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| #892557 (Received by flhurricane at: 5:04 PM 18.Jun.2017) TCDAT2 Potential Tropical Cyclone Two Discussion Number 1 NWS National Hurricane Center Miami FL AL022017 500 PM AST Sun Jun 18 2017 The disturbance over the deep tropical Atlantic has become better organized today, although a pair of ASCAT-A and B passes this morning indicated that the system does not have a closed low-level circulation, which isn`t surprising given the fast translational speed of the system. The ASCAT passes showed peak winds of near 30 kt well north of the center, and given that the environment is expected to be conducive for some additional development before the shear increases in the eastern Caribbean Sea, strengthening is forecast, and the disturbance is expected to become a tropical storm before reaching the Windward Islands in 36 to 48 hours. The National Hurricane Center now has the option to issue advisories on disturbances that are not yet tropical cyclones, but which pose the threat of bringing tropical storm or hurricane conditions to land areas within 48 hours. Under previous policy this was not possible. These systems are known as Potential Tropical Cyclones in advisory products and are numbered from the same list as depressions. Because of the threat to the Windward Islands, advisories have been initiated on Potential Tropical Cyclone Two and the appropriate warnings have been issued by the respective governments in the Windward Islands. Advisory packages will continue until the threat of tropical-storm-force winds for land areas sufficiently diminishes, although if the system becomes a tropical cyclone, the normal rules for discontinuing advisories would apply. Users should be aware that forecast uncertainty for disturbances is generally larger than for tropical cyclones, especially beyond 48-72 hours. The disturbance will be steered quickly westward to west- northwestward over the next several days to the south of a mid-level subtropical ridge over the central Atlantic. The NHC track forecast is close to a blend of the latest GFS and ECMWF tracks. There is a fair bit of across-track spread in the model guidance, which isn`t surprising given the lack of a well-defined center. However, given that tropical storm conditions are expected to extend well north of the center, users should not focus on the exact track of the center, and the track forecast is of lower certainty than usual. Most of the intensity guidance shows mostly modest strengthening during the next 36 hours, with the system moving over SSTs of 27-28C and the deep-layer shear at 10 kt or less. The NHC forecast shows the system peaking at 45 kt, in agreement with the intensity model consensus. Quick weakening is expected after that time as southerly shear increases dramatically while the system moves into the eastern Caribbean Sea. The system is forecast to become a remnant low by 72 hours and dissipate by day 4. FORECAST POSITIONS AND MAX WINDS INIT 18/2100Z 7.5N 50.4W 30 KT 35 MPH...POTENTIAL TROP CYCLONE 12H 19/0600Z 8.2N 53.3W 30 KT 35 MPH 24H 19/1800Z 9.5N 57.0W 35 KT 40 MPH...TROPICAL CYCLONE 36H 20/0600Z 10.6N 60.5W 45 KT 50 MPH 48H 20/1800Z 11.9N 64.0W 40 KT 45 MPH 72H 21/1800Z 13.5N 71.5W 30 KT 35 MPH...POST-TROP/REMNT LOW 96H 22/1800Z...DISSIPATED $$ Forecaster Brennan |
