The NMM MOS replacement for the Eta MOS

In the spring of 2002, the Meteorological Development Laboratory (MDL) implemented the Eta-based Model Output Statistics (MOS) guidance package. Four years later, the operational Eta model run by the National Centers for Environmental Prediction (NCEP) was replaced with the Non-hydrostatic Mesoscale Model (NMM) core of the Weather Research and Forecasting (WRF) system, and is now the model run in the North American Mesoscale (NAM) time slot. For many elements, applying the old Eta MOS equations to the new NMM output degraded forecast skill to warrant development of new equations. Therefore, a new suite of MOS guidance was developed for the 0000 and 1200 UTC forecast cycles of the operational WRF-NMM, to replace the previous Eta MOS. New equations were developed for 2-m temperature, 2-m dewpoint, maximum and minimum temperature, wind speed and direction, probability of a thunderstorm, conditional probability of a severe thunderstorm, probability of precipitation (PoP), and quantitative precipitation amount (QPF).

The equations were developed on a short sample consisting of one year of output from the old Eta model, and two years of output from the new WRF-NMM. Verifications show that the NAM MOS guidance is comparable in skill to that of the Global Forecast System (GFS), and in nearly all cases is better than applying the previous Eta-based equations to the NMM output. Equations for some elements (e.g., sky cover, ceiling, visibility, etc.) are still based on the old Eta model and are applied to the NAM output. These elements showed comparable skill when the original equations were applied to the WRF-NMM output. New equations for these elements will be redeveloped at a later date. The new NAM MOS guidance was implemented operationally on December 9, 2008. Additional verification results for each element will be presented at the conference.