Soon, a solid squall line forms and sweeps through the area with strong winds, intense lightning, and heavy rains. As the leading edge of the shelf cloud passes overhead, it becomes dead calm, and the underside of the shelf cloud becomes a sight to remember. Dark green, purple, gray, and yellow mingle and boil with turbulent mammatus like clouds. Then the cold downdraft follows, frequently dropping temperatures into the 40's. The visibility can drop to a few feet with hail or snow piling six inches deep.
Strong winds can topple trees and push cars and trucks off the road.
The best storms are multi-cellular which develop explosively west through south of Fresno. Within thirty minutes, a line of medium cumulus with tops to 15000 feet, become full fledged thunderstorms up to 50000 feet. The anvils generally stream toward the northeast, but the line moves to the east-southeast. Regeneration occurs on the southwest end and occasionally cloud rotation develops. Masses of scud are drawn into the cloud base.