What does the term "Snow Interval" mean?
Email response from Alex Jacques (Programming Support - MesoWest):


  The exact interpretation can vary between data providers to MesoWest,
  but in general it refers to the amount of snow that has fallen on an
  "interval board".  In several cases, stations have boards where
  snowfall is recorded, and then the board is wiped clean at set
  intervals (sometimes every 6 hours, every day, etc.).  It is a
  complementary way to measure snowfall from a storm system in addition
  to a total snow depth reading, so often snow depth and snow interval
  are paired together for stations in MesoWest.


EDIT: Why do we need snow intervals? https://what-if.xkcd.com/104/ explains: as more snow falls, the existing snow on the ground gets compressed. To measure snow depth consistently, the "snow board" must be cleared at regular intervals.