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Tropical Storm Isaac Forecast Discussion Number 5
2018-09-08 22:48:39| National Hurricane Center (Atlantic)
Issued at 500 PM AST Sat Sep 08 2018 510 WTNT44 KNHC 082048 TCDAT4 Tropical Storm Isaac Discussion Number 5 NWS National Hurricane Center Miami FL AL092018 500 PM AST Sat Sep 08 2018 The structure of the cyclone, while still a shear pattern, is improving, with the low-level center now near the convective overcast and improved outflow in the eastern semicircle. Satellite intensity estimates have increased to the 30-45 kt range, with the CIMSS satellite consensus technique now suggesting winds near 35 kt. Based on these estimates, the initial intensity is increased to 35 kt and the depression is upgraded to Tropical Storm Isaac. Isaac is now moving slowly westward, with the initial motion 275/6. The subtropical ridge to the north of Isaac is forecast to steer the system westward with an increasing forward speed for at least the next 2-3 days. After that time, there is some uncertainty in the track forecast due to the formation of a mid- to upper-level trough over the central Atlantic, with some of the track guidance showing a decrease in forward speed and a more northerly motion near the end of the forecast period. The GFS and ECMWF models show a continued westerly motion through that time, and the official forecast follows that scenario, although with a slower forward speed than in the previous forecast. The new forecast is again similar to the earlier forecast and lies near the various consensus models. Conditions appear favorable for gradual strengthening during the next 72 h as Isaac moves westward across the warm waters of the tropical Atlantic. In best agreement with the SHIPS model for the next three days, the new intensity forecast is raised from the previous forecast during this time. From 72-120 h, the system is expected to encounter westerly wind shear, which is expected to cause some weakening. FORECAST POSITIONS AND MAX WINDS INIT 08/2100Z 14.5N 36.6W 35 KT 40 MPH 12H 09/0600Z 14.6N 37.7W 40 KT 45 MPH 24H 09/1800Z 14.7N 39.5W 50 KT 60 MPH 36H 10/0600Z 14.7N 42.0W 60 KT 70 MPH 48H 10/1800Z 14.7N 44.6W 65 KT 75 MPH 72H 11/1800Z 14.5N 50.0W 75 KT 85 MPH 96H 12/1800Z 14.5N 55.5W 75 KT 85 MPH 120H 13/1800Z 15.0N 61.0W 70 KT 80 MPH $$ Forecaster Beven
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