Tropical Cyclone Discussion
679 WTPZ44 KNHC 241433 TCDEP4 Tropical Storm Iselle Discussion Number 5 NWS National Hurricane Center Miami FL EP092026 800 AM PDT Mon Aug 24 2026 Despite being affected by moderate west-northwesterly shear, Iselle's cloud pattern has continued to improve, with a core of very deep convection that has now rotated over and north of the center, and a deep convective band extending southward on the east side of the cyclone. A 24/0903 UTC AMSR-2 pass also shows the improving structure of the inner core. The latest subjective Dvorak classification from TAFB is T3.5/55 kt, and objective satellite intensity estimates range from 50 to 59 kt. The initial intensity has been set at 55 kt for this advisory, in agreement with the estimates. The initial motion estimate is northwestward, or 310/11 kt. A mid-level ridge north of Iselle is forecast to strengthen, causing Iselle to turn west-northwestward on Tuesday, and that motion should then continue through the end of the week. The latest NHC track forecast is a little faster and to the left of the previous forecast, and lies very close to the HFIP Corrected Consensus (HCCA) and the Google DeepMind Ensemble Mean (GDMI). Iselle remains over warm waters and within a moist environment. However, SHIPS and UW-CIMSS both currently analyze 17 kt of vertical wind shear. Water temperatures and moisture will remain quite favorable for another 24 h, but the shear is forecast to increase to at least 20 kt between the 12 to 24 h period. Given the recently improving structure, the NHC forecast continues to call for strengthening and system to become a hurricane in 12 to 24 h, which is at the high end of the intensity model guidance suite. However, given the increasing shear, there is some uncertainty as to whether Iselle will become a hurricane. Beyond 24 h, gradually decreasing ocean temperatures and moisture will lead to steady weakening, and the NHC intensity forecast lies in the middle of the guidance envelope from hour 48 onward. The latest ECMWF and GFS simulated satellite imagery suggest that Iselle could become a remnant low as early as Thursday. I have moved forward the forecast time for Iselle becoming post-tropical to hour 96, but it's possible the cyclone could lose its convection sooner than forecast. After that, the models indicate the remnant low opening up into a trough by day 5. FORECAST POSITIONS AND MAX WINDS INIT 24/1500Z 18.5N 117.1W 55 KT 65 MPH 12H 25/0000Z 19.4N 118.3W 65 KT 75 MPH 24H 25/1200Z 20.5N 120.2W 65 KT 75 MPH 36H 26/0000Z 21.1N 122.1W 60 KT 70 MPH 48H 26/1200Z 21.5N 123.8W 50 KT 60 MPH 60H 27/0000Z 22.0N 125.6W 40 KT 45 MPH 72H 27/1200Z 22.5N 127.6W 35 KT 40 MPH 96H 28/1200Z 23.1N 131.9W 30 KT 35 MPH...POST-TROP/REMNT LOW 120H 29/1200Z...DISSIPATED