Observations of Pressure Jump Lines in the Midwest, 10–12 August 1976

Abstract

Strong thunderstorm activity over Iowa on two successive afternoons was the apparent source of pressure-jump lines (PJL's) which moved SSE at 50 km h?1 through the nocturnal boundary layer and were detected by National Weather Service (NWS) stations as far away as Paducah, Kentucky. Rainshowers and thunder were reported at many NWS stations as the PJL's passed. The Regional Air Monitoring System (RAMS) network at St. Louis provided detailed information on the PJL'S. Arrival there was indicated by an abrupt pressure rise of 1.5 mb, a near reversal of the surface flow, and a vertical displacement of 750 m extending through the lower 4 km of the atmosphere. The passage of each PJL was coincident with the turbulent collapse of the nocturnal jet. The observations of the PJL events seem indicative of an internal bore and are similar to those of the Morning Glory seen in northern Australia. We speculate that the bore originates from a late afternoon convergence produced by thunderstorm outflow and opposing low-level winds involving the nocturnal jet.