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Tropical Storm Dorian Forecast Discussion Number 9
2019-08-26 17:00:10| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Mon Aug 26 2019 000 WTNT45 KNHC 261500 TCDAT5 Tropical Storm Dorian Discussion Number 9 NWS National Hurricane Center Miami FL AL052019 1100 AM AST Mon Aug 26 2019 Satellite imagery indicates that Dorian's convective and outflow pattern have continued to improve, with a narrow poleward outflow channel now apparent in water vapor images. Passive microwave imagery has shown a persistent low-level eye-like feature along with an intermittent mid-level eyewall forming that quickly erodes due to mid-level dry air entrainment. The initial intensity of 50 kt is based on a blend of satellite intensity estimates ranging from 45 kt to 56 kt from TAFB, SAB, and UW-CIMSS ADT and SATCON. The initial motion is now west-northwestward or 285/12 kt. Dorian is expected to continue moving west-northwestward today through Tuesday night as the cyclone moves around the southwestern periphery of a deep-layer subtropical ridge situated to its north. On Wednesday, Dorian is forecast to turn northwestward toward a weakness in the ridge, which could allow the cyclone to pass near or between western Puerto Rico and the eastern Dominican Republic. By late Thursday and Friday, all of the model guidance indicates that the ridge will build back as an upper-level trough/low weakens and lifts out to the north, which should result in Dorian turning back toward the west-northwest in the vicinity of the Bahamas. The new NHC track forecast has been nudged slightly to the right or north of the previous one and lies close to the tightly clustered consensus models HCCA, FSSE, TVCN, and TVCX. The intensity forecast is less straight-forward than the track forecast. Environmental conditions, except for the abundance of dry mid-level air surrounding and occasionally being entrained into Dorian's inner-core region, would favor at least steady strengthening due to very low vertical wind shear, SSTs of at least 29C, and the small overall circulation and inner-core wind field. The recent development of a poleward outflow channel and possible development of an equatorward channel would also support strengthening. However, until Dorian closes off a solid eyewall, only slow strengthening is likely. Anticipating when an eye will form is challenging, but Dorian could be a hurricane by the time it reaches the Windward Islands. The intensity forecast has been adjusted upward form the previous advisory, but is not as high as the statistical-dynamical SHIPS and LGEM intensity models. Interaction with Hispaniola and possibly Puerto Rico should result in some weakening in 72-96 hours, followed by restrengthening on day 5 when Dorian will be moving over the very warm waters in the Bahamas in low shear conditions. Given the unknown degree of interaction with Hispaniola, the intensity forecast at days 4 and 5 is of very low confidence. Key Messages: 1. Dorian is expected to bring tropical storm conditions to portions of the Lesser Antilles tonight and Tuesday, where tropical storm watches and warnings are in effect. Hurricane conditions are also possible in portions of the Windward Islands, and a hurricane watch has been issued for St. Lucia. Residents in these areas should refer to advice from local government officials and products from their local meteorological service for additional information. 2. Dorian is expected to produce 3 to 8 inches of rainfall from Martinique to St. Vincent, including Barbados, with isolated totals as high as 10 inches in portions of the northern Windward Islands. 3. The risk of direct impacts from wind and rainfall has increased for Puerto Rico and Hispaniola. Interests in those areas should monitor the progress of Dorian and tropical storm or hurricane watches will likely be required later today. 4. Any potential impacts from Dorian in the Bahamas and Florida later this week are highly uncertain, given the potential for the system to interact with the high terrain of Hispaniola. FORECAST POSITIONS AND MAX WINDS INIT 26/1500Z 12.3N 57.7W 50 KT 60 MPH 12H 27/0000Z 12.9N 59.4W 55 KT 65 MPH 24H 27/1200Z 13.9N 61.4W 60 KT 70 MPH 36H 28/0000Z 15.1N 63.5W 65 KT 75 MPH 48H 28/1200Z 16.4N 65.6W 70 KT 80 MPH 72H 29/1200Z 19.2N 69.3W 70 KT 80 MPH...NERN HISPANIOLA 96H 30/1200Z 22.0N 72.6W 40 KT 45 MPH 120H 31/1200Z 24.8N 76.5W 45 KT 50 MPH $$ Forecaster Stewart
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