The National Weather Service’s Weather Prediction Center now has probabilistic winter precipitation guidance. You can get maps showing
- the th percentile of the predicted distribution of accumulation for ;
- dually to that, contours on which the 10th, 20th, …, 90th percentile of the predicted distribution is a selected constant
for the next 72 hours, or for 24- or 48-hour subsets of that time period. Here’s a more in-depth description.