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Tropical Storm Isaac Forecast Discussion Number 29
2018-09-14 22:51:59| National Hurricane Center (Atlantic)
Issued at 500 PM AST Fri Sep 14 2018 779 WTNT44 KNHC 142051 TCDAT4 Tropical Storm Isaac Discussion Number 29 NWS National Hurricane Center Miami FL AL092018 500 PM AST Fri Sep 14 2018 It has been a very interesting flight of the NOAA Hurricane Hunter aircraft this afternoon. Dropsonde and SFMR data indicate that the maximum winds are around 35 kt again, and the pressure has fallen to about 1002 mb. However, the system is tilted southward with vertical height due to northerly shear, and scatterometer and Hurricane Hunter tail-Doppler radar data indicate the system is elongated from northeast to southwest, albeit closed. Note that the plane was unable to sample the southwestern quadrant well because of strong convection and intense lightning, so the flight-level winds showing no circulation were unrepresentative. The intensity forecast hasn't gotten any easier with Isaac. Vertical shear is expected to decrease a bit more, which might diminish the effects of dry air seen on water vapor images around the cyclone. However, the GFS and ECMWF continue to insist that Isaac will degenerate into a wave within the next 2 or 3 days, which is almost unanimously supported by their respective ensemble members. This is the solution that the official forecast follows. It should be noted that none of the global models show pressures this low now with Isaac, except the UKMET, and that model is the one that holds onto the circulation the longest. The reality of the situation is that the long-term future of Isaac is cloudy, and the best advice is just to follow NHC advisories every 6 hours to see if we gain any confidence. Isaac is moving westward at about 12 kt. A ridge over the southwestern Atlantic should steer the cyclone westward over the weekend then west-northwestward early next week as Isaac reaches a weaker portion of the ridge. Just like yesterday, a stronger system would likely turn more to the west-northwest earlier, moving closer to Jamaica. Since the system is forecast to be weaker, the forecast lies on the westward and faster side of the guidance, and should also be considered low confidence since it depends on the intensity. Interests in Jamaica should monitor the progress of the Isaac. FORECAST POSITIONS AND MAX WINDS INIT 14/2100Z 15.3N 68.2W 35 KT 40 MPH 12H 15/0600Z 15.4N 70.1W 35 KT 40 MPH 24H 15/1800Z 15.4N 72.2W 35 KT 40 MPH 36H 16/0600Z 15.7N 74.2W 35 KT 40 MPH 48H 16/1800Z 16.2N 76.3W 30 KT 35 MPH 72H 17/1800Z 17.5N 80.0W 25 KT 30 MPH 96H 18/1800Z...DISSIPATED $$ Forecaster Blake
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