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Tropical Storm Isaac Forecast Discussion Number 17
2018-09-11 22:34:02| National Hurricane Center (Atlantic)
Issued at 500 PM AST Tue Sep 11 2018 014 WTNT44 KNHC 112033 TCDAT4 Tropical Storm Isaac Discussion Number 17 NWS National Hurricane Center Miami FL AL092018 500 PM AST Tue Sep 11 2018 Microwave and visible satellite data show that the center of Isaac is partially exposed on the western edge of the central dense overcast. Perhaps Isaac is a little weaker than earlier, but a blend of the latest subjective and objective estimates still gives an uncertain initial wind speed of about 60 kt. We should have a better idea of the maximum winds by tomorrow morning when a NOAA Hurricane Hunter enters the storm, or if we get lucky with a scatterometer pass this evening. Isaac is barely holding on in the face of increasing shear, which is much more evident in the 300-500 mb layer than a conventional analysis at 200 mb. However, convection could redevelop near the center while the storm moves over warming SSTs into a more moist and unstable environment east of the Lesser Antilles. These conflicting factors lead to a forecast of little change in the winds of Isaac until the cyclone moves into the eastern Caribbean. I should mention that the short-term trend on satellite makes me think the intensity forecast is too high over the next 36 hours. Yet, given our limited skill in forecasting intensity changes in marginal environments, plus the 5 or 10 kt of initial wind speed uncertainty, this is the course of least regret. We can see if this trend becomes more consistent before making substantial changes on the next advisory. Model guidance is very divergent in the long range for Isaac, with the regional hurricane and statistical models showing intensification, while the global models show continued weakening. The global models have done fairly well with this cyclone, so the forecast will lean more heavily on their weaker solutions. The latest initial motion estimate is 270/15, a little faster than before. Isaac is forecast to move westward at roughly the same speed for the next few days due to a mid-level ridge over the central Atlantic Ocean. Model guidance is tightly clustered on this cycle, with the UKMET now coming into good agreement with the rest of the guidance. The only change to this forecast is to accelerate Isaac roughly 1 kt over the course of the prediction, which over the course of a 5-day forecast results in a sizable westward shift. This forecast could still be too slow, since the GFS and ECMWF are a bit ahead of the new NHC track prediction. Key Messages: 1. Isaac is expected to be at or near hurricane intensity when it approaches the Lesser Antilles on Thursday, and hurricane and tropical storm watches have been issued for several islands. Interests in those areas should follow any advice given by their local officials, and warnings will likely be issued this evening. 2. Interests elsewhere in the Lesser Antilles should continue to monitor Isaac during the next couple of days. FORECAST POSITIONS AND MAX WINDS INIT 11/2100Z 14.6N 51.3W 60 KT 70 MPH 12H 12/0600Z 14.6N 53.4W 60 KT 70 MPH 24H 12/1800Z 14.8N 56.2W 60 KT 70 MPH 36H 13/0600Z 15.0N 59.2W 60 KT 70 MPH 48H 13/1800Z 15.3N 62.3W 55 KT 65 MPH 72H 14/1800Z 15.5N 68.5W 50 KT 60 MPH 96H 15/1800Z 15.5N 73.5W 45 KT 50 MPH 120H 16/1800Z 15.5N 77.5W 45 KT 50 MPH $$ Forecaster Blake
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