It's really hard to extrapolate -29 to -40. I have noticed on extremely cold nights the temp drops like a rock, then very suddenly hits a bottom and usually rebounds some, just to crash again to a lower bottom. -40 is much harder to get than -29, however saying you wont get under 15 below next week is way too early to call and I believe there is a very high probability of it busting. Especially since the airmass will be colder AND any warming effects will be severely hampered by this current cold air.