Playing through Platinum again because I love it so much, and one of the skiers on Route 217 said that it's almost always a blizzard, but occasionally it clears up and the view is beautiful. All three times I've played through Platinum, Route 217 has always been a constant onslaught of hailstones and destruction (Hail damage kills). Does it ever stop snowing there, or is that just NPC dialogue?
No, the weather does not change.
The NPC was just trying to make conversation and the weather is always hail/snow. Source
The hail does change on a select few days each year, when it changes to 'diamond dust', which has no effect in battle.
A full list of diamond dust days can be found at http://bulbapedia.bulbagarden.net/wiki/Easter_egg#Diamond_dust