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Tropical Storm Iselle Forecast Discussion Number 4
2020-08-27 10:50:08| National Hurricane Center (East Pacific)
Issued at 200 AM PDT Thu Aug 27 2020 000 WTPZ44 KNHC 270849 TCDEP4 Tropical Storm Iselle Discussion Number 4 NWS National Hurricane Center Miami FL EP142020 200 AM PDT Thu Aug 27 2020 A sheared band of deep convection has persisted near and to the southwest and west of Iselle's center. Night-visible satellite imagery indicates that the circulation has tightened up somewhat, and the latest UW-CIMSS SATCON intensity estimate is 41 kt. Based on these satellite data, the initial intensity has been increased to 40 kt. Unfortunately, all three scatterometer passes missed Iselle's inner-core wind field. The initial motion estimate is 030/04 kt. Similar to Tropical Storm Hernan farther to the east, Iselle is forecast to remain embedded within a large monsoon gyre, which will gradually contract down as Hernan moves toward southern Baja California and weakens over the next few days. This will result in the current southwesterly flow on the south side of Iselle to become more southerly, which will gradually turn the cyclone northward and then northwestward. By 72 hours, Iselle is expected to interact with and possibly absorb the remnants of Hernan when the two systems are west-southwest of Baja California. Thereafter, Iselle is forecast to move westward to west-southwestward and slowly weaken. The new NHC forecast track is close to a blend of the HCCA corrected-consensus model and the simple consensus models GFEX and TVCE, and is similar to the previous advisory track. Although short-lived convective bursts near the center will likely continue for the next few days, no significant strengthening is forecast for the next 48 hours due to moderate-to-strong easterly vertical wind shear that will persist across the system. At 60 hours and beyond, strong shear is expected to gradually weaken Iselle, with degeneration into a post-tropical remnant low forecast to occur by day 4, with the cyclone possibly even dissipating by 120 hours. The NHC intensity forecast is similar to the previous advisory, and closely follows the IVCN intensity consensus model. FORECAST POSITIONS AND MAX WINDS INIT 27/0900Z 16.6N 116.1W 40 KT 45 MPH 12H 27/1800Z 17.2N 115.7W 40 KT 45 MPH 24H 28/0600Z 17.8N 115.2W 40 KT 45 MPH 36H 28/1800Z 18.4N 114.6W 40 KT 45 MPH 48H 29/0600Z 19.3N 114.1W 40 KT 45 MPH 60H 29/1800Z 20.5N 113.9W 35 KT 40 MPH 72H 30/0600Z 21.7N 114.6W 35 KT 40 MPH 96H 31/0600Z 22.4N 115.9W 30 KT 35 MPH...POST-TROP/REMNT LOW 120H 01/0600Z 22.4N 117.0W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Stewart
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