Abstract:A short time torrential rain occurred in the afternoon of 24 July 2016, from the central and eas tern part of Hebei Province to southern Tianjin, which was the torrential rain center. For this torrential rain both the objective and forecasters subjective forecasts had biases. In this paper the causes of the precipitation are analyzed, especially the initiation and propagation mechanism of the torrential rain, which is the most critical problem, by using the conventional surface and upper air observation, satellite images, radar data and VDRAS data that assimilated the radar and AWS data. The results show that: (1) the torrential rain occurred as the subtropical high strengthened and extended northward. From the perspective of traditional identification, the circulation pattern didnt favor the northwest upper trough at the edge of subtro pical high moving eastward and influencing the southeast of North China. It was a warm sector torrential rain which occurred under the control of the 588 dagpm. It was a non typical circulation pattern short time severe rainfall, difficult to forecast. (2) Although the distance from Tianjin to Xingtai is farther than from Tianjin to Beijing, Xingtai Sounding Station located in the southwest of the torrential rain zone has more reference value under the influence of southwest airflow which is on the northwest side of subtropical high. At 11:00 BT, Xingtai sounding data showed that the atmosphere was extremely unstable and the low level water vapor was deep and unusually vigorous. The CAPE value was 3874 J·kg-1, CIN was 22 J·kg-1 and the height of 8 g·kg-1 specific humidity extended to 600 hPa. The surface dew point temperature was nearly 30℃. (3) Near the 850 hPa warm shear line, the two surface shear lines merged and mesoscale frontogenesis was the key factor of initiation. Two cloud bands were also visible on the satellite image, enhancing convergence. So cumulus clouds developed in the convergence zone, and small cumulus clouds developed in the morning near the warm shear line. As the convergence was strengthened, the mesoscale convective system was formed in the east west direction. (4) After thunderstorm is triggered, its movement and propagation are the difficulties of forecasting, because it determines the lasting time of convective precipitation. In this case, under the influence of the convergence line and the storm gusts, new thunderstorm initiation in the west part of the shear line, and the north south cloud street met the shear line on the south side of the shear line, causing the thunderstorm to spread westward and move southward. So the propagation direction was west by south. Under the influence of southwest airflow, the convective cells moved northeastward, namely, the advection direction was east, which was opposite to the direction of propagation. Thus, the “train effect” was formed. Cloud streets showed that there was a supplement of the south warm and humid air flow under the inversion layer in the south of the shear line. In addition, the convective storm gusts triggered new thunderstorms again, making convective storms sustained.