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Tropical Storm Isaac Forecast Discussion Number 21
2018-09-12 22:58:43| National Hurricane Center (Atlantic)
Issued at 500 PM AST Wed Sep 12 2018 658 WTNT44 KNHC 122058 TCDAT4 Tropical Storm Isaac Discussion Number 21 NWS National Hurricane Center Miami FL AL092018 500 PM AST Wed Sep 12 2018 The satellite presentation of Isaac hasn't changed much in the past several hours, with the center exposed on the west side of a re-developing band of convection. On the last pass of the NOAA Hurricane Hunter aircraft, peak flight-level winds were 61 kt and SFMR values were about 50 kt. These data support an initial wind speed estimate of 50 kt. It is interesting to note that the surface center is estimated to be a tenth of a degree or two south of the well-defined swirl on satellite--a function of the fast forward speed and high shear. The shear is forecast to continue for the next day or two, and gradual weakening is anticipated, in line with the model guidance. Given the strong shear, it is possible that Isaac could degenerate into a wave near the Lesser Antilles. After that time, Isaac could escape the upper-level trough over the central Atlantic that has been shearing the cyclone, and find a more conducive environment. More of the model guidance is suggesting some reintensification of the cyclone in the western Caribbean Sea, with the biggest hold-out being the ECMWF. Continuity dictates that the forecast not change as much as the guidance, most of which is showing a hurricane at long range, so the compromise solution is no longer dissipating the cyclone and flat-lining the intensity at long range. It almost goes without saying that this is a low-confidence intensity forecast. Isaac has accelerated to near 17 kt over the past 12 hours, and the shorter-term motion is closer to 20 kt with the convection-less center. A mid-tropospheric high is forecast to persist for the next several days, which should keep the cyclone moving westward, albeit at a slightly slower pace. The model guidance has shifted a bit to the north on this cycle, but is mostly in response to the initial latitude. The NHC track forecast is moved northward as well but still lies on the south side of the consensus. At long range, it makes sense when forecasting a weaker system to stay on the left and faster side of the consensus, so that's where the new NHC prediction lies. Key Messages: 1. Isaac is expected to remain at tropical storm intensity when it moves across the Lesser Antilles on Thursday, and tropical storm warnings are in effect for Martinique, Dominica, and Guadeloupe. Tropical storm watches are in effect for Montserrat, St. Kitts and Nevis, Antigua, and Saba and St. Eustatius. Interests on those islands should follow any advice given by their local officials. 2. Life-threatening flash flooding is possible with Isaac. The storm is expected to produce total rainfall accumulations of 2 to 4 inches with isolated amounts up 8 inches across Martinique, Dominica and Guadeloupe. Rainfall amounts of 1 to 2 inches with isolated amounts to 4 inches are expected across Puerto Rico and the southern United States Virgin Islands. FORECAST POSITIONS AND MAX WINDS INIT 12/2100Z 15.4N 56.6W 50 KT 60 MPH 12H 13/0600Z 15.6N 59.0W 50 KT 60 MPH 24H 13/1800Z 15.8N 61.8W 45 KT 50 MPH 36H 14/0600Z 16.0N 64.8W 45 KT 50 MPH 48H 14/1800Z 16.0N 67.6W 40 KT 45 MPH 72H 15/1800Z 16.0N 72.5W 40 KT 45 MPH 96H 16/1800Z 16.5N 76.5W 40 KT 45 MPH 120H 17/1800Z 17.5N 80.5W 40 KT 45 MPH $$ Forecaster Blake
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