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Potential Tropical Cyclone Seven Forecast Discussion Number 1
2021-08-13 17:02:17| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Fri Aug 13 2021 000 WTNT42 KNHC 131502 TCDAT2 Potential Tropical Cyclone Seven Discussion Number 1 NWS National Hurricane Center Miami FL AL072021 1100 AM AST Fri Aug 13 2021 A tropical wave and small area of low pressure moving quickly westward across the central tropical Atlantic has been producing bursts of deep convection near and to the west of the wave axis. Scatterometer data from last evening indicated that the system did not quite have a closed circulation, but it is close. In addition, the convective organization has increased a bit since yesterday, with TAFB and SAB providing Dvorak classification of T1.5 and T2.0. Therefore, only an additional slight increase in convective organization and the low-level circulation closing off would make the system a tropical depression, and it could be at tropical storm strength when it approaches the Leeward Islands during the next couple of days. Therefore, advisories are being initiated on the system in order to issue Tropical Storm Watches for portions of the Leeward Islands at this time. Subtropical ridging extending across nearly the entire Atlantic is pushing the low along quickly toward the west, or 280/18 kt. In general this ridging is expected to persist into the middle part of next week. However, around the Sunday-Monday time frame, some mid-level troughing over the western Atlantic could allow the system to gain a little more latitude before it runs into additional ridging located near the east coast of the United States at the end of the forecast period. The GFS, which has perhaps one of the strongest solutions, is a northern outlier and shows the system turning farther into the weak troughing, while the ECMWF has a weaker solution and keeps the system on a westward track across the Lesser and Greater Antilles. At this time, the NHC official forecast lies close to the TVCN and HCCA consensus aids, and it is also relatively close to the ECMWF ensemble mean, which is farther north than its parent model. The biggest negative to the system becoming a tropical cyclone and strengthening is its fast motion of 15-20 kt. However, that motion is expected to gradually decrease in 2 to 3 days. At that same time, deep-layer shear has decreased a bit, and the system will be moving over warmer waters and toward a slightly more moist environment. Therefore, gradual strengthening is anticipated, and the NHC intensity forecast closely follows the HCCA and IVCN consensus aids through 72 hours, up until the system is near Hispaniola. After that time, land interaction is likely to disrupt the circulation, and the official forecast is below the model consensus aids on days 4 and 5. Key Messages: 1. Tropical storm conditions are possible in portions of the Leeward Islands late Saturday or early Sunday, and the risk of tropical storm conditions will spread westward to the Virgin Islands and Puerto Rico late Sunday and Sunday night. 2. Heavy rainfall with this system may produce areas of scattered flash flooding over the northern Leeward Islands Saturday into Sunday. 3. There is a risk of wind and rainfall impacts in portions of Hispaniola, the Turks and Caicos Islands, the southeastern Bahamas, and Cuba next week, and interests in those areas should monitor the progress of this system. FORECAST POSITIONS AND MAX WINDS INIT 13/1500Z 15.3N 49.3W 30 KT 35 MPH...POTENTIAL TROP CYCLONE 12H 14/0000Z 15.7N 52.0W 30 KT 35 MPH...TROPICAL DEPRESSION 24H 14/1200Z 16.2N 55.9W 35 KT 40 MPH 36H 15/0000Z 16.7N 59.6W 35 KT 40 MPH 48H 15/1200Z 17.2N 63.0W 40 KT 45 MPH 60H 16/0000Z 17.8N 66.0W 40 KT 45 MPH 72H 16/1200Z 18.4N 68.7W 45 KT 50 MPH...OVER HISPANIOLA 96H 17/1200Z 20.0N 73.5W 35 KT 40 MPH...OVER WATER 120H 18/1200Z 23.0N 77.0W 40 KT 45 MPH $$ Forecaster Berg
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