Or maybe an other effect of the orb of winters fury. similar to the warding spell it summons a snow storm trapping any units in the province and preventing units from entering it. Or another item.
That's pretty much the effect of Winter's Fury as a realm spell. I'll repost Raesene's writeup here:

Winter’s Fury
(Transmutation)

Spell Level: Divine [Kriesha] 3
Gold: 4 GB
Regency: 12 RP
Target: One province
Duration: 1 week + 2 days/level
Required Holding: Temple 3

In the cold wastes of Vosgaard, many travellers have reported sudden storms that appear out of nowhere and last for days. With this realm spell, a cleric can summon a powerful blizzard that covers an entire province, trapping invaders and potentially ending an attack if the invader is unprepared for the deadly conditions. The cleric may prevent the spell from taking effect for up to a week before its power overwhelms her control and the blizzard in unleashed.
The spell is not limited by the season and even in the middle of summer; those caught in the province take the full effects of the conditions. After the blizzard has passed, the snow dropped by the blizzard can be up to several feet deep and this could cause additional problems for invaders and the province’s population.
Army units caught unprepared by the blizzard conditions suffer one hit of subdual damage for each hour of exposure, unless they find suitable shelter. An army prepared for winter conditions must still find shelter, but have up to a day before the conditions begin to take their toll. If they are unable to find shelter after a day, units take one hit of subdual damage for each day of exposure. Soldiers in a unit reduced to 0 hits will die unless they are immediately given shelter and aid.
Individuals caught unprepared by the blizzard suffer the effects of extreme cold and severe winds, as described in the section on the environment in the Dungeon Master’s guide.
Pretty viscious, don't see much need to reinvent the wheel when Raesene has laid all the groundwork. :)