I have been through dealing with a parent with dementia/Alzheimer's twice. Very early on they may be able to read a checklist and follow it but during an emergency probably not. And I would be very worried if they tried to drive somewhere--as they may forget how to get where they were going and without a cellphone you can't contact them. Plus, they may forget the cellphone, or even just forget how to answer it when it is ringing.
If at all possible, I would call the person with dementia and tell them to stay put. Then evacuate my family and drive to the elderly person's house and evacuate them. Or, even better, if you have two drivers, then send one to help evacuate the elderly person, then the other driver can handle the rest of the family. Also, plan a place where you are going to evacuate to so various parties can meet up. This needs to be planned in advance since even cellphones will be unreliable.
I would also put an "In case of Emergency" list on the elderly person's refrigerator BEFORE they need it. Then even a helping party can do the prep for them.
I suggest also prepping for them by getting as much medical information about their medical needs as possible, including their medical issues, tests, treatments, and prescriptions PRIOR to any emergency events.
There is a lot more about this on the Listening to Katrina website that I have posted about before.
Listening To Katrina