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Potential Tropical Cyclone Six Forecast Discussion Number 3
2021-08-10 10:50:31| National Hurricane Center (Atlantic)
Issued at 500 AM AST Tue Aug 10 2021 000 WTNT41 KNHC 100850 TCDAT1 Potential Tropical Cyclone Six Discussion Number 3 NWS National Hurricane Center Miami FL AL062021 500 AM AST Tue Aug 10 2021 The disturbance moved through the southern Leeward Islands a few hours ago. Based on surface observations from the islands, the system still does not have a well-defined circulation. Although the satellite appearance shows some increase in organization, the surface data suggest no significant strengthening has occurred thus far. The current intensity estimate is held at 30 kt in agreement with a Dvorak estimate from SAB. A NOAA Hurricane Hunter aircraft is scheduled to investigate the disturbance later this morning and should provide an updated intensity estimate, and also determine whether the system has become a tropical cyclone. The disturbance continues moving west-northwestward, or about 295/15 kt. The flow on the south side of a strong subtropical ridge should maintain the west-northwestward track for the next 72 hours or so, with some slowing of forward speed by midweek. Beyond that time, a turn toward the northwest with further deceleration is likely while the system moves near the western periphery of the ridge. The official track forecast is very similar to the previous one and in good agreement with the latest simple and corrected multi-model consensus tracks. Users are reminded that the average track errors at days 4 and 5 are 175 miles and 200 miles, respectively. Dynamical conditions appear to be conducive for intensification during the next 24 hours or so, with low vertical shear and pronounced upper-level outflow over the system. The main hindrance for strengthening today and tonight appears to be some dry mid-tropospheric air in the environment, as shown by the global models. Since the projected track takes the disturbance or cyclone over the mountainous land mass of Hispaniola, some weakening and disruption of the circulation is likely in 36 hours or so. Later in the forecast period, the dynamical guidance shows some increase in westerly shear which could also inhibit strengthening. The official intensity forecast closely follows the NOAA corrected consensus forecast technique, HCCA. There is significant uncertainty in 4-5 day intensity forecasts, however. KEY MESSAGES: 1. The system is forecast to become a tropical storm as it moves over the northeastern Caribbean Sea today. Tropical storm conditions are expected in portions of the U.S. Virgin Islands and Puerto Rico beginning this afternoon, and in the Dominican Republic by Wednesday. Tropical storm conditions are possible in northern Haiti and the southeastern Bahamas by late 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 elsewhere in portions of Hispaniola, the 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 10/0900Z 15.7N 62.2W 30 KT 35 MPH...POTENTIAL TROP CYCLONE 12H 10/1800Z 16.7N 64.6W 40 KT 45 MPH...TROPICAL CYCLONE 24H 11/0600Z 17.9N 67.4W 45 KT 50 MPH 36H 11/1800Z 19.0N 70.1W 30 KT 35 MPH...INLAND 48H 12/0600Z 20.1N 72.6W 30 KT 35 MPH 60H 12/1800Z 21.1N 74.9W 30 KT 35 MPH 72H 13/0600Z 22.0N 77.0W 35 KT 40 MPH 96H 14/0600Z 24.4N 80.8W 40 KT 45 MPH 120H 15/0600Z 26.8N 83.1W 50 KT 60 MPH $$ Forecaster Pasch
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