Predecessor Snow Events associated with Extratropical Cyclones

During the winter months, the Great Lakes region is repeatedly affected by lake-effect snow events. These events often occur following the passage of a cold front, as a prevailing westerly or northwesterly wind sets up resulting in heavy accumulations to the east and southeast of the lakes. However, under certain environmental conditions, this paradigm can be reversed and lake-effect snow can occur on the western sides of the lakes. One way in which this reversal is realized is when air ahead of a warm front of an approaching extratropical cyclone is cold enough as to allow the lake-effect snow to form under easterly flow. This predecessor snow event (PSE) is subsequently exacerbated by synoptic-scale precipitation associated with the approaching extratropical cyclone. The purpose of this presentation is to introduce the term PSE while examining a PSE climatology and case study during the winter of 2013–2014 off Lake Ontario.

Five PSEs formed during the winter of 2013–2014 off of Lake Ontario. The characteristics associated with these PSEs such as distance from the extratropical cyclone, low-level environmental lapse rate, and snowfall totals varied widely. The 14 December 2013 PSE is highlighted in this presentation because it featured the most structured band of the five events. This PSE formed under well-aligned easterly flow between an approaching extratropical cyclone to the southwest and an Arctic anticyclone to the north and lasted for twelve hours. As this cyclone approached Lake Ontario, the inversion level over the lake became shallower, causing the PSE to weaken before being absorbed by the snowfall associated with the extratropical cyclone.