A Comparison of Extreme and Ordinary Freezing Rain Events of Long Duration and High Integrated Vapor Transport in Eastern North America

Freezing rain events of long duration (LD; at least 12 h) in two study domains of eastern North America gleaned from the database of McCray et al. are examined using ERA5 reanalysis-based integrated water vapor transport (IVT) to investigate how parent cyclones with high IVT (HIVT; surrogate for atmospheric rivers) contribute to severe freezing rain events. Although not necessary for an LD freezing rain event to occur, HIVT-influenced parent cyclones are associated with most (70%) events occurring in the southeastern Canada/northeastern U.S. (CAUS) study domain and with the majority (78%) of very long (at least 24 h) LD events in the region over a 38-yr period (1979-2016). Comparing extreme and ordinary LD events involving HIVT-influenced parent cyclones over the CAUS and southeastern U.S. study domains shows the importance in extreme events of slow movement and phasing between the continental parent cyclone located over eastern North America and a corresponding midlatitude anticyclone to the northeast of the CAUS domain that provides cold dry air needed in the near-surface cold layer to offset opposing diabatic effects. A corresponding subtropical anticyclone assists the parent cyclone in supplying warm and humid air at midlevels of freezing rain events to the northern reaches of the study domain, extending spatial and temporal scales of extreme events. Most ordinary LD event parent cyclones are progressive and impact the study domain only briefly. An abrupt end of ordinary LD events having a slow-moving parent cyclone occurs either when its center moves over the study domain or a second, newly dominant cyclone emerges.