Weather
This is the second article in a two-part series discussing what makes great powder and the reasons why it is found in the Cottonwood Canyons near Salt Lake City, Utah. Part 1 sets the stage and presents the facts for why Utah has some of the best powder skiing in the world. Part 2 is for the weather enthusiasts and explains why the Cottonwood Canyons are the ideal setup for consistently excellent snow conditions year after year.
Big Cottonwood Canyon (Brighton and Solitude) and Little Cottonwood Canyon (Alta and Snowbird) resorts are world-renowned ski destinations, and Utah is said to have the “Greatest Snow on Earth”.
At first, this may seem odd and, frankly, a bold statement for a state probably better known for desert towers and slot canyons.
So let’s explore what it is about the weather that makes Utah powder so magical!
To understand what makes the Cottonwood Canyons a hot spot of snowfall, we need to start with the basic ingredients of what makes a snowstorm (moisture, cold air, and energy). Once we have the background, we will get into the local factors that enhance the snowfall across the Wasatch Mountains.
The Basics: 3 Parts Every Potent Snowstorm Must Have.
Moisture refers to the amount of water vapor present in the air mass. This is measured as precipitable water, abbreviated as PWAT, which represents the amount of water that can be condensed out of the air. This is reported as a depth of water, such as “PWAT values are between 0.5 and 0.6 inches”. Notably, warm air can hold more moisture than cold air; therefore, it is often the warm tropical air masses that have the most significant PWAT values, exceeding a few tenths of an inch.
Cold air is a somewhat obvious ingredient in that you need to freeze the water vapor to get snow, but there is actually more to it than just “below 32 degrees.” The temperature and amount of water vapor dictate the type of snow crystal that forms. The type of snow crystal determines how tightly the snowflakes pack together and ultimately how dense the powder becomes. As discussed in Part 1, snow density is a crucial factor in achieving great snow. As it turns out, it is the stellar dendrites that produce the super-light snow, also known as “cold smoke.”
And the last ingredient is Storm Energy. Energy provides the means to mix the cold, dry air with the warm, moist air, making it all happen. The energy can come in as wind or convection (warm air rising into colder air), which mixes all the ingredients.
The Cottonwood Canyons Magic
Now that we know we need moisture, cold air, and energy to get a snowstorm, why do the Cottonwood Canyons receive so much snow?
Orographic Enhancement. While high-elevation terrain certainly helps collect snow, it is the change in elevation from the relatively low valleys to the high peaks over a short distance that squeezes out the snowfall. When the relatively warm, moist air at low altitudes collides with the sides of the peaks, it is forced up into the colder air at higher altitudes, and the snow begins to fall. Annual snowfall in the Wasatch increases roughly 100 inches for each 1,000 feet of elevation gain.
Convection. In addition to the mountains physically forcing warm, moist air up into colder air, convective cells can form, further mixing the moisture and cold air. This so-called convective orographic enhancement is driven by buoyant instability, where, much like a bubble underwater, a low-altitude air mass becomes less dense and shoots up into the upper portions of the atmosphere. This initiates the mixing of moisture and cold air, producing the high precipitation rates necessary for those mega 24-hour storms. The Wasatch seems to experience a high frequency of these convective storms, resulting in numerous large storms.
Location, Location, Location. The broad high terrain of the Central Wasatch sticks out of the terrain like a big snow-ning rod. The mountains are exposed to storm flow from all directions. What this means is that many different storm tracks impact the range and trigger orographic enhancement. The more often it snows, the more snow will fall.
Terrain Shape. If you look at the upper portions of the Cottonwood Canyons, especially from the northwest, there is a competent ridge that blocks flow. As the moist air flows over the range, the only way out is up through the cold air. There is not a lot of moisture that can escape by flowing around the mountains. The northwest flow is perfect for upper Little Cottonwood Canyon, which can sometimes get double the amount of snow as elsewhere in the range under these conditions.
Storm Patterns. Winter season atmospheric flows happen to hit the Wasatch Range just right. It is often the case that storms hitting the Cottonwood Canyons start with warm, moist air flowing in from the southwest, followed by cold, dry air coming from the north. This pattern, for one, brings lots of moisture and cold air to the exact location, but also very frequently results in a “right side up” structure with warmer, dense snow below cold, dry snow on top. After the front (the boundary between the warm and cold air) passes over, often the Cottonwoods are left in an unstable air mass with very low winds. The orographic effects keep the snow falling, and the cold air temperatures produce classic dendritic snowflakes. The combination results in a super-low-density topping of powder at the end of the storm.
Lake Effect. Now I’ve saved this factor for last, because there are some misconceptions about the fabled Great Salt Lake effect and its actual influence on Utah powder. First of all, lake effect primarily enhances snowfall by increasing lift and helping flow converge to create focused bands of heavy precipitation (not adding substantial amounts of heat and moisture like the Great Lakes effect in the upper Midwest). Yes, the Great Salt Lake effect is indeed real. Sometimes up to a foot of snow can fall in isolated locations in Salt Lake Valley due to lake effect. And if lake effect snow bands reach the mountains and are orographically enhanced, those storms can pack a punch, dropping multiple feet of snow over a short period. However, this weather anomaly does not significantly contribute to the overall total snow depth. University of Utah meteorologist Jim Steenburgh suggests that as little as 5% of the total annual snowfall in the Central Wasatch can be attributed to lake effect snow.
While every winter storm is made of three main ingredients (moisture, cold air, and energy), the Wasatch Mountains of Utah have a few extra factors that boost the snowfall totals. A significant elevation change drives orographic and convective instability, further mixing the moisture with cold air. The mountains are arranged in such a way that storms coming from many different directions run into the highest peaks. Storm flow from the northwest is funneled and milked dry in the upper Cottonwood Canyons. Large-scale storm patterns deliver both deep moisture and cold, dry air, and the storm evolution often leaves behind perfect dry powder as a lasting gift. And the nearby Great Salt Lake will occasionally help the skies open up and drop feet of snow on the mountains.
Indeed, other places also possess some of these factors. However, having them all together in one place is quite magical.
Honestly, I would have to say that it does make a strong case for the “Greatest Snow on Earth!”
For more technical information, weather musings, and powder snobbery, check out Jim Steenburgh’s book Secrets to the Greatest Snow on Earth: Weather, Climate Change, and Finding Deep Powder in Utah’s Wasatch Mountains and Around the World and his blog Wasatch Weather Weenies. They are great sources of knowledge and also very amusing. Much of the author’s knowledge of weather and Wasatch phenomena was fortified by Jim’s works.
For more learning about weather, check out the National Weather Service’s online learning portal The Jetstream.
