On the mesoscale, PREs occur in conjunction with low-level frontogenetical forcing along a baroclinic zone where heavy rainfall is focused. The composite analysis shows that on the synoptic-scale, PREs form in the equatorward jet-entrance region of a 200-hPa jet on the western flank of a 925-hPa equivalent potential temperature ridge located east of a 700-hPa trough. PREs are high-impact weather events that can often result in significant inland flooding, either from the PRE itself or from the subsequent arrival of the main rain shield associated with the TC that falls onto soils already saturated by the PRE. A distinguishing feature of PREs is that they are sustained by deep tropical moisture that is transported poleward directly from the TC. PREs occur most commonly in August and September, and approximately 36 h prior to the arrival of the main rain shield associated with the TC. PREs are coherent mesoscale regions of heavy rainfall, with rainfall rates ≥100 mm (24 h) −1, that can occur approximately 1000 km poleward of recurving tropical cyclones (TCs). Twenty-eight predecessor rain events (PREs) that occurred over the United States east of the Rockies during 1995–2008 are examined from a synoptic climatology and case study perspective. The median air parcel (b) pressure (hPa) and (c) relative humidity (%) for air parcels originating within the TC Erin moisture plume (red), over the southeast United States east of the TC Erin moisture plume (black), north of the synoptic front (blue), and just south of the synoptic front within the ambient moisture plume (green) are also shown. The position of TC Erin at 0600 UTC 16 Aug and 0600 UTC are labeled by white and black tropical storm symbols, respectively, while the 0600 UTC position of the synoptic front is labeled with a dash–dot–dot contour. The air parcel trajectories are overlaid on the 0600 UTC analysis of total column PW (shaded according to the right grayscale in mm). The air parcel pressure values are indicated by the contour color (according to the left color bar in hPa). The numbers identify individual air parcel trajectories, while the circles mark 0000 UTC positions. (a) Twenty 72-h backward air parcel trajectories beginning at 700 hPa at 0600 UTC.
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