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Tropical Storm Isaac Forecast Discussion Number 19
2018-09-12 10:34:06| National Hurricane Center (Atlantic)
Issued at 500 AM AST Wed Sep 12 2018 274 WTNT44 KNHC 120834 TCDAT4 Tropical Storm Isaac Discussion Number 19 NWS National Hurricane Center Miami FL AL092018 500 AM AST Wed Sep 12 2018 Isaac is a poorly organized tropical storm. Geostationary satellite and microwave images indicate that the center of the cyclone is located near the western edge of the main area of deep convection. This asymmetric cloud pattern is due to about 20 kt of west-northwesterly shear. The satellite intensity estimates have been decreasing, and an average of the Dvorak CI-numbers supports lowering the initial wind speed slightly to 50 kt. The NOAA Hurricane Hunters are scheduled to investigate Isaac later this morning, and that data should provide a better assessment of the storm's intensity and structure. Isaac continues to move due westward at about 13 kt. The track forecast reasoning remains unchanged. A fairly strong mid-level ridge to the north of Isaac should steer the system westward at about the same forward speed during the next several days. The models are now in fairly good agreement, and the NHC track forecast is just a little north of the previous one to come in line with the latest guidance. Based on this forecast, Isaac is expected to cross portions of the Leeward and Windward Islands in about 36 hours. Isaac is feeling the effects of wind shear, and the global models suggest that the shear will likely remain moderate to strong during the next couple of days. These hostile winds combined with nearby dry air will likely cause Isaac to gradually weaken during the next several days. The intensity models are in fair agreement, and this forecast is close to the model consensus, except a little lower than that aid at 96 hours. The latest runs of the GFS and ECMWF models show Isaac opening up into a trough by day 5, and the NHC forecast follows those models. 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 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. FORECAST POSITIONS AND MAX WINDS INIT 12/0900Z 14.5N 53.5W 50 KT 60 MPH 12H 12/1800Z 14.8N 55.7W 50 KT 60 MPH 24H 13/0600Z 15.0N 58.6W 45 KT 50 MPH 36H 13/1800Z 15.2N 61.5W 45 KT 50 MPH 48H 14/0600Z 15.4N 64.8W 40 KT 45 MPH 72H 15/0600Z 15.3N 70.6W 40 KT 45 MPH 96H 16/0600Z 15.2N 75.0W 35 KT 40 MPH 120H 17/0600Z...DISSIPATED $$ Forecaster Cangialosi
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