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Tropical Storm IAN Forecast Discussion Number 7
2016-09-14 04:52:57| National Hurricane Center (Atlantic)
Issued at 1100 PM AST TUE SEP 13 2016 000 WTNT45 KNHC 140252 TCDAT5 TROPICAL STORM IAN DISCUSSION NUMBER 7 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102016 1100 PM AST TUE SEP 13 2016 Ian's cloud pattern has changed little in organization since this afternoon. The low-level center remains patrially exposed to the south of a fan-shaped mass of deep convection. This structure is consistent with a deep-layer southwesterly vertical shear of nearly 40 kt associated with a mid- to upper-tropospheric cyclone to the west of the cyclone. A 0018Z ASCAT pass showed reliable surface winds near 45 kt well north of the center, which agrees with a TAFB satellite classifciation of T3.0. On this basis, the initial intensity is increased to 45 kt. The initial motion estimate has been nudged a little more to the right than earlier today and is now 345/11 kt. Ian should continue to move generally northward into a weakness in the subtropical ridge over the central Atlantic during the next 24 hours or so. A significant acceleration toward the northeast should occur in about 36 hours when a shortwave trough moving through Atlantic Canada approaches Ian. The NHC track forecast is adjusted to the right of the previous one, especially after 24 hours, and is in agreement with the the latest track consensus aids. The SHIPS output indicates that the shear should decrease, beginning in about 12 hours, roughly around the time that Ian becomes co-located with the upper low to its west. The shear is forecast to reach a relative minimum 24 hours later while the flow aloft gradually becomes more diffluent. This baroclinic forcing is expected to result in some intensification while Ian remains over warm waters, though the cyclone could possess attributes of both a tropical and an extratropical cyclone. Extratropical transition is indicated in 2 to 3 days, with absorption by a larger extratropical storm system over the North Atlantic likely around 72 hours. The NHC intensity forecast is about the same as the previous one and near the multi-model consensus. FORECAST POSITIONS AND MAX WINDS INIT 14/0300Z 27.6N 52.8W 45 KT 50 MPH 12H 14/1200Z 29.8N 53.2W 45 KT 50 MPH 24H 15/0000Z 32.3N 53.1W 50 KT 60 MPH 36H 15/1200Z 35.2N 51.3W 55 KT 65 MPH 48H 16/0000Z 39.4N 47.0W 55 KT 65 MPH 72H 17/0000Z 49.7N 33.7W 55 KT 65 MPH...POST-TROP/EXTRATROP 96H 18/0000Z...ABSORBED $$ Forecaster Kimberlain
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