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Potential Tropical Cyclone Nine Forecast Discussion Number 2
2020-07-28 22:40:20| National Hurricane Center (Atlantic)
Issued at 500 PM AST Tue Jul 28 2020 000 WTNT44 KNHC 282040 TCDAT4 Potential Tropical Cyclone Nine Discussion Number 2 NWS National Hurricane Center Miami FL AL092020 500 PM AST Tue Jul 28 2020 The overall structure of the disturbance has not changed much since this morning. New clusters of convection have developed over the northern portion of the elongated circulation and the Air Force Reserve reconnaissance aircraft has been unable to find a well-defined center. The aircraft measured some SFMR winds of 30-35 kt well to the northwest of the trough axis, and these data along with the earlier ASCAT form the basis of the 35 kt initial intensity. The lastest dynamical model guidance still suggests that the system will consolidate over the next 12-18 hours and the disturbance is expected to become a tropical storm before it reaches the Leeward Islands on Wednesday. The particulars of the track forecast remain uncertain since the system lacks a well-defined center. The initial motion estimate is a highly uncertain 285/20 kt. The overall track foreast reasoning remains the same, with the disturbance expected to move west-northwestward over the next several days to the south of a subtropical ridge that extends westward over the western Atlantic. The 12Z track guidance is in agreement on the overall scenario, but some differences are noted due primarily to the system's strength and vertical depth. Models such as the UKMET and HWRF, which depict a stronger cyclone, are along the northern side of the guidance envelope, while the weaker solutions remain more equatorward. The updated NHC track forecast is similar to the previous advisory and lies a little south of the various consensus aids. Regardless of the exact track, the system is expected to bring locally heavy rainfall to much of the Lesser Antilles, and tropical-storm-force winds to portions of the Leeward Islands, the Virgin Islands, and Puerto Rico within the next 24-36 hours. The global model guidance suggests that the system will consolidate overnight and should acquire a better defined center. As this occurs, the disturbance is likely to gradually strengthen within the low vertical wind shear environment that it is situated in. Between 36 and 72 hours, the strength of the system will largely be dependent on the amount of land it encounters. If the system moves over the Greater Antilles it is likely to be weaker than indicated in the official forecast, but a path north or south of Hispaniola could result in a stronger system. Later in the period, some southwesterly shear could also temper further strengthening. The GFS and ECMWF models generally weaken the system in the longer range due to these negative factors. The NHC intensity forecast is unchanged from the previous advisory, but an usually high degree of uncertainty regarding the future track and intensity of the system remains. Interests in Hispaniola, the Bahamas, Cuba, and Florida should continue to monitor forecasts as changes to both track and intensity are likely. Key Messages: 1. Potential Tropical Cyclone Nine will produce heavy rains and potentially life-threatening flash flooding and mudslides across the northern Leeward Islands, the Virgin Islands, Puerto Rico, and the Dominican Republic. 2. Tropical storm conditions are likely across portions of the Leeward Islands, the Virgin Islands, Puerto Rico, and portions of the Dominican Republic beginning Wednesday and spreading westward through Thursday. Tropical Storm Warnings are in effect for these areas. Do not focus on the details of the track forecast, as rainfall and wind hazards will extend far from the center of the system. 3. The details of the long-range track and intensity forecasts are more uncertain than usual since the system does not have a well-defined center and it could move over portions of the Greater Antilles later this week. However, this system could bring some rainfall and wind impacts to portions of Hispaniola, Cuba, the Bahamas, and Florida by the end of the week. Interests there should monitor its progress and updates to the forecast over the next few days. FORECAST POSITIONS AND MAX WINDS INIT 28/2100Z 14.4N 55.9W 35 KT 40 MPH...POTENTIAL TROP CYCLONE 12H 29/0600Z 15.4N 58.8W 35 KT 40 MPH 24H 29/1800Z 16.7N 62.5W 40 KT 45 MPH 36H 30/0600Z 18.2N 66.1W 45 KT 50 MPH 48H 30/1800Z 19.7N 69.7W 50 KT 60 MPH 60H 31/0600Z 20.9N 72.5W 50 KT 60 MPH 72H 31/1800Z 22.2N 75.3W 50 KT 60 MPH 96H 01/1800Z 24.7N 79.0W 50 KT 60 MPH 120H 02/1800Z 27.0N 81.0W 45 KT 50 MPH...INLAND $$ Forecaster Brown
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