Mountain Weather

NOAA






From Summitpost:


Mount McKinley is located in the subpolar low, a region where arctic air moving from the north converges with warmer air moving from the south at a latitude of 60 degrees (Denali is located at 63 degrees). This convergence creates a belt of unstable weather at this latitude. Associated with the subpolar low is a semi-permanent area of low pressure known as the Aleutian Low, located in the vicinity of the Aleutian Islands. Weather systems generated or passing through the Aleutian Low often take a straight path for the Alaska Ranges and hit Mt. McKinley with little warning. Mt. McKinley also has a reputation for streaky weather with long periods of continuous bad weather or more rarely, long periods of good weather. This explains why inexperienced, clueless climbers may reach the summit while veteran climbers die on its slopes.

As a result of its proximity to the Aleutian Low, the weather on Denali is unlike the weather on any other major mountain in the world. Extreme cold is another hallmark of Denali’s weather and temperatures routinely fall to -40 F (-40 C). Unfamiliarity with McKinley’s weather is an underlying cause of many accidents, particularly among climbers who view the mountain as “just another prize in the trophy case” or as a warm-up for Mt. Everest. Weather patterns generally fall under the following categories:

HIGH WINDS. Mt. McKinley undergoes long periods of clear skies and high winds, and these conditions are most often seen during early part of the climbing season (April and May). During these periods, many of the mountain slopes are swept clean of snow leaving behind solid blue ice and testing the cramponing skills of even the most experienced climbers. Denali Pass and the upper part of the West Rib are especially notorious sites for accidents when these conditions occur.

During such weather, many climbers are lulled by the clear skies into going for the summit. However, these winds routinely exceed 100 mph and have been known to pick climbers up and throw them down the slopes. Windstorms often come with little if any warning and are thus amongst the most feared weather patterns on the mountain. The first signs of increasing high winds are the appearance of lenticular cloud caps over the summit. Many of the accidents in 1992, Denali’s deadliest year, occurred during such a weather pattern.

LOW PRESSURE SYSTEMS FROM THE GULF OF ALASKA. These are the cyclonic weather systems that sweep in from the southwest and vary in intensity, the strongest of which have been described as hurricanes that dump snow. During the more powerful storms, it is generally unsafe to be above 14,000 feet. Fortunately, climbers usually have at least 12 hours warning before such a storm hits and the park service does a good job of informing those who don’t heed the mare’s tails in the sky. Such a weather system was responsible for the single deadliest accident on the mountain which claimed the lives of seven climbers on the Karstens Ridge route in 1967.

LOW LEVEL MOISTURE. This occurs when warm, moist air migrates inland from the Pacific Ocean and creates a period of steady drizzle in the lowlands. During these periods, the top of the cloud layer occurs between 10,000 and 14,000 feet and conditions above these altitudes may be quite good. At other times, skies above 14,000 feet will be clear, but windy. Conditions on the lower mountain, however, tend to get sloppy and may even experience periods of rainfall and whiteout.

“GOOD WEATHER”. Days in which there is not a cloud in the sky and not a breath of wind are rare on Mt. McKinley. Instead, good weather is when winds are blowing at less than 10 knots and precipitation is limited to a snow squall here and there. Many amateurs unfamiliar with Denali’s weather may misinterpret this kind of weather as marginal or a prelude to a big storm. 

One should not think that from these descriptions, weather on Denali will easily fall into one of the above four categories. Instead, the distinctions between these four patterns fall into a thousand different shades of gray. Other weather patterns occur that do not fall into these categories. For example, low pressure systems may descend from the north from the Arctic Ocean. Because these storms approach from the north, they may catch climbers on the West Buttress by surprise. Even the most experienced veterans have a difficult time forecasting the weather, but for amateurs unfamiliar with subpolar weather, reading Denali’s weather can be more akin to rolling the dice.