A high-impact predecessor rain event over the midwest associated with Tropical Storm Erin (2007)

Predecessor rain events (PREs) 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 in advance of 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.

A noteworthy example of a PRE occurred on 19 August 2007, when widespread rainfall, with local amounts exceeding 250 mm, occurred over the southern Great Lakes region from southern Minnesota to northwest Pennsylvania. This rainfall occurred in conjunction with a southerly feed of deep tropical moisture (precipitable water values > 50 mm) that intersected a northwest-to-southeast oriented stationary baroclinic zone left behind by a predecessor extratropical cyclone on 16–17 August. The deep tropical moisture streamed poleward from Tropical Storm (TS) Erin, which made landfall along the Texas coast on 16 August. A distinguishing aspect of the 19 August PRE from previously documented PREs was that it occurred in conjunction with a marginal TS rather than a mature TC. This result suggests that TC intensity is secondary to deep tropical moisture transport and the availability of a lifting mechanism well downstream of the TC in determining the likelihood of PRE occurrence, longevity and intensity.

The purpose of this presentation is to document the evolution of the synoptic-scale pattern during the PRE over the southern Great Lakes on 19 August. In particular, precipitable water analyses and air parcel trajectories will be used to show the contribution of deep tropical moisture from TS Erin to the PRE. Dynamic tropopause analyses, and meridional cross sections of kinematic and thermodynamic fields, will be used to assess the physical and dynamical mechanisms important in the poleward transport of deep tropical moisture, and the subsequent development of heavy precipitation.