Figure 1. Composite chart for Pattern A severe weather events by Evenson and Johns. Frontal boundary is position of 700 mb front. Jet's are shown at 500 mb (thin arrow) and 300 mb (thick arrow). H5 and H3 trough position (dashed lines) and ridge (zigzag lines). 850 mb thermal ridge (dotted line) and difluent flow aloft ahead of front (zigzag line).