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Potential Tropical Cyclone Six Forecast Discussion Number 1
2021-08-09 23:01:27| National Hurricane Center (Atlantic)
Issued at 500 PM AST Mon Aug 09 2021 000 WTNT41 KNHC 092101 TCDAT1 Potential Tropical Cyclone Six Discussion Number 1 NWS National Hurricane Center Miami FL AL062021 500 PM AST Mon Aug 09 2021 Deep convection associated with the area of low pressure located northeast of Barbados has continued to consolidate this afternoon, with several bands noted in both satellite and radar data from Barbados and Martinique. ASCAT data from this morning revealed a sharp trough axis, but the system lacked a well-defined circulation. However, recent visible satellite imagery hints that a better defined center may be forming just southwest of the primary convective mass. These trends suggest the system is likely to become a tropical depression or storm tonight or Tuesday when it moves near the Lesser Antilles and into the eastern Caribbean. Therefore, advisories are being initiated on the system in order to issue Tropical Storm Watches for portions of the Lesser Antilles, the U.S. Virgin Islands, Puerto Rico and parts of the Dominican Republic. Since the system is still in its formative stage the initial motion estimate is a somewhat uncertain 290/13 kt. A mid-tropospheric ridge anchored over the western Atlantic should steer the system west-northwestward through the forecast period. Although there is high confidence in the overall steering pattern over the next several days, there is lower than normal confidence in the details of the track forecast, especially in the short-range due to the lack of a well-defined center. Exactly where the center forms will have some downstream implications on the exact forecast track, especially across the eastern Caribbean. Regardless of the exact track, locally heavy rainfall and gusty winds are expected to spread across portions of the Leeward Islands, the Virgin Islands, Puerto Rico, and the Dominican Republic over the next day or two, hence the need for Tropical Storm Watches for portions of those areas. The disturbance is embedded within an area of low vertical wind shear and SSTs of around 28C. These conditions favor additional development, but the occasional entrainment of dry mid-level air located just west of the system is likely to prevent more rapid organization. By Wednesday, the system is likely to be near Hispaniola, where subtle differences in the forecast track could have large implications on the intensity of the storm later this week. A track directly over Hispaniola would likely significantly disrupt the circulation, while a track more poleward of the island could allow the system to stay more intact. An additional caveat beyond 48 hours is that vertical wind shear out of the southwest may also increase, which could limit the intensity after the system moves past Hispaniola, although uncertainty exists in how much the shear will increase given differences between the more favorable ECMWF and less favorable GFS model solutions. The NHC intensity forecast brings the system up to a 45 kt tropical storm before potential land interaction with Hispaniola and afterwards is conservative given the possibility of additional land interaction and less favorable environmental conditions towards the end of the forecast period. KEY MESSAGES: 1. The system is forecast to become a tropical storm as it moves through the Lesser Antilles tonight. Tropical storm conditions are possible in portions of the southern Leeward Islands tonight and Tuesday and in the Virgin Islands and Puerto Rico beginning Tuesday afternoon, and in the Dominican Republic by Wednesday. 2. Heavy rainfall could lead to flash, urban, and small stream flooding and potential mudslides across the U.S. Virgin Islands and Puerto Rico. The greatest threat for flooding impacts will be across the eastern and southeastern portions of Puerto Rico. 3. There is a risk of wind and rainfall impacts in portions of Hispaniola, the Turks and Caicos, the southeastern Bahamas, and Cuba later this week, although the forecast is more uncertain than usual since the system is still in its formative stage. Interests in these areas should monitor the system's progress and updates to the forecast. 4. Interests in the remainder of the Bahamas and Florida should monitor updates to the forecast for this system, but it is too soon to determine what if any impacts could occur there by late this week or this weekend given the uncertainty in the long-range forecast. FORECAST POSITIONS AND MAX WINDS INIT 09/2100Z 14.2N 59.2W 30 KT 35 MPH...POTENTIAL TROP CYCLONE 12H 10/0600Z 15.4N 61.4W 35 KT 40 MPH...TROPICAL CYCLONE 24H 10/1800Z 16.8N 64.5W 40 KT 45 MPH 36H 11/0600Z 17.9N 67.3W 45 KT 50 MPH 48H 11/1800Z 19.2N 70.2W 35 KT 40 MPH...INLAND 60H 12/0600Z 20.3N 72.8W 35 KT 40 MPH...OVER WATER 72H 12/1800Z 21.1N 75.0W 35 KT 40 MPH 96H 13/1800Z 22.8N 78.8W 40 KT 45 MPH 120H 14/1800Z 24.5N 82.0W 40 KT 45 MPH $$ Forecaster Brown/Papin
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