Extreme precipitation event and its characteristics in Yuzhong, Gansu on August 7, 2025

On August 7-8, 2025, Yuzhong County of Gansu Province suffered historically rare extreme precipitation. A cumulative rainfall of 220.2 mm was recorded at the Xinglong Mountain meteorological station in Yuzhong within 15 hours, accounting for 56% of the local annual average precipitation. The extreme torrential rain triggered severe flash floods, resulting in heavy casualties and substantial economic losses. Based on precipitation observations, radiosonde data from Yuzhong, ERA5 reanalysis datasets and GIIRS satellite retrievals, this study adopts diagnostic analysis, precipitation extremity indices and thermodynamic parameters to investigate the observed rainfall characteristics, dynamic and water vapor conditions, extreme precipitation signatures, and evolutionary features of key thermodynamic parameters associated with this event. The main conclusions are as follows. (1) This extreme rainfall event was characterized by long duration, enormous cumulative precipitation, intense short-duration rainfall rates and prominent local concentration. A total of 15 hourly extreme rainfall records were observed across Yuzhong County, with continuous short-duration heavy rainfall lasting 6 consecutive hours at Xinglong Mountain station. Among the 16 meteorological stations across Gansu Province that recorded total rainfall exceeding 100 mm (heavy torrential rain), four stations were located within Yuzhong County. (2) A two-trough-one-ridge circulation pattern at 500 hPa combined with the periphery of the Western Pacific Subtropical High (WPSH) sustained an environment with high convective available potential energy and abundant moisture. The shear line at 700 hPa served as the primary dynamic lifting trigger. Favourable synoptic conditions for this extreme rainfall were jointly established by ample water vapor transport and orographic lifting induced by the Maxian Mountain terrain. (3) For central Gansu, the total event rainfall at 78 stations, hourly rainfall intensity at 37 stations, and frequency of short-duration heavy rainfall at 79 stations all broke the all-time August records since station establishment, accompanied by highly heterogeneous spatial distribution. The core heavy rainfall centers were situated in Yuzhong County and eastern Linxia Hui Autonomous Prefecture. Daily rainfall at stations including Xinglong Mountain surpassed 100 mm, twice their historical maximum values. (4) Multiple severe convective parameters exceeded the 95th percentile extreme threshold of the past decade prior to the event, indicating extremely unstable atmospheric energy. Meanwhile, atmospheric precipitable water persistently exceeded the 95th percentile of the past decade at multiple hours. The southwest jet stream along the edge of the WPSH, coupled with orographic lifting, formed a steady transport channel for integrated water vapor and convective energy. The spatiotemporal coupling of extremely high potential instability and abnormally abundant moisture, further intensified by terrain-induced blocking and convergence acting on the shear line, constituted the core physical mechanism driving record-breaking precipitation. (5) Based on GIIRS satellite retrievals, this study proposes quantitative key forecasting criteria for localized sudden extreme precipitation under the circulation regime along the WPSH periphery: extreme precipitation is highly likely to occur when satellite-retrieved key thermodynamic parameters (e.g., CAPE, Showalter Index) exceed historical extreme thresholds and pronounced terrain convergence exists. The proposed criteria provide refined quantitative references for forecasting localized sudden extreme precipitation over complex terrain in Northwest China.