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Potential Tropical Cyclone Eighteen Forecast Discussion Number 3
2017-10-28 10:39:26| National Hurricane Center (Atlantic)
Issued at 500 AM EDT Sat Oct 28 2017 000 WTNT43 KNHC 280839 TCDAT3 Potential Tropical Cyclone Eighteen Discussion Number 3 NWS National Hurricane Center Miami FL AL182017 500 AM EDT Sat Oct 28 2017 Scatterometer data from several hours ago indicated that the disturbance consists of a broad circulation with a 100-150 n mi wide area of light winds (less than 10 kt) and an equally as large radius of maximum winds. The system still does not appear to have a well-defined center of circulation, and in fact, the ASCAT data also showed that the maximum winds are down to about 30 kt. The associated deep convection is rather disorganized but has been increasing in coverage near the estimated center during the past few hours. While the exact center is difficult to locate, the disturbance's circulation appears to be moving just east of due north, or 010/9 kt. The disturbance is beginning to accelerate in the flow ahead of a deep-layer trough located over the eastern three-quarters of the United States, and this pattern is expected to cause the system to turn northeastward and accelerate further during the next couple of days. Although the track models are in agreement on this general scenario, the overall guidance envelope has shifted a little westward, most likely as a result of a repositioning of the disturbance's current location. The new NHC forecast has therefore been nudged westward as well, lying closest to HCCA and TVCA models through 36 hours. The disturbance is currently located in its best environment shear-wise, but the system's broad and elongated structure is likely delaying intensification. Although the shear will be increasing, it should remain low enough for the next 12-18 hours to support some strengthening if the circulation can tighten up. In addition, upper-level divergence is expected to increase, which should also support some strengthening. Since the disturbance has such a broad circulation, the NHC intensity forecast closely follows the intensity trends of the GFS and is not too different from the previous advisory. The system is now expected to merge with a cold front and be extratropical by 48 hours and then dissipate by 72 hours. Even though the track forecast has shifted a little closer to South Florida and the Florida Keys, the strongest winds are expected to be well to the east and southeast of the center over the Atlantic waters and the Bahamas. Therefore, tropical storm watches or warnings do not appear necessary for Florida at this time. FORECAST POSITIONS AND MAX WINDS INIT 28/0900Z 19.6N 84.2W 30 KT 35 MPH...POTENTIAL TROP CYCLONE 12H 28/1800Z 21.7N 82.9W 35 KT 40 MPH...TROPICAL CYCLONE 24H 29/0600Z 24.9N 80.1W 40 KT 45 MPH 36H 29/1800Z 28.7N 76.5W 45 KT 50 MPH 48H 30/0600Z 34.2N 71.0W 50 KT 60 MPH...POST-TROP/EXTRATROP 72H 31/0600Z...DISSIPATED $$ Forecaster Berg
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