Introduction
Winter backcountry recreation can be an incredible way to experience the mountains during the snowy months of the year. However, colder temperatures, stormy weather, and the variety of mountain geography in the Cascades can make it challenging to know what you’ll find, whether you’re searching for firm climbing conditions or soft skiing conditions. Below, we’ve compiled some introductory concepts and most importantly, resources for you to begin your journey to better understanding weather and forecasting. While we believe the information presented below provides a good starting point, it is not a replacement for formal training, and like all skills, is developed over time through experiences and continued education. If you’re feeling impatient, you can scroll down to our Resources for a list of weather sites and links. Lastly, stay tuned for our upcoming post about how weather drives avalanche problems and influences our decision about where to go on a given day.
Weather Basics
Oh Pacific Northwest winter weather...the stuff dreams are made of! It is no secret that the PNW is, in a word, WET during the winter months. Yet Washington is a diverse state and there are some regional climatic differences we need to identify.
Climate
Climate refers to the long-term weather pattern of a place, whereas weather refers to the short-term changes on the scale of days or months. Western Washington (e.g North Bend, Mt. Baker, Mt. Rainier) is characterized by a Maritime Climate with warm, dry summers and mild, wet winters. Our proximity to the Pacific Ocean ensures that clouds are always a possibility.
However, Eastern Washington (e.g Leavenworth, Yakima, Mazama) is characterized by a more Intermountain Climate, with colder temperatures in the winter, hotter temperatures in the summer, and less precipitation throughout the year accompanied by more sun during the winter compared to the west side. And if you were wondering, there is a logic to this pattern.
Terms and Concepts
In general, the majority of our weather systems arrive from the Pacific Ocean, to the west. As a result, weather patterns, whether sunny or stormy, tend to generally travel from west to east, which is the direction of the jet stream, a narrow band of strong winds in the upper atmosphere. However, boundaries between warm and cold air cause the jet stream to wobble north and south, which forms ridges and troughs.
Differences in air pressure are primary drivers of our weather patterns. An area of High Pressure generally indicates sunny, dry weather. This is also sometimes referred to as a ridge. Air in an area of high pressure compresses and warms as it descends. This warming inhibits the formation of clouds, which results in clear, sunny skies. Sometimes, the clear, calm weather associated with high pressure systems may instead result in an inversion, where dense, cool air (i.e clouds) sink and settle into low areas like valleys where they get trapped while sunny, warmer conditions persist at higher elevations.
A classic example of an inversion; cloudy below, sunny above!
The opposite of a ridge is a trough, which is associated with Low Pressure. Areas of low pressure typically are accompanied by unsettled weather patterns, including cooler temperatures, rain, snow, and wind. You will often hear an approaching trough described as a storm front. The intensity of a storm depends on several factors, including the energy, direction, and topography. Generally, low pressure systems, and their corresponding stormy weather, are more intense during the fall and winter.
Simplistic diagram of low and high pressure and corresponding ridges and troughs.
A few more concepts will really help us understand some meteorology basics. An important fundamental rule is that rising air cools. When air is forced upwards, either by terrain or a low pressure system, it cools dramatically. Additionally, cool air cannot hold as much moisture.
Conversely, sinking air warms. As air drops, either by terrain or high pressure, it warms quickly. Because warm air can hold much more moisture, clouds dissipate and precipitation stops.
Hopefully, these concepts start to explain some typical weather events in the PNW. For example, warm air, such as the kind that accompanies large atmospheric river events, are loaded with moisture. These conditions create the “Pineapple Express” events that skiers and snowboarders dread: warm air from the tropics that can hold huge amounts of moisture, resulting in rain instead of snow. It is also why as you drive east from Seattle to Cle Elum, the clouds dissipate as you travel east of Snoqualmie Pass.
The topography of Washington also plays a big part in its weather. As you may have gathered from reading above, terrain, such as steep mountains, can force air upwards, thereby cooling it down. This process is called orographic lift, and it is the reason why the west side of the Olympic and Cascade Mountains receive such whopping levels of precipitation.
