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Tropical Storm IAN Forecast Discussion Number 1
2016-09-12 16:43:07| National Hurricane Center (Atlantic)
Issued at 1100 AM AST MON SEP 12 2016 000 WTNT45 KNHC 121443 TCDAT5 TROPICAL STORM IAN DISCUSSION NUMBER 1 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102016 1100 AM AST MON SEP 12 2016 Visible satellite images and a recent ASCAT overpass show that the low pressure system over the central Atlantic has now acquired a well-defined low-level circulation. The scatterometer data showed tropical-storm-force winds over the northern semicircle of the circulation, and the advisory intensity is set to 35 kt. Ian is not a well organized storm, with the low-level center exposed and displaced about 70 n mi to the southwest of the main area of deep convection. This is due to the effect of about 20 kt of vertical shear over the system, and this strong shear is predicted to persist for at least the next day or so. In 36 to 48 hours, the shear is forecast to relax somewhat, so some slow intensification could begin by tomorrow night. The official intensity forecast is a little above the model consensus. In 120 hours, or sooner, the system should become embedded within a frontal zone over north Atlantic and lose tropical characteristics. Ian is moving northwestward, with initial motion estimated to be about 320/11 kt. The storm is currently moving through a break in the subtropical ridge. In the next few days, a 500 mb trough approaching from the west should cause Ian to turn northward and then north-northeastward while accelerating. The official forecast track leans toward the ECMWF solution, but is also not far from the latest dynamical model consensus. FORECAST POSITIONS AND MAX WINDS INIT 12/1500Z 21.8N 50.4W 35 KT 40 MPH 12H 13/0000Z 23.4N 51.2W 35 KT 40 MPH 24H 13/1200Z 25.4N 52.4W 35 KT 40 MPH 36H 14/0000Z 27.3N 53.6W 40 KT 45 MPH 48H 14/1200Z 29.7N 54.8W 45 KT 50 MPH 72H 15/1200Z 34.5N 53.8W 50 KT 60 MPH 96H 16/1200Z 42.5N 45.5W 50 KT 60 MPH 120H 17/1200Z 53.0N 32.0W 50 KT 60 MPH...POST-TROP/EXTRATROP $$ Forecaster Pasch
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