JP3.5 A Predecessor Rain Event over the Upper Midwest Associated with Tropical Cyclone Rita (2005)

Wednesday, 3 June 2009
Grand Ballroom Center (DoubleTree Hotel & EMC - Downtown, Omaha)
Benjamin J. Moore, SUNY, Albany, NY; and L. F. Bosart and D. Keyser

Predecessor rain events (PREs) are distinct mesoscale regions of heavy rainfall that develop approximately 1000 km poleward and eastward of landfalling and recurving tropical cyclones (TCs) and approximately 24–36 h before the passage of the main rain shield of the TC. PREs develop as a continuous poleward stream of deep, tropical moisture emanating from the TC encounters a region of atmospheric lifting to produce heavy, prolonged rainfall. PREs present a forecast challenge because they have the potential to cause significant inland flooding, given that they are usually characterized by large rainfall totals (>100 mm in 24 h).

A particularly strong and prolonged PRE (rainfall totals >200 mm) occurred on 25 September 2005 in the upper Midwest as TC Rita made its way from the Texas/Louisiana coast northward into the central U.S. A widespread region of heavy rain (radar echoes ~40–50 dBZ) stretching from western South Dakota through southern Minnesota and into western Wisconsin developed and persisted for ~36 h as a substantial, poleward surge of deep, tropical moisture (precipitable water values ~40 mm) impinged upon a quasi-stationary, southwest–northeast-oriented baroclinic zone. A distinguishing characteristic of this PRE event was its quasi-stationary nature. It is hypothesized that downstream tropospheric ridging associated with TC Rita served to enhance the poleward moisture transport and acted to create a midtropospheric flow pattern that promoted slow system propagation.

The purpose of this presentation is twofold: First, it will assess the synoptic-scale environment in which the 25 September 2005 PRE occurred in the upper Midwest with particular emphasis on the central role of TC Rita in the persistent supply of tropical moisture to this region. Second, it will document how the dynamic and thermodynamic structure of the environment in the vicinity of the PRE evolved to favor exceptionally heavy rain over the upper Midwest.

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