In general, the complexity and variety of topography and geography in Washington creates very diverse weather conditions, even over small distances. Temperatures and precipitation amounts are often vastly different on either side of the mountains, and areas closer to the Puget Sound can be cloudy and mild while 25 miles inland it is cooler and sunny.
Washington Specific Weather Considerations
There are also a few specific tenets of winter Washington weather that greatly impact the amount and type of snow we receive in the mountains.
First, there is the direction that a storm is coming from. During the winter months, our storms typically arrive from the west or southwest. These storms tend to have milder temperatures, and the snow level (the elevation at which precipitation falls as snow and not rain) can be an issue. If the snow level is at 4500 feet above sea level (ASL), that means it is likely raining at Stevens Pass (approx 4,000’ ASL), and certainly at Snoqualmie Pass (approx 3,000’ ASL). In comparison, when storms come from the northwest, they are typically accompanied by colder air, lower snow levels, and lower-density snow. The direction of a storm also impacts which areas receive more snow. For example, Crystal Mountain Resort is northeast of Mt. Rainier, and so is partially blocked during storms from the southwest, resulting in less snow than other areas.
You may have also heard about the Puget Sound Convergence Zone when reading winter weather forecasts. A convergence zone forms when strong westerly winds are split by the Olympic Peninsula and re-converge on the other side, typically near the border of Snohomish and King County. The convergence of these strong winds can result in a narrow band of intense convective precipitation, resulting in enhanced snowfall rates. Oh, in case you were wondering, convection occurs when the Earth's surface becomes heated more than its surroundings and leads to significant evaporation of moisture from soil and plants into the air. Essentially, already moist air gets supercharged by an extra shot of moisture, leading to very intense, relatively short convective precipitation events. Convective precipitation is typically associated with rapid vertical rising (and therefore cooling) of the air, forcing moisture to fall as precipitation. Convergence zones often set up near Snoqualmie or Stevens Pass, and can result in significant additional snowfall compared to other areas.
Source: KOMO News
Another weather phenomenon relevant to Snoqualmie and Stevens Pass is easterly flow. In the winter, air on the east side of the Cascade crest is typically colder than on the west side. Cold air is denser than warm air, and so the resulting pressure gradient pulls the cold air from the east side over the Passes, lowering temperatures just enough for precipitation to fall as snow instead of rain. Without easterly flow, Snoqualmie Pass would likely be far less reliable as a skiing destination.
These are just some of the weather basics one must consider and understand when thinking about backcountry winter recreation in Washington. Luckily, we have tons of resources at our disposal to help us visualize, interpret, and predict upcoming weather conditions.
Resources
NOAA is the federal agency responsible for producing weather forecasts and products for the U.S. They provide seasonal outlooks, which are long-term predictions on general climate trends. However, generally speaking weather forecasts are not very reliable beyond two weeks. NOAA also provide “point forecasts,” which are forecasts for a specific location, such as Snoqualmie Pass:
These are helpful for understanding the general trend of weather and provide a baseline: will it be sunny and warm? Rainy and cold? During the winter, these forecasts also identify snow level at your selected location. However, these NOAA forecasts are typically not very accurate or detailed with regards to timing or sensitive enough to capture the nuances of places like Snoqualmie Pass. Plus, the interface on mobile is relatively poor. Lastly, a more thorough and technical narrative of incoming weather can be found in the forecast discussion. These are regional discussions that identify general weather trends and expected outcomes and impacts, and will often discuss mountain snow, especially in regards to the major highways.
NWAC is the regional forecasting avalanche forecasting center for the Olympics, Cascades, and Mt. Hood. In addition to publishing annual avalanche forecasts during the winter months, they also provide a ton of vital data and weather forecasting resources. The Mountain Weather Forecast is published twice a day (~7am and ~2pm) from around late November to late April. The forecast includes a synopsis discussion of the regional weather trends and patterns, as well as forecasts for specific mountain zones including precipitation, snow level, temperature, and winds. The 2pm forecasts include an ‘extended weather’ discussion covering the next several days as well.
