。
Things went smoothly until we ran into a blizzard.
Literal
Fierce-blizzard [to-に] encounter-[until-まで-は] smoothly proceeded.
~まで ('until') marks the endpoint of the smooth progress — 'up to the point where X happened.' Adding は makes it contrastive: 'up until that point, things were fine; after that, not so much.' 遭う (a variant kanji for 会う) is used specifically for encountering unwanted things — accidents, disasters, bad weather — where 会う is for meeting people.