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Tropical Storm IAN Forecast Discussion Number 8
2016-09-14 10:37:21| National Hurricane Center (Atlantic)
Issued at 500 AM AST WED SEP 14 2016 000 WTNT45 KNHC 140837 TCDAT5 TROPICAL STORM IAN DISCUSSION NUMBER 8 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102016 500 AM AST WED SEP 14 2016 The cloud pattern of Ian has changed little during the past several hours. The low-level center of the tropical storm is completely exposed and located more than 100 n mi south of the main area of deep convection. This poor structure is the result of about 30 kt of south-southwesterly vertical wind shear. The initial intensity is held at 45 kt, in agreement with the earlier scatterometer data and a Dvorak CI-number from TAFB. Ian is moving northward at about 13 kt, and is being steered by the southerly flow between a mid- to upper-level low to its west and a ridge to its east. A northward to north-northeastward motion at about the same forward speed is forecast during the next 24 hours as the steering pattern more or less persists. After that time, a shortwave trough is expected to move east of Atlantic Canada, and that should cause Ian to turn northeastward and sharply accelerate until the cyclone becomes absorbed by a large extratropical low in 3 to 4 days. The models are in good agreement, and the official forecast lies closest to a consensus of the GFS and ECMWF models. The strong shear that has been affecting Ian since genesis is expected to let up some during the next 24 to 36 hours. The predicted lower shear, fairly warm water, and baroclinic forcing should allow Ian to strengthen a little during the next day or two. Beyond a couple of days, Ian is forecast to move over sharply cooler sea surface temperatures and into an environment of strong shear. These conditions should cause the cyclone to lose its tropical characteristics in 2 to 3 days. The NHC intensity forecast is an update of the previous one, and lies near the middle of the guidance envelope. FORECAST POSITIONS AND MAX WINDS INIT 14/0900Z 29.3N 53.0W 45 KT 50 MPH 12H 14/1800Z 31.3N 53.2W 50 KT 60 MPH 24H 15/0600Z 34.0N 52.3W 50 KT 60 MPH 36H 15/1800Z 37.2N 49.2W 55 KT 65 MPH 48H 16/0600Z 41.6N 43.6W 55 KT 65 MPH 72H 17/0600Z 53.4N 28.0W 55 KT 65 MPH...POST-TROP/EXTRATROP 96H 18/0600Z...DISSIPATED $$ Forecaster Cangialosi
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