Here a beautiful shelf cloud heading right for me near Agate, Colorado. Shelf clouds are essentially the lead out line of an incoming storm. Cool rainy air gets caught in a downdraft and spreads horizontally as it hits the ground. Warm and wet air is lifted to the edges and condenses to create these dramatic clouds. As it approaches the wind speeds pick up considerably and then within minutes intense hail, rain lightning and rain follow. This was about 3-4 minutes before it was directly on top of me.