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Tropical Storm Isaac Forecast Discussion Number 20
2018-09-12 16:59:38| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Wed Sep 12 2018 829 WTNT44 KNHC 121459 TCDAT4 Tropical Storm Isaac Discussion Number 20 NWS National Hurricane Center Miami FL AL092018 1100 AM AST Wed Sep 12 2018 GOES-16 1-minute satellite imagery along with NOAA Hurricane Hunter data show that the center of Isaac has become fully exposed on the northwestern side of a shrinking area of deep convection. There is still a lot of wind there, however, with the NOAA plane observing SFMR and adjusted flight-level winds to support 50 kt as an initial intensity. An upper-level trough is dropping southward over the central Atlantic, which has increased the shear this morning over Isaac. This high shear is forecast to remain for the next day or two, and it is possible that Isaac will degrade into a tropical wave during that time, although the area of tropical-storm-force winds will likely not go away for a while. In a couple of days, while the wind shear could decrease over the east-central Caribbean Sea, there might not be much of a system left to take advantage of the more conducive conditions. As a compromise, the official forecast shows a slow weakening, in the direction of the model consensus, and continues to show dissipation after 96 hours in line with the GFS/ECMWF guidance. I wouldn't pay much attention to the long range forecast for now, given the uncertainties in Isaac surviving during the next day or two. The low-level center of Isaac has recently accelerated, moving westward at 15 kt, as it decoupled from the slower mid-level flow. Assuming the system stays vertically coherent, a large mid-tropospheric high is forecast to persist for the next several days. This should keep the system moving westward at a somewhat slower pace throughout the forecast period. There isn't much spread in the guidance, and the official forecast is close to the previous one after accounting for the initial position. The long-range track forecast highly depends on the intensity forecast, so it should be considered low confidence. Key Messages: 1. Isaac is expected to remain at tropical storm intensity when it moves across the Lesser Antilles on Thursday, and tropical storm warnings are in effect 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. 2. Life-threatening flash flooding is possible with Isaac. The storm is expected to produce total rainfall accumulations of 2 to 4 inches with isolated amounts up 8 inches across Martinique, Dominica and Guadeloupe. Rainfall amounts of 1 to 2 inches with isolated amounts to 4 inches are expected across Puerto Rico and the southern United States Virgin Islands. FORECAST POSITIONS AND MAX WINDS INIT 12/1500Z 15.0N 54.7W 50 KT 60 MPH 12H 13/0000Z 15.1N 56.7W 50 KT 60 MPH 24H 13/1200Z 15.3N 59.4W 45 KT 50 MPH 36H 14/0000Z 15.3N 62.6W 45 KT 50 MPH 48H 14/1200Z 15.4N 65.6W 40 KT 45 MPH 72H 15/1200Z 15.5N 71.0W 40 KT 45 MPH 96H 16/1200Z 15.5N 75.0W 35 KT 40 MPH 120H 17/1200Z...DISSIPATED $$ Forecaster Blake
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