Typical NWAC weather forecast
It’s important to understand a few key details of the NWAC forecast:
Precipitation is measured in inches of water equivalent. Water equivalent is: “is the liquid water equivalent of all precipitation types; rain, snow, ice pellets, etc., forecast to the hundredth of an inch at specific locations.” To use WE as a proxy for snowfall amounts, we generally assume a snow to water ratio of 10:1 (10 inches of snow = 1 inch WE) in the Pacific Northwest. Temperatures at or near freezing will generally have a lower ratio (heavy wet snow) and very cold temperatures can have a much higher ratio (dry fluffy snow).
So, in the above example, a forecast of 0.50 - 0.75 inches at Snoqualmie Pass would generally be interpreted as approximately 5 to 8 inches of snow, assuming typical snow ratios.
The trend is important. Are freezing levels rising or falling over the course of the day? Is more snow forecasted in the morning or the evening?
Another very powerful weather tool provided by NWAC is the historical weather data, which can be viewed via a map or a list of weather stations. Get familiar with the graph for the station closest to your preferred ski or snowshoe areas. Learning to interpret this data can greatly help with predicting potential snow and avalanche conditions. For example, consistent strong winds from a singular direction can be observed in the 24 hours prior to your proposed hike or ski tour. Or, you may notice that the temperature rose above freezing for several hours while precipitation was occurring, indicating a period of rain instead of snow. This skill takes practice and repetition, as well as verification during your outings.
Windy is a powerful weather forecasting tool with a mobile and desktop site. Windy is a more involved and feature-rich platform that requires some time to learn, but is well worth the effort! Some of the useful features include viewing animations of wind direction and speed, radar imagery, air quality, and more. Furthermore, Windy allows users to choose between different weather models. We won’t dive into weather models in this post, but this blog from Open Snow is a useful introduction.
Another visualization to explore is the Meteogram (shown above), which displays temperature, wind, precipitation type, and cloud cover (percentage and base elevation) over a short-term forecast range. This is particularly useful in the Cascades, where cloud cover and base elevation can have a huge impact on visibility in the mountains and therefore our experience! Windy also has webcams, which can provide very recent visual updates to what is happening in the mountains or on the road.
Pulling it All Together
At the end of the day, reading and understanding weather forecasts is a skill just like skiing, negotiating avalanche terrain, or packing your pack for the day. It takes practice and trial and error. One helpful habit is to regularly check the forecasts and compare your expectations of weather to what you experience throughout your day snowshoeing or skiing. To summarize, for winter recreation, some of the important factors to consider when planning an outing include:
General weather trend: will it be windy and stormy? Wet, cloudy, and warm? Or our favorite, sunny and cold?
What to check: NWAC weather forecast discussion, the NOAA point forecast, and the NOAA forecast discussion
Consider any safety concerns from the proposed weather. Will it be really cold and blustery on ridgelines? Will low snow levels make driving to the trailhead difficult? (Don’t forget to check WSDOT!) Of course, weather also drives avalanche hazard, but that is a topic for another post!
If you are expecting precipitation at your chosen destination, you will want to consider:
The amount of precipitation;
The timing and duration of the precipitation;
The predicted snow levels and any trend expected throughout the day
Other environmental or specific weather considerations:
Will visibility be difficult due to cloud level?
Utilize the Windy meteogram and NWAC forecast discussion to help anticipate visibility issues
Will east flow or a convergence zone result in significantly different conditions in certain locations?
Has an inversion formed?
Weather is a complex and endlessly fascinating topic in the Cascades and Olympics. It also takes some time to learn and become familiar with. We hope the above helps set the course for your weather education. Let us know in the comments below what was useful and what weather-related questions you may have and thanks for reading!