When it happens you’ll need to make shelter, stay warm, drink water and eat. In that order.
A fully charged cell phone is certainly a help, particularly if you can get to high ground that’s unobstructed…you may be able to get a signal. A spare battery provides added comfort.
A sharp knife, water purifier or purification tablets, metal cup, wire saw, whistle, LED flashlight, at least 100’ of nylon cord, basic first aid items, a few nutrition bars and thermal blanket (very thin/lightweight) are other essentials to add.
Then come the luxury items when weight isn’t an issue, blankets, more rope, cooking gear, tarps, sleeping bags, hatchet, freeze-dried food items, bow saw, a gallon or more of water, GPS, several lighters and plenty of paper and kindling for starting a fire.
Again, the key is shelter, fire/warmth, clean water and food.
And equally essential to all this is the knowledge on how to use it. Where to find dry wood, how to “make water”, how to use your GPS or compass and not only how to make a shelter but having actually done it. This all combines to create your menu of outdoor skills that you can continue to practice and rely on your entire life.
I’d had the pleasure of attending the birthday of an 80 year old man who had spent his life helping those younger than him hone their outdoor skills. On his 80th birthday he jumped up on a picnic table with his trusty bow, stick and block and proceeded to make fire…the old fashioned way. He wanted to show us all he could do it in 60 seconds. He didn’t…it only took him 30 seconds before he got his tinder going! Decades and decades of practice developing a skill relatively few in the world are capable of doing was his answer. The very same man years earlier had developed a walking stick made from an aluminum tube that he’d loaded with survival gear…it was genius!