Mesoscale precipitation structures accompanying landfalling and transitioning tropical cyclones in the Northeast United States

Landfalling and transitioning tropical cyclones (TCs) over the Northeast US often result in high-impact weather events with heavy rainfall and dangerous flash flooding. Mesoscale features found in these storms not only exacerbate flooding on smaller scales, but also provide an important forecast challenge. The purpose of this presentation is to demonstrate how the observed mesoscale distribution of heavy rainfall in landfalling and transitioning TCs is modulated by interactions between synoptic and mesoscale features in the presence of the complex physiography of the northeast US.

A 52-storm (1950-2006) dataset of landfalling and transitioning TCs that produced storm-total rainfalls of at least 10 cm was used to identify candidate storms for our analyses. The synoptic and mesoscale precipitation characteristics were determined by constructing analyses of daily rainfall (1200 UTC to 1200 UTC) from the NCEP Unified Precipitation Dataset for pre-2004 cases, NCEP/HPC analyzed precipitation maps, and WSI radar imagery. The 6 h NCEP/National Center for Atmospheric Research reanalysis dataset and 3 h NCEP North American Regional Reanalysis dataset, hourly archived surface data, and the NHC best-track TC dataset also were used to investigate these cases. Selected surface and upper-air maps were constructed to identify and categorize certain synoptic and mesoscale characteristics for each case. A natural coordinate partitioning of the Q vector into along- and cross-isentrope components and associated divergence fields was used to help identify areas where quasigeostrophic forcing for ascent could provide a favorable environment for mesoscale frontogenesis and heavy rainfall accompanying these TCs.

Results show that enhanced ascent associated with frontogenesis and/or orographic lifting provided a mesoscale focus for heavy rainfall that tended to occur to the left of the storm track in the majority of the cases. The interaction between a landfalling and transitioning TC, an advancing upper-level trough, and a pre-existing mesoscale boundary (e.g., a coastal front) occurred in almost every left-of-track heavy rainfall case examined. In some landfalling and weakening TCs that experienced minimal extratropical transition, corridors of very heavy rainfall were noted to the right-of-track in conjunction with training convective storms.