As rain falls out of the storm, cold rain-cooled air hits the ground (a process known as a downburst) and fans out.  At the base and slightly ahead of the downburst, air rushes out at the ground, creating strong straight-line winds (this is occurring roughly 10 miles south of the photographer).  As the downdraft spreads out, it forces warm moist air ahead of the storm to be forced upward.  When moist air rises, a cloud will form, thus creating a shelf cloud just ahead of the main storm.