Abstract: Under the constrain of conservation of moist potential vorticity (MPV), vertical vorticity may develop explosively (slantwise vorticity development) and result in severe weather such as torrential rainfall. MPV is related with stability, baroclinity and vertical wind shear. Its symbol (positive or negative) is determined by their allocation. In the middle troposphere (at about 500 hPa), the region with negative vertical component of MPV. i. e. MPV＜0, is a transition region between cold dry air and warm moist air. The zero isopleth of MPV1 or the region between 0 and 20 (0.1 PVU) can be considered as the northwest limit of a torrential rain area. The horizontal component of MPV, i. e. MPV2, is determined by moist baroclinity and vertical wind shear. To the south of the front, air is convectively unstable. Positive MPV2 centre can appear above the lower-lever jet, and torrential rain may occur in the region north of the center. To the north of the front, air is convectively stable, negative MPV2 centre can appear below the upper-lever jet, and heavy rainfall may occur in the region south of the center. The vertical circulation and the three dimensional thermal structure across the front region are given.