Abstract: An extreme warm-sector heavy rainstorm occurred in northwestern Hunan on 17–18 May 2026, breaking the historical record for hourly rainfall intensity in Hunan. Using intensive weather station observations, ERA5 reanalysis data, dual-polarization radar, and wind profile radar data, this study analyzed the formation mechanisms of this event and explored the precipitation microphysical and dynamical structural differences between the two extreme rainstorm centers (Lixian and Shimen). The results are as follows. (1) The vertical configuration of upper-level divergence, mid-to-lower-level shear lines, low-level jets, and surface mesoscale convergence lines provided a favorable circulation background for the extreme rainstorm. (2) The southerly low-level jets on the west side of the western Pacific subtropical high continuously transported moisture from the South China Sea to northwestern Hunan, serving as the primary moisture source that sustained the extreme rainstorm and enabled the record-breaking short-duration rainfall intensity. Conditional instability, intense ascending motion, and vertical secondary circulations were the key thermodynamic and the dynamic conditions responsible for this extreme rainstorm. (3) The surface mesoscale front, formed by the interaction of cold-pool outflows and warm moist inflows, triggered convection. Blocked by topography, this front remained quasi-stationary and provided favorable conditions for backward propagation, which were the key mesoscale mechanisms for the extreme rainstorm over northwestern Hunan. Through frontogenesis, uplift, and blocking effects, the topography further intensified and sustained this event. (4) Extreme rainfall intensity in Lixian was dominated by low echo centroid warm rain convective cells, with its K_DP (specific differential phase) reaching 7.2 °·km⁻¹ and Z_DR (differential reflectivity) reaching 4 dB, whereas the extreme rainstorm in Shimen was predominantly produced by back-building convective cell clusters with moderate Z_DR/K_DP (2.4 dB/1.9 °·km⁻¹) and C_C(correlation coefficient)exceeding 0.98, with their dynamic mechanisms being the mid-level jet-core (3.5 km, 26 m·s⁻¹) for Lixian and the steady maintenance of the low-level jet (at 1.6 km, 20 m·s⁻¹) for Shimen, respectively.This research provides a reference for forecasting and early warning of extreme warm-sector heavy rainstorms in Hunan.