To get a heavy Blizzard, first talk to Relj (the Viera at Mt. Bur Omisace), then exit to Paramina Rift. This triggers the heavy blizzard.
If you want to find Fafnir, who is the Mark which only appears during a heavy blizzard, you must not enter the Frozen Brook area, as that resets the weather to snow. Instead you must go the long way round to get to the Silverflow's End, where Fafnir resides. The blizzard will remain as long as you don't enter the Frozen Brook or leave Paramina Rift.