Two of my favorite weather related plot devices.

After particularly intense lightning storms, a green glowing fungi can be found on the trees and plants. New monsters seem to be springing up in the region. These storm are obviously bringing these things from other places and depositing them with the wind, rain, and goop upon the land. Kerren

Every now and again, the region encounters a dense blue fog. Every now and again, things disappear into the fog never to be seen again. The odd part is that "things", odd things from other places occasionally appear. Some of them are odd people who do not speak the local language. It seems the Fog is some extra-dimensional force that transports people and things between the worlds. Arth0xp

This one is also related to your other thread about borrowing plots from classic sources...

The Emperor is mad. His ministers are on the verge of assassinating him out of pity when he comes to the players with a request. "For months now the realisation has been with me. I have seen it in my dreams and waking nightmares. My ministers believe I am paranoid, but I know better...

I have seen the flood, the blanket pall of rain, the livid lightning arcing toward the seething bloated ocean. I know what will befall our world.

I want you to build me a ship from wood. Make it (arbitrary dimensions, probably in archaic system of measurement like cubits) high, (ditto) wide and (ditto) long. In it you will take me and my most trusted counsellors, enough food to last us (n) days and one of each pair of animal in the known world. You have seven days."

He will not explain any more than that.

Surviving on a makeshift raft for (n) days with dragons, wildebeest etc. could prove taxing to the players, trying to keep the peace...

ephe!0xp

A merchant wants to reach a good business opportunity before anyone else. You have to get him through impassable terrain, be chased by wolves, fight bad weather, and generally collect subdual damage and fatigue. Enjoy the pain.0xp

A village was separated from the world by an avalanche or something, and needs help and supplies. Get there, help as you can, and clean the road as you come back.0xp

One particular location is spared from the effects of the usual harsh winter. It snows very little, and it is surprisingly warm for this season. People are content, if every winter was like this, great! Is it only some kind of anomaly, that will pass, or is something stranger going on, possibly evil and dark, etc? (insert random demon/deity/whatever)0xp

A great storm has just finished pounding the island. Upon it shores, brought up by the cyclone, are the remains of a dozen ships lost through the centuries to the Tilda Reefs. Treasures once thought long lost are now being massaged by the lapping waves and slowly buried by the shifting sands of the beach.

Could be any number of magical/nonmagical artifacts. The treasure alone can change the fate of a nation.0xp

An unusually warm summer causes the Irongrip Glacier to melt partially, revealing the ruins of an ancient city.0xp

Melting of a glacier covers a coastal city with the extra 10 feet of water it added.

Fist size hail severly damages a city, perhaps weaking defenses or halting and destroying their construction.

A mudslide reveals a long ago covered cave system.0xp

A river floods its banks burying a small village. In the aftermath the river has cleared a bunch of land that can now be used as farming with a little work but has also moved most of the towns houses directly on top of the main road passing through.1xp

Strange winds are felt, and sometimes noises of battle and growls are heard over the wind, but not seen. Some weird natural occurence or a rip in the fabric of some plane?

Winds bring viruses of some exotic kind to a country?0xp

Every twenty-five years or so a great acid rain falls over the lands sending the people to live underground or deep in caves for many months.1xp

A sudden plague of tornados is ripping through the once peaceful valley. Crops and homes are destroyed forcing the kingdom to find somewhere to import food from or face a famine.0xp

A large earthquake ruptures through the land. A once common pass between two divided regions is now the location of a huge unpassable crevasse. Builders and supplies will be needed to create a bridge to connect these now separated areas. Problem is that the quake damaged the homes of many unsavory beasts which now plague the area. May need to protect the builders or somehow send some protection to the otherside to protect the stranded countrymen.0xp

A large volcano (or volcanoes) erupts, sending ash high into the atmosphere. This can cause a drop in temperature and an increase in precipitation. The reach is potentially global. Even if the eruption is so far away that the players will never hear of it, there can still be unseasonable weather patterns (e.g. summer snowfalls in a Mediteranean climate). I used this one as a long-term story element, to great effect.

Using cruel weather is one of my favourite evil GM tricks. See also the other major weather-related disaster, where a change in rain patterns caused wide-spread famine:0xp

The summer has been extremely hot. The valley is almost like a melting pot. In one of the hottest days a great thunder like sound is heard all over the valley. It seems that the heat has caused parts of the great glacier to fall of, revealing the ruins of an ancient city inside.

