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Tropical Storm Isaac Forecast Discussion Number 9
2018-09-09 22:32:48| National Hurricane Center (Atlantic)
Issued at 500 PM AST Sun Sep 09 2018 355 WTNT44 KNHC 092032 TCDAT4 Tropical Storm Isaac Discussion Number 9 NWS National Hurricane Center Miami FL AL092018 500 PM AST Sun Sep 09 2018 Isaac is a tiny tropical cyclone, but even still, recent SSMIS data showed that it has developed a mid-level microwave eye feature. At the time of the pass (1800 UTC), the convection was a little thin on the southern side, but geostationary satellite images suggest that it has filled in since that time. Dvorak intensity estimates are T4.0 from TAFB and T3.5 from SAB, so the maximum winds are estimated to be 60 kt. The intensity forecast philosophy has not changed since this morning. Isaac is strengthening in an environment of low shear and over warm sea surface temperatures, and for the next 36-48 hours, the NHC intensity forecast is above the bulk of the intensity models. The main reason for this is that Isaac's tiny size could allow the intensity to increase quickly within the favorable environment. However, the cyclone's tiny size will likely also be to its detriment after 48 hours when northwesterly vertical shear is expected to develop and increase to 20-30 kt. The shear will probably easily decouple the small system, causing the intensity to decrease much faster than suggested by most of the intensity models. For that reason, the official NHC intensity forecast is below the intensity consensus on days 3 through 5. It should be noted that both the GFS and ECMWF models show a weakening cyclone moving into the eastern Caribbean Sea by days 4 and 5, with the GFS even making the system an open wave by the end of the forecast period. These global model solutions lend credence to the belief that it may be difficult for Isaac to maintain hurricane status while it approaches the Lesser Antilles. Isaac is accelerating toward the west with an initial motion of 275/10 kt. Ridging to the north is expected to keep the cyclone on a westward trajectory for the entire forecast period, with acceleration continuing for the next 36 hours. The UKMET remains the biggest outlier, showing Isaac turning northwestward and northward into the central Atlantic after 48 hours. That still appears to be unlikely at this time, and the NHC track forecast is still close to the center of the guidance envelope. Only a slight northward shift was made to the new forecast based on the latest model solutions. Key Messages: 1. Isaac is expected to become a hurricane tonight or on Monday while it moves westward across the tropical Atlantic Ocean. 2. While Isaac is forecast to begin weakening by Tuesday while it approaches the Lesser Antilles, it is still expected to be at or near hurricane intensity when it reaches the islands, and the uncertainty in the intensity forecast is higher than usual. Interests in the Lesser Antilles should continue to monitor Isaac during the next few days. FORECAST POSITIONS AND MAX WINDS INIT 09/2100Z 14.5N 40.3W 60 KT 70 MPH 12H 10/0600Z 14.6N 42.2W 70 KT 80 MPH 24H 10/1800Z 14.8N 45.0W 80 KT 90 MPH 36H 11/0600Z 14.9N 48.0W 85 KT 100 MPH 48H 11/1800Z 14.9N 50.9W 85 KT 100 MPH 72H 12/1800Z 15.0N 56.5W 70 KT 80 MPH 96H 13/1800Z 15.5N 63.0W 60 KT 70 MPH 120H 14/1800Z 15.5N 69.5W 50 KT 60 MPH $$ Forecaster Berg
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