Anatomy of an Upslope Snowstorm

Anyone who has experienced winter on the northern Front Range knows that when the smell of cow manure suddenly permeates the air seemingly out of nowhere, heavy snow will start falling within the next 12 hours. As a widely accepted and honestly hilarious forecasting tool, this phenomenon is actually the result of a classic weather set-up for Colorado. Our heaviest snowfalls here on the Front Range come from a meteorological phenomenon known as an upslope snowstorm. For an upslope snowstorm to occur, certain weather patterns must be in the right place at the right time.

The first stage of the upslope snow setup is a storm system, known as a winter cyclone, forming over the Pacific. Winter cyclones usually form southeast of Alaska’s Aleutian Islands, or near the coast of Washington. These strong low pressure systems then traverse the Western US, moved along by the Jet Stream. 

For us to receive a big dumping of snow (sometimes up to 3 feet!), the Jet Stream needs to be in the right position to move this low pressure system to the southern portion of Colorado. Once there, the low pressure’s counterclockwise winds draw warm, moist air from the Gulf of Mexico and push it towards the mountains via easterly winds. These winds are what bring the smell of farms on the Eastern plains to cities along the Front Range. 

Ideal Low Pressure Positioning for an Upslope Snow

It shouldn’t come as a surprise to learn that the mountains actually influence a lot of our weather. In the case of upslope snowstorms, they act as a lifting mechanism for this moist, unstable air coming from the east. When the moist air hits the mountains, it is forced to rise where it expands and forms heavy, precipitation laden clouds. Once these clouds reach the point of being unable to hold any more water, they release moisture in the form of snow that falls heavily on the Front Range. 

How the Rockies Influence Wind Dynamics

However, a low pressure system in the south is not the only setup for an upslope storm. The clockwise winds of a high pressure system to the north of Colorado can also create easterly winds and thus, an upslope setup. But because the air associated with high pressure is much drier than in low pressure systems, this type of setup usually only results in minor accumulations for the Front Range. However, on some occasions both high and low pressure systems can be present in their optimal areas at the same time. When this occurs, the high winds from the high pressure system and the excess precipitation from the low pressure system can create blizzard conditions along the Front Range. 

Now the next time someone points out the smell before the storm, you can tell them the science behind it!

  • Written by Hayley Moser. Adventurer and Fly Fishing, Backpacking, and Hiking Guide for Colorado Wilderness Rides and Guides.