These can be problemmatic and I have a mixed track record with them myself but a few thoughts:
- Tension can be drained if the players do not feel they have any control or that they are not really threatened.
- Just as exposition can be the death of a story, a ref describing a calamity without interacting with the players can kill a session.
To turn those around, then:
- Show the event matters by having it affect things they care about, right off the bat and continuously as it unfolds.
- Execute it in stages with cycles of a small amount of referee narration followed with input from the player that effects how it unrolls.
- Setup the event over the course of some earlier sessions so that it fits seeminglessly with the game.
For example, let's say you want to do a fire storm- a wildfire that has become a raging inferno such as used to hit the US midwest in the 1800s, sometimes killing 100s or 1000s of people.
You could set the stage for this in earlier sessions during the summer where you describe drought conditions and some smaller wildfires (a fight in a small forest fire might be different at some point).
By late summer you can make it clear the drought is severe and the air is frequently smokey from many small regional fires. Let the party even investigate one if they seem inclined to intervene so you can establish that up to this point, the fires while a nuisance are nothing special.
Now the session with the firestorm. It starts with a very smokey day and higher than normal winds. The players have shown that they, like everyone else around, is used to the smoke and won't really do anything until a firestorm is upon them (much as happened to the people of Hinckley, MN when a firestorm destroyed that town and killed hundreds;
http://en.wikipedia.org/wiki/Great_Hinckley_Fire).
Distract them even with the promise of an adventure that has nothing to do with the impending event (maybe they are visiting someone to get information about this adventure, or checking out an inscription in a tomb or the like).
To this point, the session might feel like the start of any other session with some early getting gear and information together before launching into some adventure. The ref has described a few unusual things but is also doing the normal interaction with players.
Visibility drops as the smoke of the firestorm approaches, then winds part the smoke and they see fire in the forest, the entire woods on fire, flames 100 feet higher than the crowns of the trees, feel the heat.
This would be a good time to destroy something the players care about. Their own manor house in the distance, a friendly NPC in a wagon over taken by flames, the town temple on a hill outside the town. It brings it home to them and makes it clear this event is going to change things inthe campaign.
After a fairly short narrative describing this, you now ask the players, what do they do. As always, it helps to have some obvious options prepared and considered but it is good to go with the players if they take a different path. They could be among strangers and might simply just want to flee but fleeing a firestorm is hard: they have near hurricaine force winds and can move faster than a person can run. If they are good or have ties here, they will likely want to help others. Regardless, their desicions now will affect things: who or what survives, how they are perceived afterwards (heroes, villains, cowards?)
The players make their decisions, you evaluate them as appropriate to the rules in use and the game style, the firestorm progresses in repeated cycles of narration and player responses.
From here on out, it is useful ahead of time to have some idea of how you see the storm progressing. A true firestorm and other major disasters may present little opportunity for the players to really affect the overall flow of these things but it is always good to let them affect what is reasonable. Certainly at a smaller scale, their actions could save or cost many lives or structures even if they may not have the means to stop the firestorm.
It is nice to progress this as a series of short narratives from you describing what happens next with a pause for the players to make their decisions. All the while, it is nice to keep tensions up by showing the firestorm destroying most things in its path, including as events allow, more things that the players really care about.
This particular event played out at a PC's holding like his manor house near a town he cares about could be nice because there is plenty to wreck that he cares about, providing nice motivation and dramatic tension, he will care about the aftermath- how many of his tenants survive, whether his manor house does, what happens to the nearby town, and it is a major event mostly out of the players control but one that could still unfold over a fair amount of in-game time (that is, it isn't just over and done with like an earthquake).
You could do something similar with a great flood or even an invasion of barbarians or monsters of some sort.
Naturally, if you wreck the holding they have painstakingly constructed, this might cause issues but you can show the silver lining to it but having them gain stature with their overlord or community, stature that might include outright gifts or ego-stoking awards and ceremonies. And maybe the firestorm clears foliage to reveal some ruin full of treasure and adventure.