Studies using data gathered during CASES-99 show that when the near-surface nighttime wind direction shifts through the up-gully direction of a significant gully near the tower, the flow produces a pronounced but localized upward surge of vertical velocity up to at least 55 m. This surge generates an outward propagating wave packet having horizontal and vertical wavelengths on the order of 100--250 m with tilted wave fronts consistent with upward phase propagation. The wave packet is observable (with significant delays) by other sensors out to 850 m. As a working hypothesis we assume that the up-gully flow is constricted and strengthened as it progresses up the narrowing gully. We theorize that, upon exiting the gully, the flow has a pronounced and tightly confined vertical component that in turn produces a packet of internal propagating gravity waves.