Verification of Canadian avalanche bulletins including spatial and temporal scale effects

In the winters of 2004-05 and 2005-06, 235 local ratings of the current avalanche danger (“nowcasts”) at and below treeline in the Coast, Columbia and Rocky Mountains of western Canada were compared with the danger rating from the public avalanche bulletin for the region including the nowcast site. These regional bulletins are issued from three to seven times per week, and the forecast regions range from 100 km2 to approximately 29,000 km2. After identifying an observation bias and filtering the data to 192 cases, the local nowcasts agreed with the regional danger rating in approximately 59% to 64% of the cases in the Coast, Columbia and Rocky Mountains. The agreement rate was higher for small forecast regions than for larger regions. Many of the nowcasts could be compared with danger ratings published zero, one or two days previously, allowing the effect of different lead times to be assessed. For the observed range of forecast areas and lead times, spatial scale effects were greater than temporal scale effects.