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Tropical Storm Isaac Forecast Discussion Number 15
2018-09-11 10:31:59| National Hurricane Center (Atlantic)
Issued at 500 AM AST Tue Sep 11 2018 608 WTNT44 KNHC 110831 TCDAT4 Tropical Storm Isaac Discussion Number 15 NWS National Hurricane Center Miami FL AL092018 500 AM AST Tue Sep 11 2018 Isaac has not changed much overnight. Satellite images indicate that the strong tropical storm still has a central dense overcast pattern and a limited amount of outer bands. Earlier microwave data indicated that the center was not located in the middle of the convection, however, likely due to some westerly shear. Since the cyclone has changed little during the past several hours, the initial intensity is held at 60 kt, which is an average of the TAFB and SAB Dvorak final T- and CI-numbers. A nearby NOAA buoy shows that the wind field of Isaac is very compact, with tropical-storm- force winds only extending up to 40 n mi from the center. Satellite fixes suggest that Isaac continues to move westward at 13 kt. Although there remains a fair amount of spread in the models, the usually more reliable GFS and ECMWF models continue to show a westward motion during the next several days as the storm moves on the south side of a strengthening mid-level ridge. The NHC track forecast continues to follow that theme, and this forecast lies near a blend of the GFS and ECMWF aids. This forecast is a little slower than the previous one, and takes Isaac across the Lesser Antilles and into the eastern Caribbean in 2 to 3 days. Predicting Isaac's intensity has been challenging. Isaac appeared to have been in relatively favorable conditions during the past day or so, but it did not strengthen. The upper-level pattern should remain relatively favorable for another day, so modest strengthening back to a hurricane is possible during that time. Thereafter, however, an increase in shear first from a trough in the central Atlantic and then from the outflow of Florence should cause some weakening when the storm moves into the Caribbean. The models are in a little better agreement this cycle as the HWRF is no longer showing Isaac becoming a major hurricane. The NHC intensity forecast is very similar to the previous one and is fairly close to the consensus aids. Key Messages: 1. Isaac is expected to be at or near hurricane intensity when it approaches the Lesser Antilles. However, confidence in the forecast is lower than normal. 2. Interests in the Lesser Antilles should continue to monitor Isaac during the next couple of days. Watches will likely be required for portions of the Lesser Antilles later today. FORECAST POSITIONS AND MAX WINDS INIT 11/0900Z 14.6N 48.1W 60 KT 70 MPH 12H 11/1800Z 14.6N 50.1W 60 KT 70 MPH 24H 12/0600Z 14.6N 52.6W 65 KT 75 MPH 36H 12/1800Z 14.8N 55.0W 65 KT 75 MPH 48H 13/0600Z 15.1N 57.6W 60 KT 70 MPH 72H 14/0600Z 15.5N 62.8W 55 KT 65 MPH 96H 15/0600Z 15.6N 67.4W 55 KT 65 MPH 120H 16/0600Z 15.8N 72.0W 50 KT 60 MPH $$ Forecaster Cangialosi
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