Mountain Weather Forecasting for the North Shore
Background: Understanding and being able to predict the weather are crucial skills for any outdoor enthusiast. The weather is what drives the formation and evolution of the snowpack and directly influences avalanche conditions. The public avalanche forecast is only as good as the weather forecast. The more we know about the weather forecast, the more we can get out of the avalanche forecast. This blog is designed to teach you a simple procedure for forecasting the weather on the North Shore Mountains.
Disclaimer: I am not a professional meteorologist. The information contained in this blog comes from years of experience skiing on the North Shore, working as a professional avalanche forecaster, and generally being a weather geek.
What are we interested in? If we’re concerned about snow and avalanche conditions there are 4 key weather parameters that directly impact the avalanche hazard: precipitation, wind, temperature, and solar radiation (how sunny it could be). These are the parts of the weather forecast that we should zero in on. If we’re wondering whether it will be an epic powder day or miserable rainy day then you need to know where the snow line is. You also need to know a little bit about the local terrain.
The parking lot at Cypress Mountain (downhill area) is at around 915 m elevation and the top of Sky Chair sits at 1430 m. At Grouse, the top of the Skyride is close to 1100 m. Here you can ski down to around 875 m and if you trek to the top of Dam Mountain you will be close to 1350 m. Over on Mount Seymour you will start at 1025 m at the parking lot and will climb to around 1425 m at the top of Tim Jones Peak. Knowing these elevations will help you figure out whether it will be raining or snowing based on where the freezing level is.
Freezing level versus Snow Level: The freezing level is the level of the atmosphere (altitude) where the temperature is at 0°C. Above this point the air is below freezing. The snow level is where precipitation changes from snow to rain, which is often a bit lower than the freezing level. How much lower will depend on what’s going on in the atmosphere, but as a general rule of thumb you estimate it to be around 200 m lower. If the freezing level is 1000 m you could see snow accumulate down to around 800 m elevation. If the snowfall is more intense it could drive the snow line down even lower, which is pretty common in springtime convective snow flurries.
Step 1 – The Big Picture: Understanding the overall weather pattern will help us figure out whether it will be warm or cool, or wet or dry. I’m sure most of you have heard of the Jet Stream before. The Jet Stream is a relatively narrow fast-flowing air current in the upper atmosphere. It moves from west to east (in the Northern hemisphere) in a meandering path. The polar jet stream is a very important factor in our local weather. It separates air masses with different temperatures and drives our weather patterns.
If the jet stream is dipping down south and dragging a moist and warm air mass in our direction then we should expect high freezing levels and lots of rain (Pineapple Express). If the jet stream shoots up to the Arctic and then dives back south-southwest toward the BC coast then we should expect and cold and dry Arctic Outbreak. For good snow on the North Shore we would want to see the Jet Stream flowing down from the Gulf of Alaska (a Northwest flow), or a flattened Jet Stream travelling from west to east (Westerly or Zonal flow). Check out this mesmerizing website showing the upper air flow patterns over the planet. The narrow bands of super high winds (pink/purple colors) are where the Jet Stream is found.
Now that we know to look for let’s talk about where to find good information. First let’s figure out where the Jet Stream is and what kind of weather it’s bringing our way. We can use the link above: http://earth.nullschool.net/#current/wind/isobaric/250hPa/orthographic. Then we can compare this with a satellite loop. This will show us the flow pattern and the clouds and weather associated with it. This is a good link: http://www.atmos.washington.edu/~ovens/wxloop.cgi?wv_common+/24h/.
Step 2 – What’s coming down the Pipes: Knowing what the general weather pattern looks like gives us a starting point for figuring out the type of weather scenario we might be facing. Remember, the Jet Stream analysis and satellite image only show us past and current weather. We can now look ahead using computer weather models. There are a large number of different weather models available. Some are lower resolution (GFS and GEM Global) and pump out a coarser forecast for the next 7-8 days or longer. Others are high resolution (GEM LAM/ HRDPS or NAM) and provide really good weather forecasts for up to 2 days. A high resolution model is certainly our best choice for short range weather forecasting, especially in the mountains.
I recommend looking at a computer model loop. Most models allow you to view a wide variety of different weather parameters. I suggest looking at a loop that shows precipitation and at least one other useful avalanche-weather parameter. One that I recommend is the US NAM model showing the 6 hour accumulated precip, 850 mb temperature, and mean sea-level pressure. http://mag.ncep.noaa.gov/Imageanis.php. This is a 3.5 day forecast at a medium resolution (12 km). The reason I choose this one is because it’s easily accessible and it displays well.
When looking at this we’re trying to figure out where the weather is coming from (southwest, west, northwest, north) and how intense it is. The green and blue shades indicate precipitation and its intensity. The red and blue lines are isotherms (contours of equal temperature). The first blue line above the red line is the 0 degree isotherm. This means at the 850 mb altitude the temperature is 0 degrees. This is important. As we know, when we go up elevation the pressure decreases. We can roughly correlate the pressure heights to an elevation. In this case the 850 mb pressure height is roughly 1500 m, or just above the peaks of the North Shore.
Step 3 – Honing in on the Details: Once I’ve looked at a weather model loop showing me the overall pattern I will then turn to a meteogram to analyze the details. A meteogram is a graphical depiction of the weather parameters from a computer weather model for a specific point. It’s an easy way to extract the details from a computer model, but we have to realize the spatial limitations. If a weather system drifts north or south of it’s predicted path then we could see significantly less or more precipitation. This is why understanding the pattern is valuable and looking at a weather model loop can help you make a better forecast.
This is one of the best free weather websites I have come across: www.spotwx.com. There is a lot of useful information on this webpage. First step is to drop a ‘spot’ on your area of interest. Let’s drop a spot at Cypress Mountain. Then you choose which weather model you are interested in (from the selection on the left – Canadian or American weather models). For this example we can use the NAM to stay consistent with the weather model loop we just looked at. I often use the GEM LAM as an alternative.
When you click on ‘GO’ it shows you a series of graphs displaying different information. First we have to confirm the time/date and what elevation this forecast is for. In this case it’s for an elevation of 477 m. We’ll be mostly between 900 and 1400 m so we’ll have to adjust this temperature a bit. As a general rule of thumb we can estimate that the temperature decreases 0.5°C for every 100 m of elevation gain. Fortunately we also have the option to view the temperature at the 805 mb height (~1500 m, or just above ridgetop).
There are also graphs for precipitation, cloud cover, and wind (both at the elevation given and the 850 mb height). These are all the weather parameters we are most interested in for snow and avalanche forecasting. Don’t get too distracted by the rest of the data here. There’s a tonne of useful data but more than we really need.
Using this simple process you can create your own basic mountain weather forecast. At a minimum this will give you more knowledge to supplement a good public forecast. If you’re looking to save time and just want to look at a good mountain weather forecast produced by a professional meteorologist then check out this link: http://www.seatoskygondola.com/visit/current-conditions. There is an excellent 4-day weather forecast produced for the Sea to Sky Gondola by meteorologist working for RWDI. This forecast is for the top of the gondola at around 900 m.