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Tropical Storm Isaac Forecast Discussion Number 18
2018-09-12 04:59:36| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Tue Sep 11 2018 374 WTNT44 KNHC 120259 TCDAT4 Tropical Storm Isaac Discussion Number 18 NWS National Hurricane Center Miami FL AL092018 1100 PM AST Tue Sep 11 2018 A slew of recent microwave passes have revealed that Isaac's center is outrunning the deep convection by about a degree due to strengthening westerly shear. With the degradation in structure, Dvorak estimates have fallen to T3.0/3.5 from both TAFB and SAB. The initial intensity is being set at 55 kt to match the CI number (3.5), but I wouldn't be surprised if the actual intensity is closer to the final-T number. Isaac is moving quickly westward, or 270/14 kt. Ridging to the north should keep the cyclone on a similar speed and path for much of the forecast period. The only main difference among the track models is Isaac's future speed, and the NHC official forecast continues to favor the faster models--the GFS, ECMWF, and HCCA models. Since the forecast thinking is unchanged, the updated NHC forecast is not too different from the previous one. Isaac's center is expected to move across the Lesser Antilles near Martinique and Dominica on Thursday. Vertical shear is expected to increase and turn more northwesterly during the next 36 hours as Isaac moves closer to the base of an upper-level trough which extends northeast of the Leeward Islands. Therefore, deep convection is expected to remain decoupled from the low-level center, and Isaac's structure could degrade further over the next day or two. The cyclone's maximum winds are also expected to decrease, and the NHC official forecast is close to the HCCA, Florida State Superensemble, and ICON intensity consensus aids. Even though the statistical-dynamical models and the HWRF restrengthen Isaac by days 4 and 5 when the shear decreases, there may not be much left of the cyclone for any strengthening to occur. Both the GFS and ECMWF global models show Isaac weakening to a remnant low or opening up into a trough by days 4 or 5, which is the scenario favored by the NHC official forecast. As such, dissipation is now shown in the forecast at day 5. Key Message: 1. Isaac is expected to remain at tropical storm intensity when it moves across the Lesser Antilles on Thursday, and tropical storm warnings have been issued for Martinique, Dominica, and Guadeloupe. Tropical storm watches are in effect for Montserrat, St. Kitts and Nevis, Antigua, and Saba and St. Eustatius. Interests on those islands should follow any advice given by their local officials. FORECAST POSITIONS AND MAX WINDS INIT 12/0300Z 14.5N 52.3W 55 KT 65 MPH 12H 12/1200Z 14.6N 54.7W 50 KT 60 MPH 24H 13/0000Z 14.7N 57.4W 50 KT 60 MPH 36H 13/1200Z 14.9N 60.4W 45 KT 50 MPH 48H 14/0000Z 15.1N 63.7W 45 KT 50 MPH 72H 15/0000Z 15.2N 69.7W 40 KT 45 MPH 96H 16/0000Z 15.0N 74.5W 35 KT 40 MPH 120H 17/0000Z...DISSIPATED $$ Forecaster Berg
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