The Probabilistic Winter Precipitation Forecast (PWPF) is a set of probability maps for snow and freezing rain from the Weather Prediction Center (WPC). For each location, it shows the chance that accumulation will reach or exceed a set amount, such as 4 inches of snow or a quarter inch of freezing rain, over a given time window.
What the maps show
WPC's guidance page offers snowfall thresholds of 1, 2, 4, 6, 8, 12 and 18 inches, in 24-, 48- and 72-hour windows across Days 1 to 3. WPC's main winter weather page condenses this into the chance of at least 4, 8 and 12 inches of snow and at least 0.25 inch of freezing rain for each day.
WPC also produces percentile maps for 24-, 48- and 72-hour periods. They show the snowfall or freezing rain amount at a chosen point in the distribution, which gives a quick look at a reasonable low end and high end.
How the probabilities are built
The process starts with WPC's own deterministic accumulation forecast, which is set as the most likely value (the mode) of the distribution. A multi-model ensemble, drawn from global ensembles such as the Global Ensemble Forecast System (GEFS) and the European (ECMWF) ensemble plus higher-resolution models such as the High-Resolution Rapid Refresh (HRRR), supplies the spread around it.
Each member's snowfall comes from its precipitation forecast, its precipitation type and a snow-to-liquid ratio. WPC fits a binormal distribution, which allows the spread to lean toward higher or lower amounts, and reads the probabilities from it. The 48-hour windows come from adding two 24-hour accumulations together.
That is the method WPC has published. A September 2026 Service Change Notice set the Probabilistic Precipitation Portal, whose underlying ensemble information comes from the National Blend of Models, to become the operational home for these forecasts on or about 10/01/26, so the details may change.
How to read it
If a point falls inside the 40 percent contour on the 6-inch map, the chance of more than 6 inches there is 40 percent or greater. Stepping through the thresholds shows the shape of the risk.
A high chance of 4 inches with a low chance of 12 points to a fairly confident, moderate event. Similar, middling odds across several thresholds point to a wide range of possible outcomes, often tied to track or precipitation-type uncertainty.
What it does not tell you
The maps describe accumulation, not impact, and they cannot be more certain than the ensemble behind them. If every member misses the same detail, such as a narrow band or a warm layer aloft, the probabilities miss it too.
Local NWS offices issue the official winter headlines, and their criteria vary by office. WPC's guide to the Probabilistic Winter Storm Severity Index describes it drawing on the same ensemble data and turning these amounts into potential impacts.