192053 TCDAT3 TROPICAL DEPRESSION THIRTEEN DISCUSSION NUMBER 1 NWS NATIONAL HURRICANE CENTER MIAMI FL AL132016 500 PM AST MON SEP 19 2016 Satellite imagery, scatterometer data, and ship observations indicate that the large low pressure system located southwest of the Cabo Verde Islands has maintained a well-defined circulation and has developed sufficient organized convection to be considered a tropical depression. The initial intensity is set at 30 kt based on scatterometer wind data of near 30 kt along with Dvorak satellite intensity estimates T2.5/35 kt and T2.0/30 kt from TAFB and SAB, respectively. The central pressure is based on reports from ship D5ET2 during is traversal through the center of the cyclone over the past 12 hours. The depression is a large tropical cyclone with a wind field that is more than 400 n mi in diameter. The initial motion estimate is an uncertain 280/10 kt, based primarily on scatterometer and microwave satellite data. The depression is located along the southern periphery of a large deep-layer ridge that is foreast to steer the cyclone westward for the next 24 hours or so, followed by west-northwestward motion through 120 hours. The official forecast track lies close to the consensus model TVCN through 96 hours, and then leans toward a blend of the ECMWF, UKMET, and CMC models since the GFS dissipates the cyclone by 120 hours, a scenario that seems premature given the current large size of the cyclone. Vertical wind shear is forecast to remain low at less than 10 kt over the cyclone for the next 48 hours, which favors steady intensification. However, the mid-level moisture is only expected to be marginal during that time, with humidity values around 60 percent or less. The drier air along with the large size of the cyclone are expected to temper the development process, and this is reflected by the slower-than-average intensification rate. The NHC intensity forecast closely follows the intensity model IVCN. This forecast is lower than the SHIPS, LGEM, and ECMWF models, which brings the cyclone to near hurricane strength by 72 hours. FORECAST POSITIONS AND MAX WINDS INIT 19/2100Z 13.6N 28.6W 30 KT 35 MPH 12H 20/0600Z 14.3N 29.8W 35 KT 40 MPH 24H 20/1800Z 15.6N 31.8W 40 KT 45 MPH 36H 21/0600Z 16.7N 33.5W 45 KT 50 MPH 48H 21/1800Z 17.6N 35.4W 50 KT 60 MPH 72H 22/1800Z 19.5N 38.9W 45 KT 50 MPH 96H 23/1800Z 21.5N 42.5W 35 KT 40 MPH 120H 24/1800Z 23.5N 44.7W 30 KT 35 MPH $$ Forecaster Stewart
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