Storm total snowfall is all the new snow that fell during one storm, found by adding together the new-snow measurements taken through the event. It counts snow as it fell, not the depth left on the ground when the storm ends.
How observers build a total
Observers measure new snow on a snowboard, a flat board set on top of the old snow, and record the greatest accumulation since the previous observation to the nearest tenth of an inch. National Weather Service (NWS) guidelines allow the board to be cleared no more often than once every 6 hours.
When the snow ends, the observer adds the measurements from each board clearing to reach the storm total. Where wind has drifted the snow, observers take several readings in the least drifted spots and average them.
Why a sum beats one end-of-storm reading
Snowfall is defined as new snow before melting or settling. Fresh snow compacts and melts during lulls, so a single measurement at the end of a long storm can come in lower than the sum of readings taken along the way.
The 6-hour limit guards the other side. NWS guidance says never to add more than four 6-hourly observations into a 24-hour total, because more frequent clearing can inflate the number.
Totals that cross midnight
Snowfall is recorded by calendar day, so a storm that spans several days needs its daily amounts combined. After the February 2011 blizzard, NWS Chicago revised storm totals because the first tallies counted only February 1 and 2 and missed snow from January 31 that belonged to the same storm. The revised total at O'Hare was 21.2 inches.
What a total does not tell you
Ten inches over 36 hours and ten inches in four hours read the same as a total but carry very different impacts. Narrow snow bands can also make totals change sharply over short distances, so one report rarely speaks for a whole county.