You are describing where I live only that temperature is a bit low. I'm sorry to say it is neither one.
t_daily = t_average +/- 5
The highest temperature would be
t_daily = t_average + 5
t_daily = 1 + 5
t_daily = 6
================
The Lowest temperature would be
t_daily = t_average - 5
t_daily = 1 - 5
t_daily = - 4
Sounds about right for December.