Here is a forecast sounding for 03z (roughly 10-11pm eastern time) for the southern Quebec region. This depicts a vertical profile of the atmosphere between Ottawa and Montreal tonight, which is setting the stage for a classic freezing rain setup. On the bottom right you get information about the dendritic growth zone (ie where ice crystals form), which I circled on the sounding above around 575MB height (~6km). As the ice crystals form and become heavier, they begin falling towards the ground. They will then "follow" the red temperature line as depicted in the graph above. Once they cross the white line, which is the 0C line, the ice crystals will melt and form supercooled water droplets. This area is called the melting layer, which is seen to be between 750-975MB or roughly 2.5km to 0.5km.
As we can see from the graph, the red temperature line then crosses the 0C line once again near the surface (the surface here is at 1000MB). The supercooled water droplets will then cross the freezing line (this is the freezing layer) but will not have time to freeze back into ice crystals before hitting the surface since the freezing layer is so close to the ground. Therefore, these supercooled water droplets will freeze on impact as soon as they hit the ground. The large melting layer may give several hours of significant freezing rain accumulations for the Ottawa Valley and St Lawrence River Valley tonight. However, it will likely not stick very long as temperatures will warm up above 0C through the morning and remove that "freezing layer", which should give way to all rain. If temperatures remained below 0C at the ground surface, we could have seen a very significant ice storm. However, this is not anticipated for this event.
11:39 AM EST Thursday 20 December 2018 Freezing rain warning in effect for (areas in red)
Generally, between 5 and 10 mm of freezing precipitation are expected over these areas, though regions north of the St. Lawrence River can expect up to 20 millimetres of freezing precipitation.
The freezing rain should begin late this evening over Southwestern Quebec and will spread toward Central Quebec after midnight. Areas of Eastern Quebec will be affected beginning Friday morning.
The freezing rain will gradually change to rain Friday, possibly causing significant water accumulations locally.
Travel is not recommended. Surfaces such as highways, roads, walkways and parking lots will become icy, slippery and extremely hazardous.
Freezing rain warnings are issued when rain falling in sub-zero temperatures creates ice build-up and icy surfaces.