Extreme rainstorms in advance of tropical cyclones

            Extreme rainstorms in advance of tropical cyclones (TCs) that can produce more than 100 mm of rain in 24 h, known as Predecessor Rain Events (PREs), occur in conjunction with landfalling and transitioning (and near-landfalling) TCs over the eastern half of the United States (US).  A climatological analysis of PREs suggests that roughly one third of all landfalling and transitioning TCs in the eastern half of the US are associated with PREs.  The typical PRE occurs ~1000 km in advance of the TC, leads the TC by ~36 h, lasts for ~12 h, and is fed by a deep poleward transport of moisture-rich tropical air associated with the TC.

Two recent examples of PREs that occurred over the upper Midwest in conjunction with TCs Erin (2007) and Ike (2008) and produced local rainfall totals in excess of 500 mm and 300 mm, respectively, will be presented.  The Erin and Ike PREs both occurred along low-level thermal boundaries where tropical moisture was concentrated and lifted in conjunction with mesoscale and synoptic-scale frontogenetical forcing.  The Erin and Ike PREs also occurred near the equatorward-entrance regions of downstream upper-level jet streams where dynamically forced lift was concentrated.  Finally, PRE rainfall ahead of Ike was also aided by mid-level moisture from eastern North Pacific TC Lowell that made landfall over northwestern Mexico.