In the next days snow avalanches reveal more and more of this city. The area is flooded by melting water. But the PC's might be more interested in
exploring the ruin city in the mountains...0xp

OOP Rain

In an area of the world not known for its rainfall, at a time of the year that there is never any rain, there has been days and days of warm, near tropical, rain. It does not rain all day, nor does it rain every day, but it has rained 24 of the last 30 days and stayed cloudy for almost all of this time.

No one can divine the cause.

The roads are muddy. The crops are being ruined. Mold is happening everywhere. While warm, people are tired of the wet... so they are grumpy and depressed. Now, odd plants are sprouting up in abundance. Some of have been seen before in the area, one or two plants in the area. Now they there are thousands of them.

The world has changed. Now what?0xp

For the second year, the rains in the low lands were early and very light, thus the grass and plants have been abundent here, but not in the hills. Plant eating animals are coming out of the hinterlands and closer to humanity. The predators have moved down with them. Several have picked up a taste for human.0xp

A freeze, nearly a month early, has killed off or stunted most of the crops. Given the minor drought the last two years, there is no longer much food going around.

It is going to be a hard winter for everyone, poor and noble alike. And when it is a hard winter, mauraders come calling looking for food, goods, and women.0xp

21) The rain has been comming down for nearly 22 days straight. It might abated for a bit, but the clouds never quite clear. People have come concerned. The Godsworn are trying to explain it all to the peoples. Some are saying it is a sign that it is the end of the world. Others are saying that it is the tears of the spirits crying over humankind's failure to "be good". Still others are trying to forstall a panic and telling everyone it is just rain.

22) An Icestorm has swept across the land. Everything is covered in a layer of ice. People have died from the exposure. In fact, as you travel, you will see one person along the side of the road, frozen in a pointing position as they tried to ward off the last bit of ice and cold.

23) It is spring by the stars, but the spring spirits have not brought warm weather yet. The ground is too hard to dig into - as it is froze solid. The planting needs to happen soon. What can be done?

24) The snows have been heavy this year. It was a deep wet winter. Now spring has come in blessed relief. It has come warm and dry and early. The local ground is still muddy and a bit saturate. Little do people know it has warmed the mountain snows. The floods will be comming. It will be like a tidal wave coming down from the mountains along the valleys.

25) The storm has driven you to seek shelter in a cave not too far from the road. A bolt of lightning struck the hillside above you. It started a land slide, blocking your entrance/ exit. However the shifting rocks have opened up the broken debree in the back of the cave. There seems to be air comming through there. Now, you have to enter the unexpected caves hoping to get out.

26) It is fall or winter. The air is exceptionally cold and crisp. The moon is full. The brightest, most shimmering moon halo is occuring. It is a time of magic and myth. And of course, something happens - something released or escaped or just travelled here - near the characters.

27) The spring is one of wind. The wind has been constant, strong, and from an odd direction. It sometimes carries odd smells with it. Children are constantly playing with the new invention of "kites". Now new plants are begining to sprout. This is obviously the wind of change, but what does it herald?

28) A wet and gentle spring has led to creeks and rivers swelling to great size. It has also brought forth more frogs than anyone has ever seen before. Some of them are quite large, and seem to be growing larger. And what about that missing child?

29) You are in a new mansion of a noble. It is a dark and stormy night. White Blue Lightning flashes. You find yourself "elsewhere", but in the same mansion. However it is ... old... worn... dusty. There are odd people in odd garb. The candles have been partially replaced by magical lamps. The furniture is They are looking for some of their own who are "misplaced" and a possible murderer. Every time the lightning flashes, you (and some of your friends) jump between now and this "elsewhere".
Your characters jump several hundred years into the future, where the mansion still hosts the descendents of your noble. It is the 1920s and there has been a theft and a murder. Now you must play cat and mouse with a murderous fiend who is using this bridge between the two times to his own advantage.

30) It is a cold, bitterly cold, winter. However, no snow has fallen. There was some frost early on, but really nothing white has occured. The wells are begining to dry up. Something is taking the water.2xp