In the spring of 2002, the Meteorological Development Laboratory (MDL) implemented the Eta-based Model Output Statistics (MOS) guidance package. The Eta MOS guidance runs twice daily, following the 0000 and 1200 UTC runs of the Eta model, and provides forecasts for over 1200 stations in the conterminous United States (CONUS). Included in this package are 6-h, 12-h, and 24-h forecasts for probability of precipitation (PoP) and quantitative precipitation (QPF) valid at projections out to 60 hours after initialization. This first-generation Eta MOS development utilized data from a relatively coarse Eta model archive, both temporally (two cycles daily, forecasts every 6 hours out to 48 hours) and spatially (~90-km grid spacing). While our predictor data set was limited, verifications showed a significant improvement over the Nested Grid Model (NGM) MOS and some improvement over the Global Forecast System (GFS) MOS in the early projections (out to about 36-48 hours).

The current Eta MOS system is not without its deficiencies, however. Because of the limitations of the predictor data available, no forecasts are available for sites outside the CONUS, such as Alaska, Hawaii, Puerto Rico, and the Virgin Islands. Additionally, no guidance is available beyond the 60-h projection. After the initial Eta MOS implementation, an effort was undertaken at MDL to secure a new Eta model archive to correct these shortcomings. While data were only available starting in April 2000, the new model archive grid is much larger than the previous, covering all of Alaska, Hawaii, and the Caribbean Islands. The new Eta model archive also features finer spatial (32 km) and temporal (4 cycles daily, forecasts every 3 hours out to 84 hours) resolution than the original archive. Equations have been developed from the new Eta model archive and will be implemented in the upcoming cool season.

This paper discusses details from the recent Eta MOS equation developments, including verification scores. Comparisons will be made between the new and existing Eta MOS PoP and QPF systems, as well as the GFS and NGM PoP/QPF systems. In addition, we will look at the use of a higher-resolution model archive to determine what impact (if any) its use had on the skill of the guidance.