The jagged crest of the Continental Divide splits Glacier NP into two climatic regions. Two opposing systems, the Arctic Continental and the Pacific Maritime, influence the climate of the parks. The Pacific Maritime, called "Inland Maritime" here, is the dominant system. During the winter it breaks through the continental air mass, producing warm chinook winds on the east side.
West of the divide, Pacific fronts bring heavy precipitation and moderate temperatures. Moist areas west of the divide give rise to a cedar-hemlock forest in the Lake McDonald area, which is at its eastern limit here. On the eastern side, the amount of precipitation at St. Mary is about the same as at West Glacier; however high winds produce a drier, more extreme climate. Here, wind is the single most important climatic factor, one that affects every feature of the eastern slopes, particularly the vegetation that must adapt to severe drying. The wind often blows exposed areas clear of snow even during the harshest of winters, providing excellent winter range for ungulates.
Precipitation distribution is quite variable and is much higher in the upper elevations. Annual average precipitation at Headquarters near West Glacier is 29 inches (74 cm.). Annual average snowfall at Headquarters is 157 inches (398 cm).
Waterton has highly variable mountain weather, which can change quickly. Summers are brief and cool, and winters are mild and snowy, with frequent warm spells caused by chinooks (strong winds that become warm and very dry while rapidly descending the lee side of mountain slope. The park's climate is influenced by two opposing systems – the Arctic Continental (cold, dry) and the prevailing Pacific Maritime (warm, moist). The more influential Pacific system gives Waterton high precipitation and frequent chinook winds.
Wind is the most important climate factor in Waterton. After Crowsnest Pass, Waterton is the next windiest place in Alberta. Highest winds blow in January and November, with gusts of over 90 mph (145 kmph). Prevailing winds are from the SW.
Waterton has one of the highest chinook frequencies in Alberta, which contributes to it being one of the warmest areas in Alberta in winter. Even when freezing out on the prairie, temperatures may be as much as 20-30 degrees Celsius (about 35-55 degrees F) warmer in Waterton because of a strong Pacific influence from across the narrowest part of the Rockies.
Waterton receives Alberta's highest average annual precipitation levels: 60 in. (152 cm) at Cameron Lake, 42 in. (107 cm) in the townsite, and 30 in. (76 cm) at the park gate.