Chapter 4: Winter Weather Patterns in Western Canada

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.

Westerly Flow

Upslope flow, with enhanced precipitation greatest along north-south ranges and convergent ends of west-east orientated valleys and inlets along the coast.
Warm air cannot move northward, so the freezing level remains unchanged, rising briefly with each approaching system, then falling after a cold front passage.
Embedded storms are fast-moving, and often followed by periods of rapid clearing that last a few hours to a full day.
Forecasting the timing of storms is extremely difficult due to their rapid motion.
Satellite imagery shows smaller comma-shaped storms moving onshore, followed by post-frontal cellular convective clouds that form in the unstable air behind a cold front
Westerly Flow

South-Westerly Flow

Freezing levels rise dramatically to nearly 3000m as warm air floods Canada. With rising freezing levels, heavy, wet snow persists at the highest elevations, while lee-slopes and valleys remain bone-dry in subsidence breaks.
Flow is perpendicular to most ranges, maximising upward forcing of the air and the precipitation on the windward sides of the ranges. South-Westerly flows correspondingly maximise subsidence on the leeward sides of ranges, especially over the Okanagan and Interior Plateau.
Heavy precipitation is guaranteed along the coastal range, with extreme amounts in South-West to North-East orientated valleys and inlets.
Heavy snow is likely across the Eastern ranges, as the air is forced upward again by the Rockies.
Embedded storms can be fast moving, with none to very brief clearings in between. If the offshore trough digs, and a series of waves ripple along the frontal zone, a nearly stationary moisture-laden North-East to South-West orientated cloud mass can linger for one to three days, causing record rainfall

Southwesterly flow

North-Westerly Flow – The Skiers Flow

A cold airstream gathers moisture over the gold of Alaska, becoming increasingly unstable, and moves on-shore in the form of bubbling convective cells that give brief but locally heavy snowfalls from the tops of peaks to valley bottoms.
Freezing level is usually 500m or lower.
Occasionally, embedded storms appear as swirling comma-shaped convective cells moving swiftly South-Eastward. The duration of the snowfall is limited by the small scale and rapid motion of these storms, but snowfall rates can be very high.
Heaviest accumulations are along the Coast Range, but if a comma cloud crosses the Coast Range, dry powder snow can accumulate over the interior.

Northwesterly Flow

Southerly Flow

Upslope flow occurs over the Southern Interior, as air forced up from the Columbia Basic of Washington. There is no significant subsidence.
Extremely high freezing level (3500m on average) as the warm air spread to Northern BC.
Nearly stationary North-South orientated front may linger for days across the North Coast Mountains. Storms rippling along the front maintain wet conditions.
Warm Southerly air overrunning cold air in the valleys East of the front create temperature inversions. Fog and low clouds clog these interior valleys while the peaks remain warm and sunny.
Moist air flowing North can give persistent low clouds and rainfall. If arctic air is entrenched in those valleys, then significant snowfall can occur.

Southerly Flow

Northerly Flow

Very cold flow of arctic air is aligned with the ranges. Upslope flow now occurs off the Interior Plateau along the East slopes of the Coast Range.
With an increasing easterly component, upslope flows and precipitation are enhanced along the East slopes of all mountains.
Typical weather is a few flurries as the arctic front moves South, followed by rapid clearing with very cold temperatures that last days or weeks. Rarely embedded systems bring light snowfall of very dry snow.
Freezing levels lower to the surface everywhere.
This pattern gradually breaks down with a South-West shift. An arrival of mild maritime air results in heavy snowfalls to sea-level along the coast, quickly followed by milder Pacific air that spreads inland via the Fraser Canyon.

Northerly Flow

Blocking Patterns

Blocking Pattern Examples.

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.