Cold Snap/Fimbulwinter in a desert

I've been running a desert-themed campaign for a little while now. I've planned an infusion of ice monsters and cold-themed spellcasters to surprise my players, and the mechanic to allow them in-game is primarily by using the Cold Snap and Control Weather spells to keep the temperature frosty (I plan to continue this at higher levels with the Fimbulwinter spell). I have a desert town, with an ample source of water (a very prolific spring) - the ice creatures and cold-casters will be using the spring to build some impressive icy landscapes.

I realize that this may be more of a meteorology question... what happens to the environs AROUND the area of now-winter chill? I have a small (about 2 mile diameter) pocket of ice-filled, below freezing climate (maintained by repeated castings of the above spells) surrounded by a warm, dry desert. Do clouds form outside the cold ring? Rain at the perimeter? Violent inward or outward winds?

I can't fully answer the question, as I'm not well versed in meteorology, but it was my understanding that deserts typically reach very low temperatures, sometimes below freezing, over night. I don't think spells such as cold snap or control weather would cause rain or storms, unless those spells were also adding moisture to the area. Depending on how large the area you intend to keep cold is, I could imagine some "unnatural" wind in the outskirts of the area, though maybe not enough to form dust devils or other cyclones. The cold area probably has a ceiling with more warm air above it.

Magic School Bus says that weather works when warm, moisture-laden air rises into the air, forming clouds. As the clouds ascend, the moisture in them forms ice crystals, which descend to earth as ice or snow. However, as warm air moves, cold air moves in to vacate the area in question, unless there's enough warm, moist air (like in tropical areas) to keep the cold air at bay. Gotta love Ms. Frizzle, who is definitely a high-level Wizard. Shikka shikka ka-boom!
However, with your spells, you're taking warm, dry, desert air and attempting to transform it into cold, moist air, which, I'm not sure, is something the spell does, even with Cold Snap applied first.
But supposing you allowed it, and the game mechanics serviced drama as would a copper piece harlot, then yes. As your warm fronts and cold fronts met you'd have violent inward/upward winds and large cloud banks. But snow and ice wouldn't last past the spell's area.

Well, the desert is used to cold. Cold is a normal night time thing in the desert.

Unless you are moving a TONS of water, in the literal sense, then nothing happens. A lot of plants and animals die (but most just leave for better grazing) and the desert is cold for a while. Snow would require a silly amount of water to come from somewhere, and you need to be thinking lakes full of water being vaporized, and cold air doesn't vaporize water well. The hot air is dry, the cold air is also dry.

You may get tornados and wind storms if the cold air is forced up by rising heat, creating a pressure differential that collapses into high winds.

Note that just because a place is cold doesn't mean that it is necessarily wet - Antarctica has some of the dryest places in the world, after all.

If you went by real-world logic, the circle of cold air would have a lot of warm air trying to move in and fill the low-pressure area, so you get warm winds and stuff - however, in this case I'd be inclined to just stick with whatever the spells say that they do (since such a scenario would eradicate the circle of cold air completely).

Deserts already get very cold. The main "game-changer" from Fimbulwinter would be the snow, i.e. rain, for 48 weeks. When the duration is up and the sun returns, I would expect all that snow and ice to melt and have a significant impact on the climate.

Yeah, 48 weeks of rain collecting as snow would cause massive flash flooding when it melts. Anything in low ground will be inundated if it's lucky, or leveled if it's not.

Psyren is correct. The spell would maintain the weather for a duration but once that duration is up, you may have left a permanent change in the desert. Especially considering the area that Fimbulwinter encompasses.

Going with the science club here.

Place can already get cold. You're making it colder. The big change is the sudden introduction of massive quantities of (frozen) water which, for the duration of the spell, aren't going anywhere.

Come the thaw, it's going to take a lot of time to melt it all. Apart from the warm months, the desert is dry, but not always hot; in some cases, thawing snow could theoretically re-freeze at night.

As mentioned above, flooding is also a very real (and destructive) risk. A desert landscape is highly malleable to begin with; a large quantity of water moving through it would create channels extremely quickly, and these would expand very quickly. You could be looking at complete redirection of rivers, for example.

In a desert region, settlements often arise around bodies of water, such as oceans, lakes, and rivers. A lake could be dramatically altered - or even destroyed - if the melting snow creates a channel drawing water away from it. A river's source - the mountains or sea - would likely remain the same, but its path would shift to that of least resistance, which could be a dramatic change if the ice, erosion and thaw have anything to say about it.

But that's all within the radius. Outside of that, it's complicated, because, well, magic. We have a magically created radius, and you can stand directly outsideo f it and get warm summer sun. It defies meteorology to begin with. In theory, instead of the spell having a fixed radius, it would have a vanishing radius - at its worst within one area, then slowly decreasing in severity as you move out. But the spells don't say that; they just say there's a magical radius of blizzards, and then nothing.

So, basically, you have a choice when it comes to the area outside of the radius. Either you can say that the effect is so dramatic that it basically soft-extends beyond its radius, creating cold rainy weather for a ways outwards, or you can just hand-wave "A Wizard did it" and observe that people walking through the desert suddenly encounter a wall of falling snow.

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Originally Posted by Psyren

Deserts already get very cold. The main "game-changer" from Fimbulwinter would be the snow, i.e. rain, for 48 weeks. When the duration is up and the sun returns, I would expect all that snow and ice to melt and have a significant impact on the climate.

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I agree with this sentiment, but dare to quibble with the language. 48 weeks worth of water, in one form or another, is likely to change the weather. It is, going by scientific standards, unlikely to change the climate. Climate is a matter of averages over large periods of time. Colossal amounts of water will largely run off toward lower lying areas or form lakes in the low areas, where they will perhaps affect the immediate surroundings (an oasis for several seasons). But, in the long run, the return of heat and dry air will suck up crazy huge amounts of water in a matter of seasons, depending on just how dry/hot the desert normally is.

During the evaporation period, weather will likely be unstable, but these kinds of changes tend to average out as the more typical forces re-establish themselves.

Of course, all of this assumes things are going by science, much of which magic casually ignores.

Thanks everyone for your thoughts. Lot's of good discussion on how I can approach this - much appreciated.

Firstly, let me say that this is a really cool idea! I actually had a similar plot going once, but the PCs managed to prevent the BBEG from getting the spell off. I'd love to hear how all this turns out.

The way I was planning to do it was that the heat on the edge of the spell's effect would be causing the snow just within the spell's borders to evaporate rapidly shortly after sunrise, causing a ring of mists during the day which blew away during the night. The BBEG had minions with blindsight to harass interlopers in the mists during the day shift and undead for the night shift. Maybe not as scientifically accurate (or even RAW accurate?) as some of the above ideas, but it creates a great dramatic tension when the PCs have to fight difficult to see/hit skirmishers in a low visibility setting.

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Originally Posted by Gemini476

Note that just because a place is cold doesn't mean that it is necessarily wet - Antarctica has some of the dryest places in the world, after all.

If you went by real-world logic, the circle of cold air would have a lot of warm air trying to move in and fill the low-pressure area, so you get warm winds and stuff - however, in this case I'd be inclined to just stick with whatever the spells say that they do (since such a scenario would eradicate the circle of cold air completely).

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You'd probably get tornadoes up the ying yang around the outer edges.