If you're going to hiking or backpack in mountainous terrain in winter, you should learn about the prevailing weather patterns and forecasting tools that are are available for planning hikes and other backcountry tours. The weather in valleys seldom matches the weather at higher elevations, especially in terms of temperature and wind. These factors can have a big impact on the gear required for a hike, your comfort and enjoyment, and risk, since search and rescue times are much slower in poor winter weather. Mountain weather is sufficiently dynamic that you can't simply plan a hike months or weeks in advance and expect the weather to cooperate. I have cancelled many hikes due to sketchy weather and advocate you take a similarly conservative approach. The mountains will be there another day.
Weather Related Hazards
The main weather-related hazards in mountainous terrain are wind, cold temperatures, snow conditions, and water crossings.
- High wind can make walking difficult. It can accelerate heat loss from exposed areas of skin, resulting in frostbite and or create missiles, such as flying branches, chunks of ice, or tree blowdowns that can injure you.
- Cold temperatures cause ice which can make walking difficult or dangerous depending on your location. They can also accelerate hypothermia and frostbite when coupled with wind.
- Deep snow can be difficult to walk through, resulting in higher energy expenditure if you need to break out a trail with snowshoes or end up postholing because you do not have proper flotation. Blowing snow can obscure your vision in the form of whiteouts and cause navigational problems. In certain areas, snow accumulation can also produce avalanche conditions.
- Unbridged water crossings become much more risky, since getting wet can quickly lead to frostbite or hypothermia in freezing weather. River rocks that are dry during three season weather may be covered with snow or ice, making crossings far more hazardous.
Mountain Weather Concepts
Mountain weather results from the interplay between topography, wind, and moisture. Being able to read and interpret the elevation changes shown on topographic maps becomes much more important in winter when deciding on routes to take and whether to hike or not on any given day.
Mountains Create Their Own Weather
When the wind hits the mountains, it speeds up as it flows over mountain tops. This can create numerous microclimates, depending on the geography of the terrain, which can have very different weather despite being relatively close together. For example, if the wind hits a mountain's east side, it's not unusual for the valley on its west side to receive substantial snowfall. Moisture carried by the wind hits the mountain and cools as it's deflected upwards. This results in snowfall on the opposite side and potential avalanche conditions, if the valley's sides have a slope angle between 30 and 45 degrees.
High Elevations are Colder
For every thousand feet of elevation you climb, the temperature drops 3 to 5 degrees. Called the atmospheric lapse effect, this not only has temperature consequences, but can also explain why the weather on top of mountains differs from valleys and lower elevations. It also explains why the risk of frostbite and exposure increases at higher elevations when coupled with increasing wind speeds.
Prevailing Winds
The wind blows across most mountain ranges in a consistent direction: for example, from west to east or east to west. Significant changes in wind direction are often an indication of a significant change in the weather pattern. In winter, this can often mean increased precipitation if the wind passes over a major body of water, or extreme cold, if it comes from the north.
Fronts Bring Bad Weather
Fronts – both cold fronts and warm fronts – define the dividing line between two air masses, one of which pushes the other out so its way. When fronts collide there is a change in the weather pattern, usually with bad weather, high winds, and a change in weather direction. Here's what the interaction between these two types of fronts looks like on the ground.
Warm fronts, associated with areas of high pressure, affect local conditions gradually and often provide observable clues, such as wispy clouds, 24 hours before their arrival. Cold fronts associated with areas of low pressure move in much more rapidly, as fast as 35 mph, and causes rapid dramatic storms, followed by cooler and clearer weather. Knowing which is headed your way and when they will arrive can help you decide where you want to be when they make their influence on the weather felt.
Forecasting
There are a lot of information sources you can tap into for weather information and forecasts, but nothing trumps local knowledge of a mountain region's weather patterns. That's why it always makes sense to hike with a guide or experienced local who is familiar with the local weather patterns and can double-check your assumptions. Many mountain areas also have local weather and avalanche forecasters that can help you determine what conditions will be like in the mountains and wilderness areas. It's important to find out about these and track them for a few weeks before any major expedition, so you can anticipate bad weather patterns ahead of time and change your plans to avoid them. Observing weather trends is the most important part of good forecasting, because it lets you observe the formation and development of bad weather systems as they develop and provides a context to assess the conditions that will be present during your trip.
If you hike in the United States, NOAA (weather.gov) publishes detailed forecasts and instrument readings for free, along with several very useful graphical tools for displaying the information. This same information reformatted and dumbed down by many commercial websites, which is why I prefer to get it directly from NOAA instead.
The two NOAA tools that I use the most are the point forecast, which lets you position a cursor on the your region of interest and surrounding areas to get an idea of local weather patterns. This tool is quite precise, so you can drill down to the summit of a mountain if you want to discern weather differences on the basis of elevation or terrain. The point forecast tool is locate don the bottom right, below. It's also quite accurate within a 12 hour window, in my experience.
The 48 hour Key Metric Forecast is also very useful for trend analysis. This is a sub-report in the lower right hand corner of NOAA's 7 day forecast report and one of the most useful forecasts you can find for predicting key weather metrics 48 hours into the future. The report is based on the location of the current point forecast and let's you browse multiple variables including wind speed, gust speed, temperature, dewpoint, sky cover, thunder, and precipitation forecasts. I have found this report useful for forecasting when winds will increase or decrease above-treeline.
Other Resources
That's a brief introduction to winter weather forecasting for winter hiking, mountaineering, backcountry skiing and other winter pursuits. The best way to learn how to forecast local winter weather patterns is to find a friend who knows how to do it and can show you the ropes. It is an important skill for winter recreation, especially if you're after Type II routes that require a greater deal of self-sufficiency.
See Also
- Mountain Weather: Backcountry Forecasting for Hikers and Backpackers
- Staying Alive in Avalanche Terrain
- Winter Navigation and Escape Route Planning
The post Winter Weather Forecasting in Mountainous Terrain appeared first on Section Hikers Backpacking Blog.
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