The National Weather Service's Meteorological Development Laboratory (MDL) has issued Model Output Statistics (MOS) guidance forecasts at stations for over three decades. Recently, gridded MOS (GMOS) forecasts have been added to the product suite. For many elements, GMOS forecasts are produced from an objective analysis of MOS forecasts at stations to points on a fine resolution grid, with a successive correction method. GMOS guidance is available for most elements found in the National Digital Forecast Database (NDFD), including temperature, dewpoint, wind, and probabilistic guidance. These various forms of gridded guidance are part of the National Digital Guidance Database (NDGD).

An important element in the NDFD is the element named "weather." This element contains forecasts of precipitation coverage or probability (e.g., scattered, chance, likely), precipitation type (e.g., rain, snow, ice pellets, thunder), precipitation intensity, obstruction to vision (e.g., fog), and a few other attributes. To date, gridded guidance for weather has not been available within NDGD. A GMOS weather grid is being developed to support NWS Weather Forecast Offices (WFOs) as they prepare the NDFD.

Here, we present a method that uses a collection of GMOS forecasts to generate several precipitation grids to support the GMOS weather grid. The precipitation probability component is supported by precipitation potential index (PPI). PPI values range from 0 to 100 and resemble POP12 values in magnitude. We generate PPI from GMOS forecasts of 12-hour probability of precipitation and 3-hour probability of precipitation occurrence. Multiple precipitation types may be present in the weather grid (e.g., "definite rain and chance of snow"). We use PPI, temperature, conditional probabilities of freezing and frozen precipitation, and thresholds for precipitation type best category to determine the precipitation probability for each of freezing, frozen, and liquid precipitation types. Precipitation type also may include thunderstorm and severe thunderstorm categories, which we generate from a combination of 3-, 6-, and 12-hour probabilities of thunderstorms and 3- and 12-hour unconditional probabilities of severe thunderstorms. Finally, we assign precipitation intensity from 6-hour quantitative precipitation forecasts.

The resulting precipitation probability, type, and intensity grids are combined to generate GMOS weather guidance. Though there is no current GMOS obstruction to visibility grid to support that component of the weather element, the GMOS weather grid remains credible without it. We expect these grids will be valuable to forecasters at NWS WFOs and throughout the weather enterprise.