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Hurricane ISELLE Forecast Discussion Number 20
2014-08-05 10:55:33| National Hurricane Center (East Pacific)
Issued at 200 AM PDT TUE AUG 05 2014 000 WTPZ44 KNHC 050855 TCDEP4 HURRICANE ISELLE DISCUSSION NUMBER 20 NWS NATIONAL HURRICANE CENTER MIAMI FL EP092014 200 AM PDT TUE AUG 05 2014 Although Iselle's eye remain large and distinct, cloud tops have been warming significantly since the previous advisory. Also, some westerly shear is beginning to impinge on the cyclone, and the CDO has become elongated a little in a northeast-southwest orientation. The aforementioned observations all suggest weakening, and this is noted by a decrease in the Dvorak T-numbers from TAFB and SAB. The initial intensity is, therefore, lowered to 110 kt for this advisory, which is a blend of subjective and objective satellite intensity estimates. The initial motion estimate is 270/07 kt. After a slight jog to the west-southwest, Iselle appears to have returned to a due west course. The NHC model guidance remains tightly clustered and in excellent agreement on Iselle moving slowly westward for another 12-24 hours, and then turning more west-northwestward at a faster forward speed as the ridge to its north gradually strengthens. The new NHC forecast track is just an update of the previous advisory track, and is close to the very reliable consensus model, TVCA. On the forecast track, the center of Iselle should move near or over portions of the Hawaiian Islands in 72-84 hours. Since Iselle is an annular hurricane, only slow weakening is forecast, especially since the vertical shear is forecast to remain 5 kt or less for the next 72 hours or so. The GFS-based SHIPS model is showing more significant weakening and makes Iselle a tropical storm in 36 hours, which seems premature. This is due in part to the SSTs being about 0.5 degree cooler in the model than what is currently observed along the cyclone's path. The official intensity forecast follows the consensus model, ICON, but is slightly higher to account for the low bias of the SHIPS model. Remnant low status is expected by Day 5. Interests in the Hawaiian Islands should closely monitor the progress of Iselle. However, it is important not to focus too closely on the exact track and intensity forecasts because the average track error 72 hours out is about 100 miles, the average intensity error is about 15 kt, and because the hazards of a tropical cyclone can extend over a broad area often well away from the center. Iselle is expected to cross into the Central Pacific Hurricane Center's area of responsibility around 1800 UTC today. Once this occurs, the Central Pacific Hurricane Center in Honolulu, Hawaii, will begin issuing advisories on Iselle. FORECAST POSITIONS AND MAX WINDS INIT 05/0900Z 15.9N 138.6W 110 KT 125 MPH 12H 05/1800Z 16.2N 140.1W 100 KT 115 MPH 24H 06/0600Z 16.7N 142.7W 85 KT 100 MPH 36H 06/1800Z 17.3N 145.8W 70 KT 80 MPH 48H 07/0600Z 18.1N 149.2W 60 KT 70 MPH 72H 08/0600Z 19.7N 155.6W 50 KT 60 MPH 96H 09/0600Z 21.1N 161.4W 40 KT 45 MPH 120H 10/0600Z 21.8N 166.3W 30 KT 35 MPH...POST-TROP/REMNT LOW $$ Forecaster Stewart
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