A torrential rain and hail stroke Shijiazhuang on July 26，2011． The strong and cold air flow on 500 hPa and above resulted in sharp shifts of synoptic situation at high altitude in twelve hours，with short wave trough moving southward rapidly，which caused the failure of radiosonde observation and numerical forecast． Based on other monitoring data，obvious characteristics of the dramatic changing weather were analyzed． Firstly，according to satellite images，the comma cloud above the Great Bend of the Huanghe River had long and stout tail，with a tendency of developing southward． The dark area at the rear part of clouds indicated the intrusion of dry cold air，which developed into convective clouds with warm and moist air at low altitude． The tail fracture suggested the acceleration of moving southward of cold air． Secondly，the Shijiazhuang single station element variation showed that the surface potential pseudo equivalent temperature was 8 ℃ higher than the normal value，which indicated abnormal instability energy． Strong convective weather with the largest precipitation was located in the intensive zone of pseudo equivalent potential temperature，which reached 90 ℃ at the energy center． The difference of pseudo equivalent potential temperature within 100 km was more than 25 ℃． Finally，the radar echoes arranged zonally from the southwest to the northeast with the strongest echo of 65 dBZ in the front． Gust front formed before the strongecho，its maximum positive and negative speed were both larger than 20 m/ s． The squall line moved from west to east and its path and intensity ditermined the path and intensity of precipitation and hail． The transformation of wind direction and formation of surface convergence line were 21 min and 30 min ahead of the precipitation，respectively．