Synoptic-scale environments and dynamical mechanisms associated with predecessor rain events ahead of tropical cyclones

Predecessor rain events (PREs) are distinct mesoscale regions of heavy rainfall that develop approximately 1000 km poleward and in advance of landfalling tropical cyclones (TCs), and approximately 24–36 h before the passage of the main rain shield associated with the TC. PREs develop as a continuous poleward-moving 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 typically characterized by large rainfall totals (>100 mm in 24 h). Additionally, an increased risk of flooding is posed if the TC rain shield subsequently passes over the region previously affected by the PRE. The objectives of this presentation are to: (1) document, through PRE-relative composite analysis, key synoptic-scale features in the environments of PREs in order to establish distinctive scenarios for PRE development, and (2) examine, through a case study of a PRE associated with TC Ernesto (2006), mechanisms for PRE development and maintenance.

In order to investigate their preferred synoptic-scale environments, PREs occurring during 1988–2008 are stratified into three distinct categories: “jet in ridge,” “southwesterly jet,” and “downstream confluence,” based upon the configuration of the upper-tropospheric flow within which the TC is embedded. PRE-relative composites are presented to elucidate key dynamical processes in each category. While the composites indicate the importance of a poleward-moving stream of deep moisture from the TC, a low-level baroclinic zone, and an upper-level jet streak, the location, orientation, and magnitude of these key synoptic-scale features differ markedly among the three PRE categories. Our results therefore are suggestive of three distinct flow configurations favoring PRE development.

Dynamical and physical mechanisms associated with PREs are elucidated through a case study of a high-impact “downstream confluence” category PRE (event rainfall totals >100 mm), which occurred during 30–31 August 2006 in advance of TC Ernesto. This PRE developed in central and eastern North Carolina and Virginia as a poleward stream of deep tropical moisture (precipitable water values ~50 mm) interacted with a stalled cold front and a region of cold-air damming beneath the equatorward entrance region of a 200-hPa jet streak. Through the duration of the PRE, lifting within a mesoscale region of enhanced baroclinicity on the southeastern flank of an evaporationally generated cold pool provided a focus for the continuous development of stratiform and convective rainfall. Subsequently, significant flooding occurred as TC Ernesto (and its associated rain shield), aided by weak southwesterly steering flow, moved poleward over the region that had been affected by the PRE ~24 h earlier.