Freezing Rain: an Analysis of Rare Events Associated with Northwesterly Winds at Montreal, Quebec (CYUL) and Overall Impacts of Freezing Precipitation on the Aviation Industry

Freezing rain (FZRA) poses a significant threat to the safety and operation of aircraft in and around affected hubs and airports. Accurately forecasting even short-lived events saves the industry millions of dollars over the course of one winter. Therefore, understanding both the mechanisms that lead to any duration of FZRA and its impacts on operations is crucial to aviation forecasters.

Though long-lived FZRA events in North America have been well documented, much rarer, short-lived occurrences of FZRA at CYUL, observed under a northwesterly wind configuration, have not and are thus the focus here. These rarer, non-classical occurrences are not a result of warm air advection aloft coupled with an underlying layer of cold air at the surface; the classic signature of FZRA in Quebec. Instead, they are the result of a cold front and trailing arctic air mass undercutting a much warmer, sub-tropical air mass. The location of the low-pressure system and its associated vertical structures are of significant importance to the subsequent occurrence of freezing precipitation. Using a 39-year dataset (1979-2018) and the North American Regional Reanalysis (NARR), we will compare these rare events to events that occurred under similar synoptic conditions and whose frontal passages yielded no FZRA at CYUL.

Finally, the effect of FZRA on operations within the aviation industry, regardless of duration, will be presented. This includes feedback from dispatchers with both small and large airlines, pilots, air traffic controllers, grounds crew at airports, and Nav Canada's National Operating Centre (the Canadian version of the Air Traffic Control System Command Centre-ATCSCC). This discussion serves to underline the impacts of FZRA from various points of view in the industry, and to provide forecasters with a better understanding of its effects on those who rely most on our forecast products.