Module 12 | Scenario 3: Forecasting Snow with Computer Models

Key Concepts

Forecasts produced by computer modeling systems are available on many websites, often for free. Some examples include:

• http://weather.rap.ucar.edu/model/
• https://www.pivotalweather.com/model.php
• https://wxcharts.com/

There are tons of products you could potentially examine, but we're going to focus on special snowfall forecasting produced by the University of Utah Department of Atmospheric Sciences and available at http://weather.utah.edu. These products use statistical techniques based on machine learning as well as a technique known as downscaling to generate high-resolution snowfall forecasts for the western United States. 

 a. HRRR-Snow Forecasts

Snowfall forecasts from the National Centers for Environmental Prediction High-Resolution Rapid Refresh (HRRR) are available on weather.utah.edu for 1-hour, 6-hour, 24-hour, and 48-hour accumulation periods (Note: If no snow is forecast for the forecast period, these plots won't show anything). An example of the 48-hour accumulation period is shown for the Intermountain West below. The header at the top and left tells you the date and time that the HRRR forecast started (HRRR Initialized) and at the top and right when the forecast is valid. The 48-hour accumulation periods ends at the valid time, so this is a forecast of 48-hour accumulated snowfall from 12:00 UTC 19 February through 12:00 UTC 21 February 2024. During this period, accumulations of 4 – 12 inches are forecast across the Wasatch Range and parts of Wyoming, Idaho, and Nevada, with higher accumulations of 24 inches (2 feet) or more forecast along portions of the Sierra Nevada in California.

b. Downscaled Utah Snow Ensemble forecasts

The Utah Snow Ensemble is an ensemble modeling system that predicts snowfall across the western United States.  Developed by Dr. Jim Steenburgh (yes, the instructor for this class!) and his research group, the Utah Snow Ensemble combines two of the world's best ensemble forecast systems, the 31-member Global Ensemble Forecast System (GEFS) and the 51-member European Center for Medium Range Weather Forecasting (ECMWF) Ensemble Forecast System (ENS), into one big 82-member "superensemble."  They use downscaling and machine learning approaches to transform these lower resolution ensemble forecasts in highly detailed (800-m grid spacing) snowfall forecasts that you can use to anticipate your next big powder day.  The Utah Snow Ensemble forecasts are available in the left-hand navigation bar on http://weather.utah.edu. Look for "Utah Snow Ensemble (Experimental)" and click on it.  Click on a region of interest and select a variable of interest, such as total snow, which will show the total accumulated snowfall since the beginning of the forecast period. 

Below is an example 10-day (240-hour) total snowfall forecast for the Wasatch Front Region during a very active storm cycle.  It provides a spatial summary of the ensemble forecasts.  I have added red and blue boxes around Snowbasin and Alta Ski Areas for reference. In the top left panel is the forecast for one member of the ensemble, known as the ECMWF ENS Control run.  It is producing 42" of snow at Snowbasin and 39.8" of snow at Alta.  At top right is the average (also known as the mean) of all 82 forecasts which for Snowbasin and Alta are 47" and 56.6" respectively. T he bottom left and bottom right panels show the ensemble minimum and maximum total snowfall based on the lowest and highest values at each gridpoint, respectively.  These are extremes and not necessarily realistic outcomes, but can give a forecaster context as to how big the range of possibilities are.  For Snowbasin and Alta the minimums are 8.1" and 27.0" respectively, whereas the maximiums for Snowbasin and Alta are 79.0" and 103.5", respectively.  Wouldn't the latter be awesome!  If you want to learn more go to https://wasatchweatherweenies.blogspot.com/2024/09/the-utah-snow-ensemble.html. 

Also available are Utah Snow Ensemble "plumes" for many locations (click on "Plumes" on weather.utah.edu). Below is the plume for Alta-Collins during the same forecast period as above. Focus on the top-right panel, which shows lines of accumulated snow from the beginning of the forecast period to the end. Here, you can see that the range of total snowfall produced by the 82 ensemble members is between 27" and 103.5".  The average produced by the ENS members (thick magenta line) is about 55" and the average produced by the GEFS members (thick blue line) is about 70".  This is a pretty exciting forecast for Alta skiers!