Forget about my definition, its the car's definition. Extreme weather is what the car thinks not me. Sometimes 31F is OK other times its not. Depends if the car is already warm or if the car is ice cold. In mean if you've been driving on the engine, and the coolant is already warm, and you continue on EV power, the ICE might not come on in auto mode. However, if the car is stone cold, and you turn on heat wanting to warm the 1C coolant up to 65C then the ICE could very well start.
At the end of the day use EV now if you're trying to use heat in the winter without the ICE, or don't use the heat and waste the battery, just tough it up and drive with your coat on. Whatever works for you. And if that doesn't work start the engine and drive with the heat and burn some gas.