Even Env. Can. is not believing their own model. Just for kicks, I checked out the Canadian model raw temps for Sunday morning and they have Ottawa well into the -20s C. See map below. So why is their official forecast for Sunday morning still -17C? Well, without asking them personally, I've read that it is because they take into account the climatology, ie the "normal" temperatures for a particular place and date. If a model output is too many standard deviations from the "normal", they adjust temperatures back closer to normal. And what do you know, the "normal" low temperature for Ottawa is -17C.
TWN forecast low for Sunday morning is currently -19C
Accuweather forecast low for Sunday morning: -19C
Raw GFS and Euro temps for Sunday morning: between -25 and -30C