Winter in Montreal is generaly cold, very cold somedays, the temperature could go down until -30 or more, but usualy it is around -10 to 5 which is okey.
If you come in winter you better wear warm clothes and don't play with cold ;))
All building are heated inside so no problem about this, however outside, if it is realy cold you have just to wear warm and don't walk for a long time.
Sometimes there are ice storm during winter, that I hateeee because you even can't walk outside except if you are professional in ice skating ;)
Beware, temperature can easily drop down to -20 C in the winter. With the humidity, your body will crisp. And with the wind, it can feel like - 50 C !
Better bring a thick jacket and good boots.
Don't worry if you see a local with no winter hat or gloves...it's normal !
Winter in Montreal can be really hard at times. Gone are the winters where just the mountain would stick out of a huge snowbank, but the snow still can fall heavily.
If you drive a car in Montreal during the winter slow the F down. There's no need ot drive like and idiot. The snow can make roads treacherous as well as hard to find a parking spot.
The undergound cities are awesome in winter since you don't need to drive around in the snow, can take off your 800 pound coat, and just chill out.
Nature covered by a thick layer of ice is beautiful... once you forget that some branch collapsed on a car, house, power is lost, road are not safe, etc...
And it doesn't get that bad most of the time.
When you have only 2-3 mm of ice, it's like having crystalized gardens, forests.
When you get 5-10 mm, then trouble begins.
OK not everyone supports cold the same. A hint, carry a t-shirt with a V-neck in your luggage, should it get colder, go into any washroom and put under whatever you wear without having to wear a jacket and be the only one freezing while all the canadians are happy about the 'warm' weather...
Winter is great! I love it for the winter sports, skating, skiing, etc...
When nature goes crazy, it can decide that although ground temperature is below zero, the temperature in the clouds allow rain to fall instead of snow.
What happens is that it will freeze upon contact to the first below-zero object it encounters.
That is exactly what happened during the worst ice storm in memory in the Montreal region. Freezing rain fell for a few days until it covered everything with such a layer of ice that most of the trees had broken branches, but also electrical pylones collapsed like dominos, cutting power to about 1.5 million people from a few hours to 45 days.
I was in the medium/lucky people who lost power for 8 days.
Winter in Montreal is very cold (sometimes less than minus 15 degrees Celsius), so do be prepared especially the thick winter clothings including gloves and headgear.
When I was at Montreal in Feb 2009, it was really cold but luckily I did not go out so often because I was there to attend a meeting held at the hotel I was staying (Delta Hotel Montreal).
Winter can get cold in Montreal, especially in January. Be sure to bring warm clothes, especially if you're not used to cold weather. Also, be careful of ice on the roads and if you have to climb a stairway to enter a building (many homes in the city have this), be careful that you don't slip if the stairs are icy.