Your missing the point. The amount of snowfall is only one factor the MO have to consider when issuing warnings. "Severe" weather can be prolonged, but a 10 minute shower which provides a covering coupled with gusty winds must be classed as severe too. Warnings are issued for the safety of the public, and for the older generation especially, 2-5cm's widespread is more than enough to cause problems and be classed as "severe". The depth of snow is generally determined with how prolonged the snowfall is, unless the snowfall is extremely heavy. However, if someone is driving down a motorway at 70MPH+ in the middle of a 20 minute heavy snow shower which accumilates around 2CM's - during that 20 minutes the weather would be considered severe.