For a given wind flow, it is possible to generalise about future weather. In Western Canada, the most common flows are from the West. Westerly winds supply abundant pacific moisture, that then fall as snow in the mountains. The source of that air determines the moisture content, and therefore the precipitation amount.
The topography of the terrain is important in determining the precipitation rates of a given wind flow. ‘Onshore-upslope’ flows of sub-tropical marine air cause enormously wet snowfalls, whereas ‘Offshore-downslope’ flows of dry continental air cause cold sunshine. The following paragraphs discuss the characteristic weather patterns associated with winter flows in Western Canadian mountains.
Most winters experience a strong anti-cyclone lasting 7-14 days. Areas under the high centre experience prolonged sunny skies and calm weather. Areas under the low centre experience extensive clouds and precipitation. There are three types of blocking patterns:
A: Omega Block. This typical pattern is a low-high-low from West to East.
B: Rex Block. A high situated North over a Low. The isobars form a completely closed system.
C: Cut-off High; or Cut-off Low. When a pressure system remains stationary because it is cut-off from the upper flow.
Blocking patterns can have a significant impact on the snowpack. An extended high typically results in good stability, however may result in extensive faceting or surface hoar growth, which could become a persistent weakness.
Water vapour satellite images are the best tool for identifying blocking patterns. These images show atmospheric circulation at all levels.
Weather models often develop blocking patterns too slowly, and dissipate them too rapidly. They often persist longer than the forecast.