As some of you might of heard by now, some forecasts are calling for a modest snow event this weekend. I don’t want to completely curve your excitement for this possibility, BUT it is also important to remain realistic. Forecasting snowfall in the northern Wasatch is extremely difficult 5-7 days out. Snowfall in the Wasatch is strongly dependent on moisture availability, flow direction, flow speed, and 700 mb temperatures. All of these above parameters are dependent on the exact location, trajectory, and strength of the upper level trough as it moves into Utah. Forecasting the exact location of the upper level trough 5-7 days out is unrealistic, and thus, forecasting exact snowfall 5-7 days out is nearly impossible.
What models can tell us 5-7 days out though is the likelihood for a pattern change. Model ensembles can show us if the deck of cards is stacked in our favor for potential unsettled weather. Below is an image of the European Ensemble 500 hPa anomaly (blue to purple colors represent lower pressure while orange colors represent higher pressure), which shows the potential for a longwave trough over the western U.S this weekend.
Before getting to excited, we should investigate the uncertainty within the forecast. Below is an image from the Canadian Ensemble 500 mb heights for Sunday morning. The black lines represent the mean height field while the red letters represent locations of the upper level trough for each ensemble members. Where these red letters are closer together, there is a higher confidence in the location of the trough and where they are further apart, there is less confidence. Notice how the red letters below are far apart across Oregon, Washington, And Idaho? What this boils down to is there will likely be a trough across the western U.S. this weekend, but there is no way to tell if the trough will dig down into Utah – giving us snowfall and colder temperatures – or merely give northern Utah a glancing blow.
What’s the bottom line?
We will see a pattern change this weekend with the potential for unsettle weather across northern Utah and we will likely see a storm system impact Utah this weekend, but we can not estimate the exact strength of the storm, and associated snowfall accumulations, at this time.