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Hurricane Isaac Forecast Discussion Number 10
2018-09-10 04:56:17| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Sun Sep 09 2018 684 WTNT44 KNHC 100256 TCDAT4 Hurricane Isaac Discussion Number 10 NWS National Hurricane Center Miami FL AL092018 1100 PM AST Sun Sep 09 2018 Isaac's IR satellite presentation has generally improved since the last advisory. While there is no evidence of an eye in conventional satellite imagery, the most recent microwave imagery around 2100 UTC indicated the presence of a pinhole eye. Dvorak intensity estimates are T4.0 from both TAFB and SAB, and on this basis Isaac has been upgraded to a hurricane, the 5th of the 2018 Atlantic season and 3rd in the past few days. I must stress that all aspects of the forecast are highly uncertain at this time. The dynamical track model spread is nearly 1000 miles at day 5, while the intensity guidance spread is nearly 100 kt. Although the NHC track and intensity forecasts have not been changed very much, this should not be interpreted as increasing confidence in the forecast. With regard to track, there is a clear split in the model guidance regarding the speed of Isaac. The GFS and ECMWF models forecast that a mid-level ridge to the north will accelerate the hurricane westward for the next 36 h, with a westward motion continuing through day 5 as the ridge is reinforced in the wake of Florence. On the other hand, the UKMET and HWRF models show a weakness developing in the ridge which causes Isaac to move much slower, and eventually turn northward. The NHC forecast has been favoring the former solution, and continues to do so with this advisory since I see no reason to make a drastic change to the forecast at this point. Isaac is a very small hurricane, which makes the intensity forecast particularly tricky since small storms are susceptible to rapid changes of intensity, up and down. Since the wind shear is expected to remain low, and Isaac is forecast to remain over fairly warm SSTs for the next 48 h, the current forecast of further intensification seems reasonable, and the NHC forecast is still at the top end of the guidance envelope. From 72 h onward, there could be a sudden, and substantial increase in wind shear associated first with an upper-level trough to the north, and later with the outflow of Hurricane Florence. Assuming this shear affects Isaac as expected, the tiny hurricane would likely weaken quickly, perhaps even faster than currently shown in the forecast. By the end of the forecast period, the NHC forecast remains a fair amount below the intensity consensus, closer to the weakening solutions of the GFS and ECMWF. Key Messages: 1. Isaac is a small hurricane and uncertainty in the forecast is higher than normal. Although Isaac is forecast to begin weakening by Tuesday when it approaches the Lesser Antilles, it is still expected to be at or near hurricane intensity when it reaches the islands. 2. Interests in the Lesser Antilles should continue to monitor Isaac during the next few days. FORECAST POSITIONS AND MAX WINDS INIT 10/0300Z 14.5N 41.6W 65 KT 75 MPH 12H 10/1200Z 14.7N 43.5W 75 KT 85 MPH 24H 11/0000Z 14.9N 46.3W 80 KT 90 MPH 36H 11/1200Z 15.0N 49.3W 85 KT 100 MPH 48H 12/0000Z 15.0N 52.0W 85 KT 100 MPH 72H 13/0000Z 15.2N 57.3W 75 KT 85 MPH 96H 14/0000Z 15.5N 63.5W 60 KT 70 MPH 120H 15/0000Z 15.5N 70.0W 50 KT 60 MPH $$ Forecaster Zelinsky
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