Remember that the in-game world is smaller than that in the books, and that
the area of Middle-earth covered in the books is equivalent to a smallish part
of Western Europe.
I remember in Asheron's Call, in its early days when we had Advocates and I was
one, getting /a messages from someone stationed in Eastham (eastern shore)
that it was raining where he was, and knowing that in ten or fifteen minutes it
would be raining in Yaraq (on the western shore of the Inner Sea). The whole
island of Dereth was stated by a dev to be forty miles across, so say from Eastham
to Yaraq was twenty miles. That makes perfect sense.
I see no reason why a carefully-designed weather system should not precipitate
from Falathlorn to the Misty Mountains, dropping rain or snow depending on the
Eruanne - Shards of Narsil-1 - Elendilmir -> Arkenstone