Abstract: A heavy rainfall event that occurred in southern Shanxi region from 20 May to 21 May 2023, was simulated and analyzed using the high-resolution Weather Research Forecasting (WRF) model. On this basis, the sensitive area for targeted observations of this heavy rainfall process was identified using the conditional nonlinear optimal perturbation (CNOP) method, and the physical importance of this area was analyzed. Finally, the effectiveness of the sensitive area identified by the CNOP method was investigated through the observing system simulation experiments, and the underlying reasons were analyzed. The results show that the WRF model can simulate heavy rainfall in southern Shanxi Province; however, the local rainstorm has a location deviation, a slightly lower intensity, and a westward shift in the areas of moderate and heavy rainfall, while the extent of light rain is relatively large. The sensitive area identified by the CNOP method is mainly located near the center of the northeast cold vortex, reflecting the important influence of the vortex on the rainstorm event. Furthermore, according to the distribution of local maximum energy associated with the CNOP-type initial error, three secondary sensitive regions were selected for comparative analysis. These regions correspond to the Qinghai–Xizang Plateau low-pressure system, the shear line in the Shanxi Province, and the Western Pacific subtropical high. The results show that assimilating simulated observations within the sensitive region yields the highest threat scores (TSs) for heavy rain and rainstorm forecasts, followed by assimilation over Shanxi and its surrounding areas. Assimilation over the Qinghai–Xizang Plateau low-pressure belt and the northern region of the Western Pacific subtropical high also improves the TSs, although to a lesser extent. Further analysis reveals that assimilating simulated observations within the sensitive area substantially affects the wind forecast over southern Shanxi. Combined with the complex terrain of southern Shanxi, the assimilation promotes wind convergence in multiple regions, thereby expanding the area affected by heavy rainfall and improving the forecast accuracy of this event.