Abstract: Comparison analysis were carried on using large-scale circulation,dynamic and vapor condition,instability stratification,and radar data to evaluate the formation reasons of the two severe convection events on August 24 and September 1,2014 over Shanghai,and a further warning research was conducted as well.The results show that the two events happened at the edge of subtropical high with obvious low-level jets.The events start times in forecasting were earlier than real conditions.As for the differences,severe convection appeared in front of the trough,accompanied by squall line and mesoscale gust front,causing the thunderstorm on August 24,while the deep moist tongue appeared in severe convection due to low-level vortex moving eastward,causing the long-time rainstorm on September 1,2014.Vertical wind shear is a fine indicator to the maintenance and development of a thunderstorm.The convection in front of the trough coupled with severe vertical shear tends to cause regional thunderstorm,but the convection due to low-level vortex moving eastward coupled with weak vertical shear tends to cause a regional convective rainstorm.Since the affecting time of convection system in front of trough is short,it is necessary to analyze the development of mesoscale and small-scale low pressure convergence.The start time of the convective rainstorm due to low-level vortex moving eastward is related to the warm-zone convective rainstorm.

Key words: Severe convection, Cold front, Squall lines, Low-level vortex, Forecasting and warning