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Pineapples, powder and precipitation - winter weather in the Coast Mountains

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Meteorological Service of Canada

A meteorologist’s motto says that "… if you can predict the wind you can predict the weather." It’s not quite that simple in the mountains – or elsewhere – but given certain wind directions or "flow patterns", you can make some basic predictions about the weather.

Pineapple Express Bringing warm rain

Today’s computer models do an excellent job forecasting the winds or flow at the level of the mountaintops and these charts are available on the Internet. If you love to ski, ride or tour in the mountains and know the difference between the Pineapple Express and the Powder Pattern, you can pick and plan your play-days better.

In addition to flow direction, the key variables in mountain weather are:

1) the strength of the flow,

2) the stability of the air,

3) the moisture content of the air, and

4) the terrain.

Weather is created by the vertical motions of rising and sinking air. Rising air cools, creating clouds and eventually precipitation if the air is moist and the updrafts strong. Conversely, sinking air warms and dissipates clouds, creating fair weather.

Weather forecasts are really just predictions of where the air will be rising (clouds, low pressure systems) and where the air will be sinking (clear skies, high pressure systems). Any process that helps "lift" the air will intensify precipitation and storms. Any process that results in subsiding or sinking air or that suppresses lift will do opposite.

Some important facts to remember:

1) strong flows have more energy for storms,

2) unstable air rises easily,

3) moist air can produce more snow than dry air, and

4) terrain can enhance vertical motion.

The two most important questions regarding the weather on the B.C. Coast are:

1) Is the flow upslope or down slope?

2) Is the flow onshore or offshore?

Air flowing toward a sloping mountainside is forced upward, creating heavier precipitation. Onshore flows bring moisture-laden air to the mountains. Offshore down slope flows bring dry air that warms and clears skies as it sinks.

In the Coast Mountains, the most common flows are from the west where the Pacific Ocean supplies the moisture for precipitation. The heaviest snowfalls occur when onshore winds blow extremely moist unstable air directly up the slopes. When the flow is from the southwest, warm air with high freezing levels floods up Howe Sound, converging as the valley narrows, producing heavy rain with snow at the highest elevations.

When the flow is from the northwest, cold air from the northern Gulf of Alaska sweeps over the warm waters of the Pacific, gathering moisture and heat from the ocean surface. When that cold moist and unstable air meets the Coast Mountains, heavy snowfalls of West Coast Powder result.

Upcoming Events:

"Pineapples, Powder and Precipitation." Thursday, Jan. 19, 7 p.m. at Millennium Place. Learn more about local weather patterns and avalanche hazards with David Jones and local avalanche forecasters Anton Horvath and Tony Sittlinger. Admission by donation. Presented by the Whistler Naturalists.

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