Why snowstorms and extremely cold weather still occur in some regions under global warming?

The cold event in one place at one time (say, a week or a month) is just weather, and says nothing about climate. Global warming refers to a long term rising trend of globally averaged temperature attributed to human activity since the 20^th century, in addition to natural climate variability. Snowstorms and extremely cold weather are extreme climate events against a background of rising temperatures. Such events are part of natural climate variability and are not precluded by global warming. However, global warming has reduced the frequency of occurrence of extremely cold weather over past few decades. The frequency of extremely cold events is expected to decrease further if the global temperature keeps rising in the future.