Winter Weather Patterns in Western Canadian Mountains

A meteorologist's motto says – if you can predict the wind direction you can predict the weather. It's not quite that simple in the mountains or elsewhere, but given certain wind directions or "flows", it is possible to generalize about expected weather. In addition to the direction, the strength of the flow, the source and stability of the air, and the topography are key factors in mountain weather.

To make a forecast, you need detailed knowledge of the topography because the most important questions regarding the future weather are:

1. Is the flow upslope or downslope?
2. Is the flow onshore or offshore?

In a storm, dynamic processes cause air to rise and cool, forming clouds and precipitation. Air in a storm encountering a sloping mountainside is further accelerated upward, causing heavier precipitation. On the downwind (lee) slopes the opposite process occurs: subsiding air dissipates the clouds and warms as it descends – the familiar Chinook wind.

Coastal BC Weather Patterns

Across the mountains, the most common flows are from the west – a consequence of latitude. The "Westerlies" are so-named, supplying abundant Pacific moisture that falls as winter snow in the mountains. The source of the air establishes the moisture content and thus the precipitation rate on the windward slopes. Onshore upslope flow of sub-tropical maritime air across the Coast Mountains unleashes monsoonal rainfall rates with enormous wet snowfalls in the high alpine. Conversely, an offshore downslope flow of continental air fosters sunshine downwind of the mountains.

What follows are some examples of the characteristic patterns associated with typical BC winter weather. The examples are from particularly dynamic and strong flows that produced intense storms. In this article the flow refers to the wind at 700 millibars (mb) (a unit of atmospheric pressure), a pressure which is roughly at the mountaintops or about 3000 metres above sea level. The flow is represented neatly by the orientation and spacing of what meteorologists call the height contours (a line on a map joining areas of equal elevation of the 700 mb pressure surface) on a "700 mb chart". The area most affected by poor weather and storms lies within the closely-spaced contours. This "packing" of the contours reflects the strength of the temperature contrast across the storm track or frontal zone: the stronger the temperature contrast, the stronger the flow and the greater the energy available for embedded storms (a storm in the strong flow) to exploit.

I hope this discussion and the examples provided give you a better understanding of how the "flow" relates to winter weather in the mountains. When all else fails, go with the flow!

Characteristics of Westerly Flow

Westerly flow at 700 mb

• Upslope flow and enhanced precipitation is greatest along north-south oriented ranges and especially at the convergent ends of west-east oriented valleys and inlets along the coast.
• Warm air cannot move northward in this flow so the freezing level remains relatively unchanged, rising only briefly with each approaching system before falling again on cold frontal passage. Warm 'noses' of air ahead of systems are sometimes pinched off entirely and slump southeastward, maintaining low freezing levels, while sustaining heavy accumulations of snow above 1000 to 1500 metres.
• Embedded storms are fast-moving and often followed by periods of rapid clearing that may last for a few hours but can persist for a full day. Timing of systems beyond day two is extremely difficult due to their rapid motion, so confidence in the forecast beyond day two is low. Satellite imagery shows smaller comma-shaped systems moving onshore followed by post-frontal cellular convective clouds (bright cauliflower shaped clouds that) form in the unstable air behind a cold front.

Characteristics of Southwesterly Flow

Southwesterly flow at 700 mb

• Southwest flow is perpendicular to most mountain ranges, maximizing the upward forcing of the air and the precipitation on the upslope (windward) side of the range. Southwest flow correspondingly maximizes subsidence on the lee (downwind) side, especially over the Okanagan and the Interior Plateau. Heavy precipitation is guaranteed along the Coast Range, with extreme amounts in southwest to northeast oriented valleys and inlets. Heavy snow is likely across the eastern ranges as the air is forced upward again by the towering Rocky Mountains. With rising freezing levels heavy wet snow persists only at the highest elevations while lee slopes/valleys remain bone-dry in subsidence breaks.
• Freezing level rises dramatically to over 3000 metres as a steady flow of warm stable air floods western Canada.
• Embedded storms can be fast-moving with very brief clearing (or none) between. If the offshore trough digs and a series of waves ripple along the frontal zone, a nearly stationary moisture-laden northeast to southwest-oriented cloud mass can linger for one to three days causing record rainfalls and flooding - the Pineapple Express - when air originates in the sub-tropics.

Characteristics of Northwesterly Flow

Northwesterly flow at 700 mb

• The skier's flow. A cold airstream gathers moisture over the Gulf of Alaska, becomes increasing unstable, and moves onshore in the form of bubbling convective cells (bright cauliflower shaped clouds that form in unstable air) that give brief but locally heavy snowfalls from the tops of peaks to near valley bottoms.
• Freezing level usually drops to 500 metres or lower.
• Occasional embedded storms appear as swirling comma-shaped conglomerations of convective cells moving swiftly southeastward. The duration of snowfall is limited by the small scale and rapid motion of these storms but snowfall rates can be very high.
• Heaviest accumulations along the Coast Range but if the comma cloud crosses the Coast Range, dry powder snow can accumulate over the interior.

Characteristics of Southerly Flow

Southerly flow at 700 mb

• Upslope flow occurs over the southern interior as air is forced upward from the Columbia Basin of Washington State. There is no significant subsidence.
• Extremely high (3500 metres) freezing levels as warm air spreads to northern BC.
• A nearly stationary north-south oriented front may linger for days across the North Coast Mountains. Storms rippling along the front maintaining wet, mild conditions.
• Warm southerly over-running of cold air in valleys east of the front creates persistent temperature inversions with little weather associated. Fog and low clouds clog interior valleys while the peaks of mountains remain relatively warm and sunny. Moist air flowing northward can give persistent low clouds and rainfall to the otherwise dry southern interior valleys. If arctic air is entrenched in those valleys then significant snowfalls occur.

Characteristics of Northerly Flow

Northerly flow at 700 mb

• Dreadfully cold flow of arctic air is aligned with the mountain ranges and valleys. Upslope flow now occurs off the Interior Plateau to the eastern slopes of the Coast Range. Associated weather usually constrained to a few flurries as the arctic front moves southward followed by rapid clearing and bitter cold conditions that can persist for days or weeks. On rare occasions, an embedded system from the north brings light snowfalls (5 to 10cms) of exceptionally dry snow.
• Freezing level lowers to the surface everywhere.
• This pattern breaks down with a gradual shift to southwest flow and the arrival of maritime air resulting in heavy snowfalls to sea-level on the coast. Quickly followed by a rapid transition to milder Pacific air that spreads inland via the Fraser Canyon in developing southwest flow.

Appendix

• 700 mb Analysis Charts can be found at:
  http://weatheroffice.ec.gc.ca/analysis/index_e.html
• Animated 700 mb (or 700 hPa) Analysis Charts can be found at:
  http://cirrus.unbc.ca
  Under Weather Links, select WX Viewer, then ANALYSIS, and 700hPa
• 700 mb Forecast Charts can be found at:
  http://weatheroffice.ec.gc.ca/model_forecast/global_e.html
  (The lower left-hand panel is the 700 mb level)
• Animated 700 mb Forecast Charts can be found at:
  http://cirrus.unbc.ca
  Under Weather Links, select WX Viewer, then PROGNOSIS, and GEM OOZ or GEM 12Z (The lower left-hand panel is the 700 mb level)