The general processes leading to the development of convective precipitation ahead of a major cyclone are discussed, including the effects of horizontal advection and vertical motions on the creation and triggering of potential instability.
  By use of hourly rainfall charts, the history of development of a relatively narrow streak of heavy rainfall is illustrated. Heaviest rain occurred where several individual large rainstorms moved successively over nearly the same path. These rainstorms formed in a rather small area over a period of 16 hours. While the earliest formation took place in the region of greatest instability and general upward motions, no reason could be found for the successive formation of rainstorms in one locality. The analysis suggests quite definitely that the answer does not lie in any stationary feature of the general large-scale flow field.