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Potential Tropical Cyclone Nine Forecast Discussion Number 3
2019-09-13 10:58:17| National Hurricane Center (Atlantic)
Issued at 500 AM EDT Fri Sep 13 2019 000 WTNT44 KNHC 130858 TCDAT4 Potential Tropical Cyclone Nine Discussion Number 3 NWS National Hurricane Center Miami FL AL092019 500 AM EDT Fri Sep 13 2019 Satellite imagery and surface observations indicate that the tropical disturbance is currently comprised of a trough elongated from west-southwest to east-northeast, with a vorticity center near a loosely curved convective band at the eastern end of the trough. While the system has become a little better organized since the last advisory, it does not yet have a well-defined circulation center. The initial intensity remains 25 kt based on satellite intensity estimates and earlier scatterometer data. The disturbance is now moving a little faster, with the initial motion of 315/5. The system is to the south of a low- to mid-level ridge over the southeastern United States, and it is to the east of a mid- to upper-level low over the central Gulf of Mexico. The large-scale models forecast the ridge to shift eastward into the Atlantic during the next 24-48 h, with a weakness developing near the southeastern coast of the United States and the east coast of the Florida peninsula. The track guidance indicates that the disturbance will move generally northwestward for 48 h or so, followed by a turn to the north and eventually to the northeast as the system moves through the weakness in the ridge. There is some spread in the guidance, with the GFS model taking a weaker system into the Florida peninsula while the UKMET and ECMWF models show a stronger cyclone farther offshore. Overall, there has been an eastward shift of the guidance since the previous advisory, especially after 72 h. The new forecast track is thus also nudged a little to the east and now calls for the system to spend less time over the Florida peninsula than previously forecast. The new forecast track is to the left of the center of the guidance envelope and the consensus models, and additional adjustments may be necessary once the circulation center becomes better defined. The disturbance is currently in an environment of moderate southwesterly shear and upper-level divergence caused by the upper-level low and a shortwave trough extending from the low eastward to the southeastern Bahamas. The shear should gradually diminish during the next couple of days, which should allow the disturbance to develop into a tropical cyclone and intensify. The large-scale models forecast the shear to increase after 72 h as a mid-latitude westerly trough moves through the southeastern United States and approaches the system. The new intensity forecast is similar to the previous forecast through 72 h, then it is stronger than the previous forecast due to the forecast track keeping the system more over water. Key Messages: 1. The system is expected to become a tropical storm and bring tropical-storm-force winds and heavy rainfall to portions of the northwestern Bahamas today and Saturday. Significant storm surge is not expected in the northwest Bahamas from this system. Residents there should follow any advice given by local officials. 2. The system could bring tropical-storm-force winds to portions of the Florida east coast over the weekend. Residents there should monitor the progress of this system and follow any advice given by local officials. 3. Heavy rainfall is possible from Potential Tropical Cyclone Nine across portions of eastern Florida into this weekend, which could produce flash flooding. There is greater uncertainty than usual in heavy rainfall prospects farther north across the Carolinas. 4. Note that forecast uncertainty for these disturbances is generally larger than for tropical cyclones, especially beyond 48-72 hours. FORECAST POSITIONS AND MAX WINDS INIT 13/0900Z 24.6N 75.2W 25 KT 30 MPH...POTENTIAL TROP CYCLONE 12H 13/1800Z 25.1N 76.2W 30 KT 35 MPH...TROPICAL DEPRESSION 24H 14/0600Z 26.0N 77.8W 35 KT 40 MPH 36H 14/1800Z 27.0N 79.3W 40 KT 45 MPH 48H 15/0600Z 28.2N 80.4W 45 KT 50 MPH 72H 16/0600Z 30.3N 81.1W 50 KT 60 MPH 96H 17/0600Z 31.0N 80.5W 50 KT 60 MPH 120H 18/0600Z 32.0N 78.5W 55 KT 65 MPH $$ Forecaster Beven
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