Check out The Weather Network's 14-day forecast for Yellowknife this morning. (We arrive in YK on the 19th.) I'll continue checking the weather obsessively, multiple times a day, from three different sources. I'll be relieved if it isn't in fact -40, but then again I don't want it to be TOO warm, or the snow will be harder to run in. Tough to please, I know!
If I could choose, I'd ask for lows of -22 to -18; highs of -12 to -8 (all in °C); clear, sunny skies; minimal wind; no Ontario-like, core-permeating humidity; and lots of northern lights dancing at night.