A narrow cloud band developed over Kanto Plain on 22 April 1992 in association with the passage of an extratropical cyclone. Its unique features include the rapid formation and appearance of an arc-shaped solid cloud wall, rather than a line of a number of isolated convective cells. It evolved along the southern edge of a much broader weak rain area in a manner similar to the embedded areal type as identified by Bluestein and Jain (1985). A mesoscale analysis of conventional observation data reveals that a local cold air mass propagated southeastward from the northwestern part of Kanto Plain. It was driven by the horizontal pressure gradient force associated with the eastward-propagating cyclone. The cloud band was found to be forced to form when and where the cold surge collided with the synoptic-scale southwesterly flow which was prevailing over the southern part of Kanto Plain. It is further revealed that the cold surge was a leading part of a low-level mesoscale air mass which was termed as the "forerunner". It was so named since it existed in front of the synoptic-scale northwesterly flow and beneath the southwesterly flow ahead of the upper-level trough.