Comparison on characteristics and formation mechanisms of Grade 12 gale events in Yichun of Jiangxi during two successive Derecho events

On April 21, 2025, two successive Derecho events with an interval of approximately 4 hours occurred sequentially along the northern Hunan-northern Jiangxi-Zhejiang line. Both events intensified notably over Yichun, Jiangxi, where Grade 12 gales breaking historical records were observed at four stations, resulting in severe disaster losses. To deepen the understanding of the formation mechanisms of Derecho events and extreme gales, this study compares the characteristics, causes and intensification mechanisms of the two Grade 12 gale episodes in Yichun using upper-air and surface observations, dual-polarization radar data and ERA5 reanalysis datasets.The results are summarized as follows:(1) The extreme weather was triggered by two sequential Derecho events processes. Favorable environmental conditions for Derecho events occurrence include low-level warm advection, mid-level dry layers, deep 0–8 km vertical wind shear of 30.7 m·s⁻¹, and a mean entrainment-layer wind speed of 19.2 m·s⁻¹.(2) From 15:30 to 15:50 Beijing Time, the first Grade 12 gale in Yichun was jointly induced by multiple successive downbursts and density currents associated with cold pools with a 1-hour temperature drop of 10 °C, after the merging and intensification of supercells and small convective cells.(3) The second Grade 12 gale in Yichun during 19:00–19:30 Beijing Time was generated by combined effects of downbursts and storm-scale momentum downward transport. Such momentum transport was characterized by a temporally continuous high-wind core of 27 m·s⁻¹ that rapidly descended from 4–7 km altitude down to the ground.(4) In spring nowcasting services over the south of the Yangtze River, Attention should be paid to the rapid intensification of local gales when downbursts, momentum downward transport and intense cold pools appear simultaneously or exert successive impacts at a given location. Under the environmental conditions of strong wind speeds within the entrainment layer, multi-cell storms can also produce surface Grade 12 gales under pronounced momentum downward transport.