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Tropical Storm IAN Forecast Discussion Number 3
2016-09-13 04:42:44| National Hurricane Center (Atlantic)
Issued at 1100 PM AST MON SEP 12 2016 000 WTNT45 KNHC 130242 TCDAT5 TROPICAL STORM IAN DISCUSSION NUMBER 3 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102016 1100 PM AST MON SEP 12 2016 Ian remains a sheared tropical cyclone, with the strongest convection displaced to the northeast and east of the center. However, moderate convection has recently developed over and just north of the center due to the vertical wind shear's backing from a southwesterly to southerly direction. Data from a recent partial ASCAT-B overpass indicate that the 34-kt wind field has expanded to at least 200 nmi in the northeastern quadrant, and that peak winds have also increased to more than 35 kt. Dvorak satellite intensity estimates are T3.0/45 kt from TAFB and T2.5/35 kt from SAB, and an average of these values yields an advisory intensity of 40 kt. Ian has maintained a steady motion of 330/11 kt. The latest NHC model guidance is in excellent agreement on the storm continuing to move north-northwestward through a break in the Bermuda-Azores High for the next 24 hours, followed by a turn to the north at 36-48 hours. After that, Ian is expected to gradually accelerate as the cyclone gets captured by a strong mid-latitude shortwave trough. The new official forecast track is essentially just an update of the previous advisory track and lies close to the GFS-ECMWF model consensus, and the multi-model consensus TVCN. The vertical shear is forecast to decrease to 15-20 kt in 36 to 48 hours when an upper-level low, currently located about 400 nmi northwest of the cyclone, is forecast by the GFS, UKMET, and ECMWF models to move over and become colocated with Ian's low-level center. The combination of decreasing vertical wind shear and some infusion of baroclinic energy associated with this complex interaction is expected to produce at least some slight strengthening by days 2-3. Around 120 hours, Ian is forecast to interact with a frontal zone over the north Atlantic and be transformed into an extratropical cyclone. FORECAST POSITIONS AND MAX WINDS INIT 13/0300Z 23.5N 51.5W 40 KT 45 MPH 12H 13/1200Z 24.9N 52.4W 40 KT 45 MPH 24H 14/0000Z 26.9N 53.4W 40 KT 45 MPH 36H 14/1200Z 29.4N 54.1W 45 KT 50 MPH 48H 15/0000Z 31.8N 54.0W 50 KT 60 MPH 72H 16/0000Z 38.0N 49.7W 50 KT 60 MPH 96H 17/0000Z 48.0N 38.0W 50 KT 60 MPH 120H 18/0000Z 56.5N 25.2W 50 KT 60 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart
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