According to data, derechos are most common between May and August, when 70% of all derechos occur. As the line of storms strengthens, updrafts continue on the edge of the storm and the mass of rain-cooled air at the surface expands the storm horizontally, pushing it forward more quickly and generating higher wind speeds. The tornadoes may occur with isolated supercells (rotating thunderstorms) ahead of the derecho producing squall line, or they may develop from storms within the squall line itself," the Storm Prediction Center notes.