From AMS Glossary
(also called snow depth.) A measurement of the depth of snow on the ground made either since the snow began falling or since a previous observation.
The total accumulation is equivalent to the total snow depth during a storm, or after any single snowstorm or series of storms. Snow accumulation can vary due to settling and melting and will therefore vary depending on how often it is measured. For example, if new snow is measured every hour during a relatively long duration storm, it is likely that the summed accumulations may exceed a total snow accumulation measured only once at the end of the storm.