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Tropical Storm Isaac Forecast Discussion Number 16
2018-09-11 16:58:31| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Tue Sep 11 2018 079 WTNT44 KNHC 111458 TCDAT4 Tropical Storm Isaac Discussion Number 16 NWS National Hurricane Center Miami FL AL092018 1100 AM AST Tue Sep 11 2018 One-minute visible satellite data from GOES-16 indicate that the center of Isaac remains on the westward edge of the central dense overcast. The initial wind speed is kept at 60 kt, which is a compromise between the lower subjective estimates and higher ADTs and microwave estimates. This is a more uncertain estimate than normal due to the spread of the intensity fixes. The initial wind and 12 ft seas radii have been expanded out to the north based on TAFB estimates and microwave data. The intensity forecast continues to be tricky. Westerly shear should increase over the next couple of days, which would normally cause some weakening. However this effect could be tempered by increasing SSTs, mid-level moisture and instability in the path of the storm. Given our limited skill in forecasting intensity changes in marginal environments, plus the 5 or 10 kt of initial wind speed uncertainty, the official forecast will show no significant change in intensity for the next couple of days. Because of these uncertainties, a Hurricane Watch has been issued for many of the Leeward Islands, even though Isaac is not explicitly forecast to be a hurricane. Some weakening is indicated at long range after considering the global model trends in intensity, which have done fairly well for Isaac. The latest initial motion estimate is 270/14. Isaac is forecast to move westward at about the same speed for the next few days due to a mid-level ridge over the central Atlantic Ocean. Other than the UKMET model, there is not much spread in the guidance, except with the forward speed. Given the strength of the ridge, the cyclone isn't likely to slow down much, so the official forecast is basically along the previous forecast track but faster. The uncertainty increases at long range, and is somewhat dependent on the intensity of Isaac, but faster seems to be way to go given the weakening trend anticipated at that time. Key Messages: 1. Isaac is expected to be at or near hurricane intensity when it approaches the Lesser Antilles on Thursday, and hurricane and tropical storm watches have been issued for several islands. Interests in those areas should follow any advice given by their local officials. 2. Interests elsewhere in the Lesser Antilles should continue to monitor Isaac during the next couple of days, as additional watches could be needed for other islands. FORECAST POSITIONS AND MAX WINDS INIT 11/1500Z 14.6N 49.7W 60 KT 70 MPH 12H 12/0000Z 14.6N 51.8W 60 KT 70 MPH 24H 12/1200Z 14.7N 54.4W 65 KT 75 MPH 36H 13/0000Z 14.9N 57.1W 60 KT 70 MPH 48H 13/1200Z 15.1N 59.9W 60 KT 70 MPH 72H 14/1200Z 15.5N 65.5W 55 KT 65 MPH 96H 15/1200Z 15.5N 71.0W 50 KT 60 MPH 120H 16/1200Z 15.5N 75.0W 50 KT 60 MPH $$ Forecaster Blake
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