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Tropical Storm Iselle Forecast Discussion Number 7
2020-08-28 04:36:27| National Hurricane Center (East Pacific)
Issued at 800 PM PDT Thu Aug 27 2020 000 WTPZ44 KNHC 280236 TCDEP4 Tropical Storm Iselle Discussion Number 7 NWS National Hurricane Center Miami FL EP142020 800 PM PDT Thu Aug 27 2020 Iselle continues to produce bursts of deep convection in an environment of moderate easterly shear, with the latest round of convection completely covering the low-level center. Since satellite classifications haven't changed much since the previous advisory, the initial intensity is left at 45 kt based on the earlier ASCAT data. There is some uncertainty in exactly how strong the shear currently is, with SHIPS diagnostics showing nearly 30 kt while UW-CIMSS is analyzing a little under 20 kt. A recent SSMIS pass showed some defined curved banding, so I'm inclined to think the shear is closer to the lower estimate. With that said, the SHIPS diagnostics do show the shear relaxing just a bit during the next 12-24 hours, which could allow Iselle to strengthen a little, and this scenario is supported by the latest HWRF, HMON, COAMPS-TC, HCCA, and GFS model solutions. After that time, the shear is expected to strengthen again, and then Iselle will be moving over cooler waters. Therefore, the new NHC intensity forecast is very close to the previous one from 24 hours and beyond. Iselle is likely to lose all of its convection and become a remnant low in about 3 days and then open up into a trough by day 5. Iselle is embedded in an elongated mid-level trough which extends northeastward to Tropical Storm Hernan near the coast of Mexico, and its initial motion is north-northeastward, or 030/3 kt. The interaction between the two tropical storms is expected to cause Iselle to continue moving north-northeastward or northward during the first 2 days. Once Hernan dissipates and Iselle weakens, a low-level ridge over northern Mexico should cause Iselle to turn toward the northwest and west on days 3 and 4. The track models have shifted a bit to the northeast after 48 hours, leaving the previous forecast near the left side of the guidance envelope, so the new NHC forecast was adjusted closer to the multi-model consensus aids during that period. FORECAST POSITIONS AND MAX WINDS INIT 28/0300Z 17.6N 115.5W 45 KT 50 MPH 12H 28/1200Z 18.2N 115.0W 50 KT 60 MPH 24H 29/0000Z 19.1N 114.5W 45 KT 50 MPH 36H 29/1200Z 20.1N 114.2W 40 KT 45 MPH 48H 30/0000Z 21.7N 114.2W 35 KT 40 MPH 60H 30/1200Z 23.2N 114.7W 30 KT 35 MPH 72H 31/0000Z 23.9N 115.7W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 01/0000Z 23.9N 117.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 02/0000Z...DISSIPATED $$ Forecaster Berg
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