Abstract
Hourly surface observations from more than 200 stations, together with routine radar and rawinsonde data, are used to elucidate the mesoscale conditions for the extraordinarily widespread severe convection in the central United States on 3 April 1974. Strong mesoscale pressure fluctuations on an hourly time scale, particularly intense north of the Ohio River, were superposed on a substantial synoptic-scale pressure fall associated with the northeastward advance of a deep cyclone toward the Great Lakes. Most of this mesoscale variation was associated with 10 wayepackets of considerable spatial and temporal continuity. The character of the accompanying wind and weather fluctuations was highly suggestive of gravity-wave activity. The packets were aligned approximately along the low-level wind flow and propagated to its right at speeds of ∼30 kt.
Three major regions of organized convection were present, also oriented approximately parallel to the low-level flow and separated by ∼150 n mi. These regions propagated to the right of this flow at speeds of ∼20 kt and appeared to arise from a source other than the presumed gravity waves. Rather, the convection appeared to initiate the waves, in at least some instances. More generally, the overtaking of a slow-moving convective region by a rapidly moving wave caused an increase in intensity of convection, a reformation of a line of radar echoes within the region, and an increase in the pressure amplitude of the wave.
