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Potential Tropical Cyclone Six Forecast Discussion Number 5
2021-08-10 23:07:49| National Hurricane Center (Atlantic)
Issued at 500 PM AST Tue Aug 10 2021 965 WTNT41 KNHC 102107 CCA TCDAT1 Potential Tropical Cyclone Six Discussion Number 5...Corrected NWS National Hurricane Center Miami FL AL062021 500 PM AST Tue Aug 10 2021 Corrected speed of motion in second paragraph. Satellite imagery this afternoon continues to show that the disturbance has an organized convective pattern, with satellite intensity estimates of tropical-storm strength from SAB and TAFB. However, the circulation still appears to not be well defined, with the San Juan WSR-88D Doppler radar showing multiple mid-level centers and several convective cells with small-scale rotation. Based on this, the system will not be upgraded to a tropical cyclone at this time. The initial intensity remains 30 kt based on mainly on continuity from earlier data. It should be noted that squalls with short-lived winds to tropical-storm force have been reported in bands over the northern Leeward Islands. The initial motion remains west-northwestward or 295/15 kt. A strong low- to mid-level ridge to the north should steer the system generally west-northwestward for the next 72 h with some decrease in forward speed after 24 h. Beyond that time, a turn toward the northwest with an further decrease in forward speed is likely while the system moves near the western periphery of the ridge. There has been some increase in the spread of the track guidance from 72-120 h, with the GFS shifting to the right and the other models not changing very much. The new NHC forecast track has only minor changes from the previous one and lies near the various consensus models. Conditions continue to appear favorable for strengthening before the system reaches Hispaniola in about 18 h, although there may be dry air entraining into the system in the southeastern quadrant. The intensity forecast will follow the previous prediction in calling for the disturbance to become a tropical storm before landfall, followed by weakening to a depression. From 24-60 h, the models have come into better agreement that the system will encounter westerly vertical wind shear, and based on that and the possibility of land interaction the intensity forecast shows little re-intensification during that time. After 60 h, the models still disagree on the evolution of the upper-level winds near the system, but they are in better agreement that conditions over the eastern Gulf will become more conducive for development. Thus, the intensity forecast calls for intensification during that time. The new intensity forecast is almost the same as the previous one. KEY MESSAGES: 1. The system is forecast to become a tropical storm as it moves over the northeastern Caribbean Sea tonight. Tropical storm conditions are expected in portions of the U.S. Virgin Islands and Puerto Rico tonight, and are expected to begin 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, along with possible rapid river rises and the potential for mudslides across the U.S. Virgin Islands, Puerto Rico, and the Dominican Republic. The greatest threat for flooding impacts will be across the eastern and southern portions of Puerto Rico. 3. There is a risk of wind and rainfall impacts elsewhere in portions of Hispaniola, the Bahamas, and Cuba during the next several days, 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. There is a risk of wind and rainfall impacts in portions of Florida beginning Friday through the weekend. However, given the uncertainty in the long-range forecast it is too soon to determine the timing, location, and magnitude of any potential impacts. Interests in Florida should monitor updates to the forecast. FORECAST POSITIONS AND MAX WINDS INIT 10/2100Z 16.9N 65.5W 30 KT 35 MPH...POTENTIAL TROP CYCLONE 12H 11/0600Z 17.8N 67.8W 35 KT 40 MPH...TROPICAL CYCLONE 24H 11/1800Z 19.0N 70.4W 35 KT 40 MPH...INLAND 36H 12/0600Z 20.2N 72.8W 30 KT 35 MPH...OVER WATER 48H 12/1800Z 21.2N 75.0W 30 KT 35 MPH 60H 13/0600Z 22.1N 77.2W 35 KT 40 MPH 72H 13/1800Z 23.1N 79.1W 35 KT 40 MPH 96H 14/1800Z 25.5N 82.5W 40 KT 45 MPH 120H 15/1800Z 28.5N 84.5W 50 KT 60 MPH $$ Forecaster Beven
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