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Tropical Storm Iselle Forecast Discussion Number 3
2020-08-27 04:58:36| National Hurricane Center (East Pacific)
Issued at 800 PM PDT Wed Aug 26 2020 000 WTPZ44 KNHC 270258 TCDEP4 Tropical Storm Iselle Discussion Number 3 NWS National Hurricane Center Miami FL EP142020 800 PM PDT Wed Aug 26 2020 Satellite imagery indicates that a convective burst has formed a little closer to the center of Iselle during the past few hours, although the circulation remains at least somewhat elongated. Various subjective and objective satellite intensity estimates are near 35 kt, so that remains the initial intensity. The imagery, as well as model analyses, show that Iselle continues to be undergoing strong easterly shear. Iselle is embedded in a large monsoon gyre, and the southwesterly flow on the south side of the gyre should be the main steering influence for the next 60 h or so. This should result in the cyclone continuing its current slow motion toward the northeast. After that time, the cyclone should separate from the gyre, allowing a mid-level ridge to the north and northeast of the system to become the dominant steering mechanism. This should cause Iselle to turn to the north, then northwest, then eventually to the west. The track guidance is in good agreement with the general scenario, although there is some spread on when and where there turns will occur. The new forecast track is close to the HCCA corrected consensus mode and has only minor adjustments from the previous forecast. Present indications are that the current strong shear will persist for several days, and after 72 h the forecast track takes the cyclone over cooler sea surface temperatures. Based on this, significant strengthening appears unlikely, although short-lived spin ups due to convective bursts are possible. The new intensity forecast is unchanged from the previous forecast in showing little change of strength for 72 h, followed by weakening to a depression and a remnant low. An alternative scenario is that the shear causes Iselle to weaken to a trough at any time during the next 120 h. FORECAST POSITIONS AND MAX WINDS INIT 27/0300Z 16.2N 116.2W 35 KT 40 MPH 12H 27/1200Z 16.7N 115.8W 35 KT 40 MPH 24H 28/0000Z 17.3N 115.3W 35 KT 40 MPH 36H 28/1200Z 17.9N 114.7W 35 KT 40 MPH 48H 29/0000Z 18.6N 114.1W 35 KT 40 MPH 60H 29/1200Z 19.4N 113.6W 35 KT 40 MPH 72H 30/0000Z 20.7N 113.7W 30 KT 35 MPH 96H 31/0000Z 22.5N 115.5W 30 KT 35 MPH...POST-TROP/REMNT LOW 120H 01/0000Z 22.5N 116.5W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Beven
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