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Potential Tropical Cyclone Seven Forecast Discussion Number 1
2018-09-02 22:46:46| National Hurricane Center (Atlantic)
Issued at 500 PM EDT Sun Sep 02 2018 713 WTNT42 KNHC 022046 TCDAT2 Potential Tropical Cyclone Seven Discussion Number 1 NWS National Hurricane Center Miami FL AL072018 500 PM EDT Sun Sep 02 2018 The area of disturbed weather associated with a tropical wave that the NHC has been tracking for the past few days has become better organized today. Strong winds previously associated with an upper- level trough just west of the system have decreased significantly during the day, and the upper-level flow across the disturbance has become more anticyclonic. Earlier scatterometer wind data indicated that the system does not yet have a closed surface circulation. However, the same data indicate that winds of 25-28 kt exist outside of thunderstorm activity, with some higher gusts likely present. The initial intensity is being set at a conservative 25 kt for this advisory. The initial motion estimate is an uncertain 300/13 kt. The global models are in excellent agreement that the strong subtropical ridge to the north of the disturbance will remain entrenched across the southeastern U.S. and mid-Atlantic states throughout the forecast period. This flow pattern should keep the system moving in a west- northwestward to northwestward motion until landfall occurs along the central Gulf coast in 60-72 hours. The model tracks are tightly packed, so the NHC official forecast track essentially lies down the middle of the guidance envelope. On the forecast track, the system is expected to move across the Florida Keys Monday afternoon, and reach the central Gulf Coast by Tuesday night or Wednesday morning. The aforementioned upper-level anticyclonic has been steadily increasing during the day today, with weak cirrus outflow now occurring in all quadrants. The disturbance is forecast to move across the very warm waters of the Gulfstream late tonight and early Monday morning where local diabatic heating should enhance deep convection near the mid-level circulation, causing the vortex column to build downward to the surface. Once a closed surface circulation develops, the combination of low to modest vertical shear and SSTs of at least 30C along the track should allow for at least slow but steady strengthening. Although the official intensity forecast shows weakening at 72 hours, this is due to the system expected to be inland at that time. Conditions will favorable for continued strengthening after the 48-h period until landfall occurs, and a peak intensity of around 55 kt around 60 hours is possible. The SHIPS and LGEM models were the only intensity guidance available for this package, and the official forecast is just a little below an average of those models. The HWRF and HMON models will be forthcoming for the next advisory package, so some adjustments to this first intensity forecast may be required as more guidance becomes available. Key Messages: 1. Potential Tropical Cyclone Seven will bring heavy rainfall and gusty winds to portions of the Bahamas, South Florida, and the Florida Keys tonight and Monday, and interests in those areas should monitor the progress of this system. 2. A Tropical Storm Watch has been issued for portions of the central Gulf Coast, and tropical storm conditions are possible in those areas Tuesday night and Wednesday. Heavy rainfall from this system will affect portions of the central Gulf Coast later this week, including areas that are currently receiving heavy rainfall from a different weather system. Interests in these areas should monitor products from their local National Weather Service office. 3. Users should be aware that forecast uncertainty for potential tropical cylones is generally larger than that for tropical cyclones, especially beyond 48-72 hours. FORECAST POSITIONS AND MAX WINDS INIT 02/2100Z 22.7N 77.3W 25 KT 30 MPH...POTENTIAL TROP CYCLONE 12H 03/0600Z 23.7N 79.3W 30 KT 35 MPH...TROPICAL DEPRESSION 24H 03/1800Z 25.1N 81.9W 35 KT 40 MPH 36H 04/0600Z 26.6N 84.5W 40 KT 45 MPH 48H 04/1800Z 28.0N 87.1W 50 KT 60 MPH 72H 05/1800Z 30.6N 91.6W 35 KT 40 MPH...INLAND 96H 06/1800Z 33.0N 94.0W 25 KT 30 MPH...INLAND 120H 07/1800Z 34.0N 95.5W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Stewart
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