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Tropical Storm Isaac Forecast Discussion Number 8
2018-09-09 16:33:11| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Sun Sep 09 2018 266 WTNT44 KNHC 091433 TCDAT4 Tropical Storm Isaac Discussion Number 8 NWS National Hurricane Center Miami FL AL092018 1100 AM AST Sun Sep 09 2018 WindSat microwave data from several hours ago show that Isaac has developed a well-defined low-level ring in the 37-GHz channel, and the center is embedded in the middle of a small central dense overcast. TAFB and SAB Dvorak estimates have increased to T3.5 given Isaac's improved structure, so the initial intensity is raised to 55 kt. Isaac's future intensity is the most challenging part of the forecast. Since sea surface temperatures will be warm (between 27-28 degrees Celsius) and vertical shear will be almost non-existent (less than 10 kt) for the next 36-48 hours, the tiny storm will be prone to a possibly significant increase in intensity during the next couple of days. For that period, the NHC intensity forecast is above the guidance, and above the previous forecast, due to relatively high numbers from the various rapid intensification indices. However, in 2-3 days, the global models are showing northwesterly shear steadily increasing and potentially reaching a maximum of around 30 kt by Thursday. The big question is the timing of this shear and how much it will affect Isaac's intensity before the cyclone reaches the Lesser Antilles. The HWRF, which brings Isaac to major hurricane intensity despite the forecast shear, appears to be an outlier, and is therefore pulling up the ICON intensity consensus and HCCA. As a result, the NHC intensity forecast is below ICON on days 4 and 5. This new forecast shows a more peaked maximum intensity, with Isaac possibly increasing and then decreasing in strength at faster rates than shown here. While the microwave data indicate that Isaac's center is slightly south of previous estimates, the current motion estimate is 270/8 kt. A low- to mid-level ridge is expected to move westward in tandem with Isaac for much of the forecast period, causing the cyclone to accelerate toward the west, reaching a forward speed of around 15 kt by 36 hours, and then holding that speed through day 5. With the exception of the UKMET and HWRF models, the guidance envelope is tightly packed from north to south, and the speed differences noted over the past few days have decreased. The updated NHC track forecast is just a little south of the previous one to account for the adjusted initial position, and lies slightly south of the multi-model consensus in the direction of HCCA and the Florida State Superensemble. FORECAST POSITIONS AND MAX WINDS INIT 09/1500Z 14.3N 39.1W 55 KT 65 MPH 12H 10/0000Z 14.4N 40.8W 65 KT 75 MPH 24H 10/1200Z 14.4N 43.3W 75 KT 85 MPH 36H 11/0000Z 14.4N 46.3W 85 KT 100 MPH 48H 11/1200Z 14.3N 49.1W 85 KT 100 MPH 72H 12/1200Z 14.2N 54.8W 80 KT 90 MPH 96H 13/1200Z 14.5N 60.5W 70 KT 80 MPH 120H 14/1200Z 15.0N 66.5W 60 KT 70 MPH $$ Forecaster Berg
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