WPC produces probabilistic forecasts for the threat of snow in a different manner for medium range time frames than more predictable short range time frames. The days 1, 2, and 3 forecasts issued at night (usually by 0900 UTC) are valid for the 24 hour periods corresponding to each day (today, tomorrow, the day after tomorrow). The next set of days 1, 2. and 3 forecasts issued during the day (usually by 2000 UTC) are valid at times starting at 12 hours later than the forecasts issued the previous night.

The days 4-7 forecasts are issued by 0900 UTC and 1930 UTC for 24 hour forecast periods corresponding to 4, 5, 6, and 7 days into the future. In contrast to the days 1-3 forecasts, the daytime forecasts for days 4-7 remain for the same valid times as those days 4-7 periods valid from the prior night's forecasts. In this series of forecasts on the web site, The 1930 UTC issuance produces a 12 hour overlap of the short range day 3 forecast with the day 4 forecast. These are independent products and the overlap is removed again with overnight product issuance.

The description for the days 1-3 winter precipitation forecasts is available at: http://origin.wpc.ncep.noaa.gov/pwpf/about_pwpf_products.shtml
The description for the days 4-7 winter precipitation forecasts is available at: http://origin.wpc.ncep.noaa.gov/wwd/pwpf_d47/WPC_WinWx_PDD.pdf