A nice way to do this is a Wizard 5/Red Wizard 5/Incantatrix 10
"I'm terribly sorry for what I'm about to do..."
Spells: Fimbulwinter
Metamagic: Fell Drain, Extend Spell, Flash Frost Spell, Energy Substitution (Cold), and Lord of the Uttercold
Metamagic Reducer: Arcane Thesis (Fimbulwinter).
Maximize Spell and Empower Spell are gained from Circle Magic
Fimbulwinter (8)+Fell Drain (+2-1-1)+Flash Frost Spell (+1-1)+Extend Spell (+1-1)+Lord of the Uttercold (+0)
Energy Substitution (Cold) is required for Lord of the Uttercold.
Make sure you've raised your CL to 40.
Wait until winter. Cast Frostfell to drop the temperature so that it's a cold climate, for a +10 to your d20 roll for the effects of Fimbulwinter.
Cast Fimbulwinter. You might want Spell Thematics to make this even more epic. Maybe have it look like a wolf is eating the sun, or some such.
10 minutes later, an area 40 miles in radius, up to 20 miles away from where you cast this, is in the depth of winter.
But this winter is different... for one thing, it will last for at least 100 weeks, or up to 144 weeks at most. For the people that don't want to do the math, that's a span from a little less than 2 years to a bit less than 3 years.
For another, its cold is infused with the deadly embrace of the Negative Energy plane. Every six seconds, regardless of the actual conditions within the circle, everything takes 8 cold damage and 8 negative energy damage. And everything damaged gets dealt a negative level. Wights begin to rise within a day of the start of this storm.
And the winter's conditions themselves are a horror. The depth of the snow can range from 9 inches to 10 feet of snow. And the winds...
Well they can be anywhere from 11+ to 31+ miles per hour.
I'm actually planning for something like this happening to a populous city in the history of my campaign world. A city of culture is destroyed within days, while hordes of wights roam through the area and into the countryside itself. And even a minute or two of exposure without adequate protection can result in another wight joining those numbers...
Of course, as of the present, this happened a century ago. But they're still hunting the wights...
But, yeah, find a way to make this immune to dispelling, and I would say this is worse than the locate city bomb and AftS. Because they don't last as long, and don't have the horrible effect of forcing the survivors to watch their loved ones, children and the elderly first, die from the supernatural cold, and knowing from the icy feel of the drain inside your chest that you would be next...
And there is the horror of effectively making that 40 mile radius almost completely unclaimable; an explorer would probably see it as merely being a field of winter, and even if he was 20th level (probably a monk,from the lack of paranoia and foresight necessary), he would die after a mere 600 feet of travel.
Drop this on a major trade route, and watch as wars break out...
Happy Halloween.