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Tropical Storm Iselle Forecast Discussion Number 9
2020-08-28 16:47:33| National Hurricane Center (East Pacific)
Issued at 800 AM PDT Fri Aug 28 2020 000 WTPZ44 KNHC 281447 TCDEP4 Tropical Storm Iselle Discussion Number 9 NWS National Hurricane Center Miami FL EP142020 800 AM PDT Fri Aug 28 2020 GOES-16 shortwave infrared imagery and a recent AMSR2 microwave overpass indicate that deep convection has been developing near the surface center during the past several hours. In fact, the microwave image showed a small, compact inner core defined by a partially closed eye-like feature. My initial thoughts were that this cloud feature is in the mid-portion of the atmosphere, but the lower 37 GHZ frequency confirmed very little vertical tilt. Subjective and objective satellite intensity estimates are based on the shear scene-type which would yield a slightly lower intensity estimation. Consequently, the initial intensity is held at 50 kt, but it could certainly be a little stronger based on the aforementioned polar low-orbiter pass. The FV3/GFS and ECMWF SHIPS statistical-dynamical intensity models show 20 to 30 kt of northeasterly shear persisting through the next few days, however, the UW-CIMSS shear analysis reveals less than 20 kt. For now, based on what the large-scale models and the SHIPS models agree on, gradual weakening should begin by early Saturday morning and continue through Monday morning as the cyclone traverses decreasing oceanic temperatures and moves into a more thermodynamically stable surrounding environment. The NHC intensity is an update of the previous advisory, and calls for Iselle to weaken to a tropical depression on Sunday and degenerate into a remnant low on Sunday evening. Based on the 0920 UTC AMSR2 pass, the initial position was adjusted to the northwest of the previous position and the forward motion is estimated to be northeastward, or 035/4 kt. A northward direction should commence by early Saturday morning, then a turn north-northwestward to northwestward is forecast during the 48-60 hr period. As Iselle continues to weaken and become a more shallower system, a turn toward the west-northwest, well offshore of the southern coast of the Baja California peninsula, is forecast to occur Monday morning. The NHC forecast is nudged to the left of the previous track forecast due the adjusted position, and is based on the HCCA and TVCE multi-model consensus aids. FORECAST POSITIONS AND MAX WINDS INIT 28/1500Z 18.3N 115.3W 50 KT 60 MPH 12H 29/0000Z 19.2N 114.8W 50 KT 60 MPH 24H 29/1200Z 20.6N 114.6W 45 KT 50 MPH 36H 30/0000Z 22.2N 114.9W 35 KT 40 MPH 48H 30/1200Z 23.5N 115.7W 30 KT 35 MPH 60H 31/0000Z 24.1N 116.6W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 31/1200Z 24.4N 117.3W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 01/1200Z...DISSIPATED $$ Forecaster Roberts
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