Explanation of Outflow
  Outflow zones are produced in thunderstorms as the result of a downdraft hitting the ground and spreading out in a horizontal direction.  When this occurs  a mini cold front is created thus pushing cold dense air out in front of the system. This cold dense air shoves the warm moist air upward in the atmosphere which results in the formation of a cloud.  This cloud is know as a shelf (arcus) cloud.