A multiscale analysis of three sequentially linked flood-producing heavy rainfall events during August 2014

In this study, a multiscale diagnostic analysis of three sequentially linked flood-producing heavy rainfall events in eastern Michigan, Long Island, New York, and eastern Maine, respectively, during 11–14 August 2014 is performed. In addition, the Long Island event, during which a persistent “training” convective line produced ~345 mm of rain in less than 12 h at Islip, setting the all-time New York State 24-h precipitation record, is examined using an ensemble of convection-resolving Weather Research and Forecasting (WRF) model simulations.

It is found that the three heavy rainfall events were preceded by the development of a Rossby wave train across the North Pacific and North America in response to a perturbation of the jet stream by a coherent tropopause disturbance over the western North Pacific. The Rossby wave train development culminated in strong cyclogenesis over the Gulf of Alaska, ridge amplification over western Canada, and the formation and amplification of an upper-level potential vorticity (PV) streamer (i.e., trough) over the eastern U.S. The PV streamer contributed to cyclogenesis and to the poleward transport of moist air over the Great Lakes during 11–12 August, supporting heavy convective rainfall (>100 mm) and flooding in the Detroit, Michigan, metropolitan area. During 12–13 August, continued amplification of the PV streamer was linked with the formation of a secondary low along the eastern U.S. coast and with the transport of an exceptionally moist air mass into the northeastern U.S. The secondary low tracked northeastward and interacted with a coastal front over New Jersey and Long Island, facilitating the development of the extreme-rain-producing convective line over Long Island. Thereafter, the low progressed into New England, producing heavy convective rainfall (>100 mm) and flooding across eastern Maine on 14 August.

To diagnose the mesoscale processes associated with the heavy rainfall event over Long Island, the members of the WRF ensemble are ranked according to the correlation between the forecast and observed accumulated precipitation distributions in the vicinity of Long Island. A comparison of accurate and inaccurate members highlights the importance of strong frontogenesis driven by the interaction between a southeasterly low-level jet (LLJ) associated with the secondary coastal low and a coastal front over New Jersey and Long Island for forcing training convection across Long Island. Notable differences between the accurate and inaccurate members with regard to the mesoscale structure and evolution of the coastal low and the LLJ are evident, corresponding to less robust frontogenesis and less-organized, shorter-lived convection over Long Island in the inaccurate ensemble members.