From Glossary of Meteorology
Snow lifted from the surface of the earth by the wind to a height of 2 m (6 ft) or more above the surface (higher than drifting snow), and blown about in such quantities that horizontal visibility is reduced to less than 11 km (about 7 statute miles).
As an obstruction to vision, it is encoded BS in a surface aviation weather observation and as BLSN as an obstruction to vision in a METAR or SPECI observation. Blowing snow can be falling snow or snow that already accumulated but is picked up and blown about by strong winds. It is one of the classic requirements for a blizzard.