Mesoscale Conditions for the Severe Convection of 3 April 1974 in the East-Central United States

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 U.S. on April 3, 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 wave packets 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 similar to 30 knots. Three major regions of organized convection were present, oriented approximately parallel to the low-level flow and separated by similar to 150 n.mi. These regions propagated to the right of this flow at speeds of similar to 20 knots and appeared to arise from a source other than the presumed gravity waves. The convection appeared to initiate the waves, at least in 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.