Use of National Blend of Models for Forecast Consistency in the Western US Forecast Offices

The National Weather Service (NWS) is progressing towards using the National Blend of Models (NBM) as a starting database for NWS forecasts. The NBM is a set of forecast grids based on the output of over 150 models and ensemble members. The goal is to have a high quality initial forecast which NWS offices can adjust, as needed, to make important changes to the message of the forecast. Having a common starting point promotes better consistency among offices allowing offices to better collaborate changes for targets of opportunity. Later versions of the NBM will include more probabilistic information.

The various challenges of using the NBM in complex terrain will be discussed. Some issues include identifying cold pools, precipitation type forecasts, identification of local wind features, and the handling of shallow cold air masses. A proposed common methodology for creating forecasts, called ForecastBuilder (FB), will also be discussed. FB provides a common forecast preparation strategy while allowing for differences in methodologies and grid definitions among many offices. FB faces the challenges of being flexible to allow for varying forecast preparation strategies and grid configurations. However, it also cannot be too complex to configure or allow for too much diversity resulting in differing grids among offices.