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Subtropical Storm Leslie Forecast Discussion Number 11
2018-09-29 04:52:18| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Fri Sep 28 2018 020 WTNT43 KNHC 290252 TCDAT3 Subtropical Storm Leslie Discussion Number 11 NWS National Hurricane Center Miami FL AL132018 1100 PM AST Fri Sep 28 2018 Convective banding has been consolidating to the south of Leslie's center since the afternoon, but recent scatterometer data indicate that the maximum winds have decreased to 40 kt and the area of gale-force winds has decreased in size. The convective structure suggests that Leslie may be taking on some tropical characteristics, but since wave vapor imagery still shows the cyclone embedded within a complex deep-layer low, Leslie is still being designated as subtropical. Leslie is moving west-southwestward, or 255 degrees at 10 kt. Leslie is entrenched between several mid-tropospheric highs located to its east and west, and these features are expected to push Leslie slowly southwestward for the next 3 days. The track guidance is tightly clustered during this period, and the NHC forecast is very similar to the previous one. After day 3, Leslie is likely to meander on days 4 and 5 in weak steering, and the updated official forecast has been adjusted southward and eastward at the end of the forecast period to account for the latest model solutions. Global model fields indicate that Leslie has migrated to the northwest of its parent upper-level low, which is putting it under a regime of moderate north-northeasterly shear. For the next 48 hours, this shear is expected to continue, and phase-space diagrams suggest that Leslie will be straddling the line between shallow and deep warm core. As a result, only modest strengthening is anticipated during this period, and the official forecast maintains Leslie as a subtropical storm through 48 hours. However, the transition to a tropical storm could occur any time during the next day or two. After 48 hours, Leslie should definitely be deep warm core, and more significant strengthening is expected, with the cyclone forecast to reach hurricane intensity by day 4. This scenario is shown by the various intensity models, and the NHC intensity forecast is very close to the HFIP Corrected Consensus aid and the Florida State Superensemble. If the statistical- dynamical models are correct, Leslie could be stronger by the end of the forecast period than is indicated in the NHC forecast. Large swells generated by Leslie when it was a stronger extratropical low have already reached Bermuda, will soon reach the Lesser and Greater Antilles, and should reach portions of the east coast of the United States later this weekend. These swells could cause life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 29/0300Z 35.7N 49.4W 40 KT 45 MPH 12H 29/1200Z 34.9N 50.8W 45 KT 50 MPH 24H 30/0000Z 34.0N 52.0W 45 KT 50 MPH 36H 30/1200Z 33.5N 53.0W 50 KT 60 MPH 48H 01/0000Z 33.1N 53.9W 50 KT 60 MPH 72H 02/0000Z 32.2N 55.4W 60 KT 70 MPH...TROPICAL CYCLONE 96H 03/0000Z 30.5N 56.0W 65 KT 75 MPH 120H 04/0000Z 31.5N 56.5W 70 KT 80 MPH $$ Forecaster Berg
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