Based on NCEP (1°×1°) reanalysis data， the large sale circulation features as well as thermal and dynamic conditions for two types （5 cases for each type） of severe convective weathers under ahead of trough pattern during summer in Anhui Province are analyzed. The results show that： the nontornadic type mainly causes strong wind and short time heavy rainfall which is characterized by deep upper trough. However, the tornado is usually triggered by short wave trough with the low level jet which causes strong vertical wind shear. By comparing the composite thermal and dynamic parameters， it is demonstrated that the moisture condition is good for both types of severe convective weathers and the average precipitable water during summer is above 55 mm. However， the vertical wind shear at middle to low levels is very strong when tornado occurs and the 0-1 km vertical wind shear is about 3 times higher than that of nontornadic type. Because of larger vertical wind shear, the storm relative helicity (SRH) of tornadic type is stronger than that of nontornadic type. According to the thermal and dynamic conditions，the CAPE of tornadic type is not very large while the energy helicity is large and significant differences are found in storm relative helicity. Therefore, more attention should be paid to the vertical wind shear and SRH in forecasting the tornado under ahead of trough pattern.