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| #939968 (Received by flhurricane at: 4:47 PM 13.Sep.2018) TCDAT4 Tropical Storm Isaac Discussion Number 25 NWS National Hurricane Center Miami FL AL092018 500 PM AST Thu Sep 13 2018 The center of Isaac has been mostly exposed during the past several hours as temporary bursts of convection fire just east of the center. Assuming some spin down of the circulation after the plane left, the initial wind speed is set to 35 kt. Since there haven't been any reports of sustained tropical-storm-force winds in the Lesser Antilles and the center is moving away, all watches and warning have been lowered. However, there remains a threat of flash flooding due to a large area of rain that has yet to move through the Lesser Antilles. Strong shear and mid-level dry air is forecast to persist for the next day or so, which should cause Isaac to drop back to tropical depression status on Friday. However, the shear is forecast to weaken by the weekend, and moisture values are expected to rise as the system gets deeper into the Caribbean Sea. These conditions would normally allow for some restregthening, but the initial character of the system is pretty weak, and it is uncertain whether there will be much left of Isaac to take advantage of the more conducive conditions. Model guidance has been inconsistent on the future of Isaac, which speaks to the lack of predictability of this situation. The bulk of the guidance show Isaac staying weak or degenerating into a tropical wave over the next few days, so the official forecast follows that consensus. At this point it is just best to keep an eye on the long-range forecast on each advisory to see if there are any changes. Isaac is moving westward at about 15 kt. A mid-level ridge is forecast to persist over the southwestern Atlantic, steering Isaac westward for the next few days, with a turn to the west-northwest possible at long range. The future track is very uncertain at long range because it is dependent on the intensity. If Isaac degenerates into a wave, the remnants will likely move south and west of the guidance mean, and if the cyclone is stronger, it will probably move slower and more to the right. The new NHC forecast assumes that Isaac will stay weak, and therefore is on the left and faster side of the guidance. FORECAST POSITIONS AND MAX WINDS INIT 13/2100Z 14.9N 63.2W 35 KT 40 MPH 12H 14/0600Z 14.9N 65.4W 35 KT 40 MPH 24H 14/1800Z 14.9N 68.2W 30 KT 35 MPH 36H 15/0600Z 14.9N 70.8W 30 KT 35 MPH 48H 15/1800Z 15.0N 73.0W 30 KT 35 MPH 72H 16/1800Z 16.2N 77.4W 30 KT 35 MPH 96H 17/1800Z 17.5N 81.5W 30 KT 35 MPH 120H 18/1800Z...DISSIPATED $$ Forecaster Blake |
