Distant effects of recurving tropical cyclones on rainfall production in midlatitude convective systems

Recent research has identified ``predecessor rain events'' (PREs), in which mesoscale regions of heavy rainfall occur ~1000 km poleward and eastward of recurving tropical cyclones (TCs). PREs typically occur ~24–36 h prior to the arrival of the main rainshield associated with the TC, and frequently result in significant flooding. A distinguishing feature of a PRE is that it is sustained by deep tropical moisture directly associated with the TC that is transported well poleward ahead of the TC. This study will quantify the effects of this tropical moisture on a record-breaking rain and flood event over the northern Great Plains and southern Great Lakes region on 18-19 August 2007.

In this event, which occurred ahead of Tropical Storm Erin, a southerly stream of deep tropical moisture (precipitable water values >50 mm) moved poleward and intersected a northwest-to-southeast oriented quasi-stationary baroclinic zone beneath the equatorward entrance region of an upper-level jet streak over the upper Midwest. A slow-moving convective system developed and produced widespread heavy rainfall, with local amounts exceeding 380 mm (15 inches) that resulted in destructive and deadly flooding in Minnesota and Wisconsin. Observations and numerical simulations using the Weather Research and Forecasting (WRF–ARW) model indicate that low-level frontogenesis was maximized during the overnight hours of 19 August 2007 and provided the forcing for vigorous ascent during the mature stage of the PRE. A simulation in which the moisture attributable to TS Erin was removed shows a 25% reduction in the total precipitation and a 50% decrease in the maximum precipitation over the PRE region compared to the control simulation. The extent of this decrease in total precipitation in the “dry run” underscores the importance of moisture originating from TS Erin in transforming a heavy rain event into a high-impact, record-breaking rain event.