Need Help: Tornado Encounter

I want to add a tornado encounter to a homebrewed campaign, but not sure how exactly.

Conditions:

• The world is tidal locked with its sun..one side always faces away from it.
• The players are on the dark side, where temperatures are well below zero, and there are constant blizzards.
• Because of the blizzards and the wind, movement is halved...unless the players have woolly mounts of some sort.
• Snow everywhere, no trees, but chunks of ice? Stone?

I figure lightning could reveal the presense of the storm. There will also be moonlight, but the blizzard limits visabilty.

Not sure what level exactly they will be when it happens...maybe 5-7?

When it happens, there will be a place to take cover in...but it may be hard to gain access (mostly finding the entrance).

In theory, to tornado will travel at about 100 feet per round? That should allow time after spotting it to find some cover. The shelter is sort of hidden though.

Ideas?

If it helps, there’s a spell called whirlwind that’s basically a tornado. But it’s a 7th level spell so I wouldn’t use it as written on a level 5-7 party. Might give you a starting point, though. 

What kind of encounter is the tornado?

Is it to be a ticking-clock escape situation? i.e. they spot it in the distance and then have limited time to find shelter, but once they find shelter they have basically "won" the encounter.

Is it a trap/survival situation? i.e. the tornado is suddenly upon them and they need to work together to keep each other alive and get everyone out of the tornado.

Is it a combat situation? i.e. the tornado is being created by an enemy they can find and kill.

Quote from Agilemind >>

What kind of encounter is the tornado?

Is it to be a ticking-clock escape situation? i.e. they spot it in the distance and then have limited time to find shelter, but once they find shelter they have basically "won" the encounter.

Is it a trap/survival situation? i.e. the tornado is suddenly upon them and they need to work together to keep each other alive and get everyone out of the tornado.

Is it a combat situation? i.e. the tornado is being created by an enemy they can find and kill.

 It's a ticking clock situation intended to lead them to a shelter that contains a dungeon and important clues.

Leaving aside that tornadoes are formed by a mix of hot air rising and cool air falling, and don't tend to happen in places without a source of hot air... This sounds like a skill challenge encounter. Describe that they see the tornado and tell them they'll need to find shelter, then ask the players what skills they're going to use to try and navigate their way to a shelter. Oh, what a coincidence, whatever shelter they find has the next plot point.

You don't need a map for this and don't need to worry about the speed or damage of the tornado, although you will need to decide what setbacks will happen for failures on the skill challenges, especially if they botch them all.

For ticking clock, there are some good ideas in various sets of "chase" or "skill challenge" rules. Basically have a series of escalations for the tornado approaching: e.g.

Stage 0: High perception characters notice the tornado forming.
Stage 1: The tornado is clearly visible ahead of them.
Stage 2: Strong winds pick up, meaning the player characters must shout to hear each other and any dropped / thrown items will get blown away by the wind.
Stage 3: Winds increase again, skills using DEX are at disadvantage, lightweight/weak player characters might need to be held by the strong player characters to avoid falling over.
Stage 4: the wind is so strong characters have to succeed on a STR save or have something they are holding / carrying be ripped away and sucked up by the tornado.

Each "round" - i.e. after every player has attempted one thing to progress towards the shelter, you increase the Stage of the storm.

Now for the solution of reaching shelter, leave it open ended, but with clear problems the party need to solve. e.g.

Challenge 1: Get to the shelter building.
Challenge 2: Get inside the shelter.
Challenge 3: Seal up the shelter to keep them safe inside.

For each challenge have 2-4 suggested skills they can use to solve them, or they can suggest other skills to use. Some choices can provide Advantage or other bonuses on the skill checks of other party members - e.g. Performance to cheer on another party member. Then set appropriate DCs and the number of successes required to complete the challenge (critical success is worth double). e.g.

Challenge 1: baseline DC = 13, number of successes = number of characters in the party. Suggested skills: push/sprint Athletics (DC 10), find a safe path Survival (DC 12), dodge snow banks Acrobatics (DC 13)
Challenge 2: baseline DC =15, number of successes = number of characters in the party -1. Suggested skills: picking the lock Sleight of Hand (DC 12),  identify an opening Investigation (DC 13), force open a window Athletics (DC 15)
Challenge 3: baseline DC = 18, number of successes = number of characters in the party -2. Suggested skills: spot the gaps / weak points Perception (DC 15), use furniture to block up the gaps Acrobatics or Survival (DC 18),  hold the door/window closed Athletics (DC 20)

Maybe the question is, do you want the tornado to be a deadly threat that might kill PCs?  Or is it just supposed to be a challenge to wear them down for the next phase of the adventure?  I mean, if the party rolls nothing but natural 1s, should they die?

If you're going for a realistic threat, a tornado would probably kill them, so go ahead and have it do more and more damage (probably cold + bludgeoning) in addition to tossing them around, each round, until they've found shelter or run out of HP.

If it's not supposed to be that bad, then make it a skill challenge with lower amounts of HP damage, with the "total failure" outcome of losing (all or most of) their supplies, having to huddle under some ice outcroppings to avoid being blown away, and getting sealed in by the storm.  Once it passes, they have to dig their way out, low on HP, possibly fatigued, possibly low on spell slots, with no way of getting a long rest because their tents and food are gone.  So they're at a disadvantage now.  But luckily, they can now see a cave nearby where maybe they can find shelter... hey, are those tracks leading into it?  Maybe someone in there can help...

The goal is to make them feel endangered without killing anyone. Going for a feeling of intensity