Advice on transitioning from pads to going potty outside?

We just got a 6 month old labradoodle pup from our local shelter. She was surrendered by owners that figured out they didn't have time for a puppy. (Ugh!) Apparently, they did little to no training but at least got her to potty on pads while they were out.

Now that she is with us, we'd like her to go potty outside like our other dog. Some progress was made in that, as I hoped, she will go outside with the other dog or with one of us. She has even started to go out on her own if the doggy door is available. However, now that the weather in Alaska is getting below freezing we can't just leave the doggy door available.

Any advice on how to transition our pup from pads or going inside to telling us when she needs to go out?