Storyline simulations of Hurricane Helene (2024) rainfall under future warming scenarios

Abstract Assessing how a warmer climate alters tropical cyclone-related extreme rainfall is key to strengthening resilience on the Gulf Coast. We use storyline simulations with the Community Atmosphere Model (CAM), configured at ∼28 km over the North Atlantic and initialized from NOAA GFS and Hadley OI SSTs. We examine how rainfall from Hurricane Helene (2024) changes under the current climate (actual) conditions and under potential future warming of +2, +3, and +4 K above preindustrial (counterfactuals), generated by thermodynamic perturbations to temperature, humidity, and sea surface temperature fields. Analysis of track error (calculated as a function of forecast hours) for the 20-member ensemble shows that the median error remains below 50 km during the first 24 h (when initialized on 25 September 2024, at 0Z) for the actual simulation. Mean landfall timing varies modestly among the experiments, occurring near 51 h in the actual and 4 K ensembles, but shifting slightly earlier to about 49 h in 2 K and 48 h in 3 K. Median accumulated rainfall from 26 to 28 September agrees well with satellite and radar estimates under current climate, while counterfactuals show enhanced rainfall near the Florida coast and substantially increased probabilities of intense 3-hourly rainfall over land. Peak rainfall lies west of Helene’s track over Florida and shifts to the east of the track as the storm moves into the southern Appalachians. Changes in 3-hourly accumulated rainfall are sensitive to the storm’s location and the 4 K produces the largest rainfall rates at both near-landfall and inland regions. For the 99.9 th percentile, the percentage change in the inland region is 10%–30% in the 4 K scenario compared with the actual scenario, while near landfall it is 32%–44%. Results show that storyline analysis is more challenging inland, where longer lead times can produce non-monotonic changes in 3-hourly rainfall with each 1 K increase in warming.