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Hurricane Isaac Forecast Discussion Number 13
2018-09-10 22:31:55| National Hurricane Center (Atlantic)
Issued at 500 PM AST Mon Sep 10 2018 407 WTNT44 KNHC 102031 TCDAT4 Hurricane Isaac Discussion Number 13 NWS National Hurricane Center Miami FL AL092018 500 PM AST Mon Sep 10 2018 Isaac has not become better organized today. The system continues to display a ragged CDO with limited banding features. Dvorak T-numbers from both TAFB and SAB continue to support an intensity of 65 kt. It is not clear why Isaac has failed to strengthen. Cirrus motions and microwave imagery suggest some shear over the tropical cyclone, which may have had some influence. Isaac should not experience a significant increase in shear until about 48 hours, so some strengthening is called for up to that time. Thereafter, the dynamical guidance shows a significant increase in shear, which should reverse the intensity trend. The HWRF model continues to be an outlier in predicting significant intensification over the Caribbean, whereas the other models do not. The official intensity forecast is above the model consensus over the first half of the period, and below the consensus during the latter part of the period, considering the HWRF could be making the consensus too high at days 3-5. Isaac continues moving westward or 270/12 kt. There continue to be a few differences among the track guidance, in particular the U.K. Met, which shows the system turning northwestward to northward well to the northeast of the Leeward Islands. The GFS takes the system into the Caribbean but shows a more west-northwestward motion. The official forecast stays close to the ECMWF and the corrected consensus predictions, which is very similar to the previous NHC track. Given the spread in the guidance, the confidence in the details of the track forecast beyond the first couple of days is larger than usual. Key Messages: 1. Isaac is a small hurricane and uncertainty in the intensity forecast is greater than normal. Although Isaac is expected to begin weakening when it approaches the Lesser Antilles, it is still likely 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. Watches will likely be required for portions of the Lesser Antilles tomorrow. FORECAST POSITIONS AND MAX WINDS INIT 10/2100Z 14.4N 45.0W 65 KT 75 MPH 12H 11/0600Z 14.5N 47.1W 70 KT 80 MPH 24H 11/1800Z 14.6N 49.8W 75 KT 85 MPH 36H 12/0600Z 14.6N 52.3W 75 KT 85 MPH 48H 12/1800Z 14.8N 54.7W 75 KT 85 MPH 72H 13/1800Z 15.3N 60.2W 70 KT 80 MPH 96H 14/1800Z 15.5N 65.5W 60 KT 70 MPH 120H 15/1800Z 15.5N 70.5W 50 KT 60 MPH $$ Forecaster Pasch
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