ESRL/PSD Seminar Series

Abstract

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The aim of this presentation is twofold. First, an overview of predecessor rain events (PREs), including a composite analysis of all PREs that occurred over the eastern third of the United States during 1995-2008, will be presented. Second, a case study of a high-impact PRE that occurred over the northern Great Plains and southern Great Lakes region in advance of Tropical Storm (TS) Erin on 19 August 2007 will be presented.

Predecessor rain events are coherent mesoscale regions of heavy rainfall (>100 mm in 24 h) that can 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. 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. PREs are high-impact weather events that frequently result in significant inland flooding, either from the PRE itself or from the subsequent arrival of heavy rain associated with the TC that falls onto soils already saturated by the PRE rainfall.

In the case of TS 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 rainfall, with local amounts exceeding 250 mm that resulted in record 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 preliminary finding of interest from a simulation using the WRF-ARW model in which the moisture attributable to TS Erin was removed shows a 31% 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.

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