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Tropical Storm Isaac Forecast Discussion Number 14
2018-09-11 04:43:56| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Mon Sep 10 2018 269 WTNT44 KNHC 110243 TCDAT4 Tropical Storm Isaac Discussion Number 14 NWS National Hurricane Center Miami FL AL092018 1100 PM AST Mon Sep 10 2018 Several microwave images that arrived after the issuance of the last advisory revealed that the low-level center of Isaac has raced ahead of the main convective mass, and the system is poorly vertically aligned. The system also recently passed very near NOAA buoy 41041, which confirmed that the center is on the western side of the central dense overcast. The maximum winds observed by the buoy were only about 30 kt, with a minimum pressure of near 1001 mb. Due to the western adjustment of the center location, the latest Dvorak classification from TAFB is lower, and a blend of the TAFB final-T and CI-numbers supports a lower initial intensity of 60 kt. It is worth noting that this estimate is more uncertain than normal due to limitations of the Dvorak technique, and most of the other intensity estimates are higher. Given the high pressure and light winds at the buoy, I am inclined to favor the lower estimate at this time. Confidence in the track forecast is slightly higher now, and the UKMET is the only outlier that does not bring Isaac westward into the Caribbean within the next 72 h or so. There is still some disagreement on the speed of Isaac, especially once it crosses the Leeward Islands, and this seems largely related to the cyclone's intensity. The ECMWF and GFS depict a weakening storm that simply continues westward, while the HWRF and CTCI show strengthening as Isaac slows down and turns more west-northwestward. The NHC track forecast for now continues to favor the typically more reliable global models and shows a steady westward motion through day 5. The updated NHC forecast is a little faster than the previous advisory, due primarily to the westward adjustment of the initial position, and remains close to HCCA. A slight adjustment has been made to the intensity forecast, but confidence in the intensity forecast remains quite low. Due to the current structure of Isaac, it seems less likely that the tropical storm will intensify more than what is shown by the intensity guidance over the next 24-48 h. The NHC forecast has therefore been lowered slightly, but it is still near the top end of the guidance, and calls for Isaac to be a hurricane when it approaches the Leeward Islands later this week. It is also worth noting that the small size of Isaac could make it more susceptible to sudden short-term changes in intensity that are nearly impossible to forecast. The intensity spread is tremendous by the end of the forecast period, with multiple models showing Isaac becoming a major hurricane around day 4 or 5, and others showing outright dissipation. The NHC forecasts have been favoring a weaker solution thus far due to an expected increase in shear beginning in a couple of days, and I see no reason to make a dramatic change at this point. 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 few days. Watches will likely be required for portions of the Lesser Antilles tomorrow. FORECAST POSITIONS AND MAX WINDS INIT 11/0300Z 14.5N 46.9W 60 KT 70 MPH 12H 11/1200Z 14.5N 49.0W 60 KT 70 MPH 24H 12/0000Z 14.6N 51.7W 65 KT 75 MPH 36H 12/1200Z 14.8N 54.3W 65 KT 75 MPH 48H 13/0000Z 15.0N 56.9W 65 KT 75 MPH 72H 14/0000Z 15.4N 62.4W 60 KT 70 MPH 96H 15/0000Z 15.5N 68.0W 55 KT 65 MPH 120H 16/0000Z 15.5N 72.5W 50 KT 60 MPH $$ Forecaster Zelinsky
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