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| #892660 (Received by flhurricane at: 4:55 PM 19.Jun.2017) TCDAT3 Potential Tropical Cyclone Three Discussion Number 1 NWS National Hurricane Center Miami FL AL032017 400 PM CDT Mon Jun 19 2017 A pair of ASCAT passes between 15 and 16Z showed that the disturbance over the Gulf of Mexico has an area of tropical-storm- force winds within a large area of deep convection 100 to 150 n mi northeast of a broad low center. The ASCAT data and visible satellite imagery show that the center is not well defined, and in fact multiple low-level swirls are evident in the latest imagery. An Air Force Reserve Hurricane Hunter aircraft is currently investigating the system and will provide more data on its wind structure. The NWS now has the option to issue advisories, watches, and warnings for disturbances that are not yet a tropical cyclone, 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 central Gulf coast, advisories have been initiated on Potential Tropical Cyclone Three and the appropriate watches/warnings have been issued. Users should be aware that forecast uncertainty for disturbances is generally larger than for tropical cyclones, especially beyond 48-72 hours. The initial intensity is set to 35 kt based on the ASCAT data. The initial pressure is set to 1002 mb based on extrapolated data from the aircraft. Only modest strengthening is expected given the moderate to strong southwesterly shear of around 30 kt expected to persist over the system during the next couple of days. The NHC intensity forecast is close to the latest intensity consensus aid. Given the broad nature of the low, the initial motion estimate is a rather uncertain 350/08. During the first 12 to 24 hours the system will move northward to north-northwestward due to the a weakness in the subtropical ridge over the northern Gulf and then the system should slow and turn more northwestward as it interacts with an upper-level low currently situated off the Texas coast. The details of this interaction will have significant implications on the track of the system, and there remains a fair amount of model spread. The HWRF is on the right side of the guidance envelope, with a track toward southeastern Louisiana while the UKMET and ECMWF show a solution toward southwest Louisiana or extreme eastern Texas. For now, the NHC forecast lies in the middle of the guidance envelope and a little left of the TCVA consensus aid. Given the disorganized nature of the circulation and the fact that the wind and rain hazards extend well north and east of the center, users are encouraged to not focus on the details of the track forecast. The primary hazard from this disturbance is expected to be heavy rainfall over portions of the central and eastern Gulf Coast. FORECAST POSITIONS AND MAX WINDS INIT 19/2100Z 24.7N 88.7W 35 KT 40 MPH...POTENTIAL TROP CYCLONE 12H 20/0600Z 26.0N 89.5W 35 KT 40 MPH 24H 20/1800Z 27.2N 90.6W 40 KT 45 MPH...TROPICAL CYCLONE 36H 21/0600Z 27.9N 91.4W 40 KT 45 MPH 48H 21/1800Z 28.7N 92.3W 40 KT 45 MPH 72H 22/1800Z 32.0N 93.5W 30 KT 35 MPH...INLAND 96H 23/1800Z 35.5N 90.5W 25 KT 30 MPH...POST-TROP/INLAND 120H 24/1800Z...DISSIPATED $$ Forecaster Brennan |
