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Potential Tropical Cyclone Nine Forecast Discussion Number 2
2019-09-13 04:57:14| National Hurricane Center (Atlantic)
Issued at 1100 PM EDT Thu Sep 12 2019 000 WTNT44 KNHC 130257 TCDAT4 Potential Tropical Cyclone Nine Discussion Number 2 NWS National Hurricane Center Miami FL AL092019 1100 PM EDT Thu Sep 12 2019 An Air Force Reserve reconnaissance aircraft earlier this afternoon found a closed but very broad cyclonic circulation along with 1000-ft flight-level and SFMR surface winds that supported an intensity of 25 kt. Recent ship reports also support an intensity of 25 kt, although stronger winds could be present in some of the fragmented convective bands located in the northeastern quadrant. The initial motion is an uncertain 310/02 kt. The disturbance has been meandering over the southeastern Bahamas the past several hours due to southwesterly mid-/upper-level winds associated with an east-west oriented shortwave trough impinging on the west side of the circulation. A large upper-low located over the central Gulf of Mexico that has produced this shortwave trough is expected to spin out another trough on Friday that will move across South Florida and the Straits, impinging on the disturbance and hindering any significant northwestward motion. By late Friday and Saturday, however, the upper-level is expected to have moved into the western Gulf of Mexico, ending any negative effects in the disturbance. This expected to result in the formation of a tropical cyclone with a deeper vortex column that should get steered basically northwestward in combined deep-layer southeasterly flow between the upper-low to the west and a ridge to the northeast. The new NHC track forecast is basically just an extension of the previous forecast track, albeit a little slower, and lies down the middle of the widely divergent guidance envelope, close to the various simple consensus models, and to the right of the NOAA-HCCA consensus model. The aforementioned complex steering flow pattern will also play a major role in the intensification process. The two shortwaves troughs are expected to hinder significant convective organization for the next 24-30 hours due to their localized increase in the vertical shear and intrusions of dry air. By 36 hours and beyond, however, those negative factors are forecast to abate, resulting in the disturbance moving underneath an axis of strong upper-level difluence and speed divergence near or just to the east of the Florida east coast. This more favorable upper-level outflow regime should allow for convection to develop and organize fairly quickly around the center before the system reaches the Florida east coast, which should result in the formation of a moderate tropical storm. Although the center is forecast to be inland at 72-96 hours, the center is expected be close enough to the coast to keep tropical-storm-force ongoing over the coastal waters through 120 hours. The official intensity forecast is identical to the previous advisory and follows a blend of the consensus models HCCA and IVCN. Key Messages: 1. The disturbance is expected to become a tropical storm and bring tropical storm force winds to portions of the northwest Bahamas within 36 hours. Note that forecast uncertainty for these disturbances is generally larger than for tropical cyclones, especially beyond 48-72 hours. 2. The system is expected to bring tropical-storm-force winds and heavy rainfall to portions of the northwest Bahamas on Friday 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. 3. The system could bring tropical-storm-force winds and rainfall to portions of the Florida east coast over the weekend. Residents there should monitor the progress of this system. FORECAST POSITIONS AND MAX WINDS INIT 13/0300Z 23.8N 74.5W 25 KT 30 MPH...POTENTIAL TROP CYCLONE 12H 13/1200Z 24.6N 75.6W 30 KT 35 MPH...POTENTIAL TROP CYCLONE 24H 14/0000Z 25.5N 77.2W 30 KT 35 MPH...TROPICAL CYCLONE 36H 14/1200Z 26.5N 78.8W 40 KT 45 MPH 48H 15/0000Z 27.5N 80.2W 45 KT 50 MPH 72H 16/0000Z 29.8N 81.8W 40 KT 45 MPH...INLAND 96H 17/0000Z 30.8N 81.4W 40 KT 45 MPH...INLAND 120H 18/0000Z 31.8N 79.8W 45 KT 50 MPH...OVER WATER $$ Forecaster Stewart
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