The evoluation of low-level flow upstream of the Continental Divide (Rocky Mountains) from being blocked, i.e. unable to surmount the barrier, to becoming unblocked and blocked again is studied observationally and numerically.

Satellite photographs during the summer months frequently reveal a weather situation in which strong convective development is observed over the Rocky Mountains and several hundred kilometers to the east of the mountains, while the region immediately to the lee is essentially cloud free. It is proposed that an orographically induced mesoscale wave phenomenon may produce this situation.