Precipitation
First and foremost, you need to predict the amount of new snowfall and its snow water equivalent across your forecast area.
Snow water equivalent: Usually it’s easiest to start by forecasting the amount of liquid precipitation (SWE) expected to fall over your forecast area. To do so, you’ll need to evaluate Quantitative Precipitation Forecast (QPF) output from one or more weather models.
- Forecast for 12-hour periods at the most, determining the 12- and 24-hour amounts of liquid equivalent precipitation for each period
- Use even shorter periods (such as 3 or 6 hours) to calculate snowfall intensity rates and identify the heaviest snowfall periods
- Modify the model results based on potential areas of orographic enhancement or the effect of mountain shadowing; note that model results can vary based on a model’s coordinate system and its ability to accurately identify a place’s location and altitude; for more information, see the COMET module Impact of Model Structure and Dynamics module at http://www.meted.ucar.edu/nwp/model_structure/
New snow amount: Some weather forecast models explicitly forecast new snow depth. If you don’t have such guidance, estimate snowfall amount for each time period using the forecasted amount of liquid precipitation and the forecasted temperature at the desired elevation. Temperatures can be estimated from model forecasted temperatures on constant pressure surfaces nearest to elevations of interest. Snow falling within certain temperature ranges has an approximate density range.
For example, you can estimate the amount of snow for a designated time interval from the model QPF amount and average forecasted air temperature at the forecast elevation.
- Use the table below to estimate new snow density based on the temperature forecast
- Divide the density into the forecasted liquid water amount to determine new snowfall depth
