National Weather Service United States Department of Commerce

The National Weather Service (NWS) recently upgraded the latest forecast guidance to help forecasters deliver the most consistent and accurate weather forecasts and warnings to the Nation. The NBM is a nationally consistent suite of calibrated forecast guidance based on a blend of both NWS and non-NWS model output. On February 19, 2020, the latest version of the National Blend of Models (NBM), version 3.2, went operational to NWS meteorologists around the country! The goal of the NBM is to create a highly accurate, skillful, and consistent starting point for forecasts and is a foundational component in evolving NWS capabilities to achieve a Weather-Ready Nation.

NBM Motivation:
The NWS operates 122 individual weather forecast offices across the United States and its territories and each of them is responsible for providing forecasts and warnings for their areas of responsibility, along with Impact-Based Decision Support Services (IDSS) to their partners in emergency management and government. Each office looks at all available observations and model guidance to create forecasts tailored to their analysis of the data. However, even when looking at the same data, forecasters with different areas of expertise and areas of focus can come up with slightly different forecasts over the same areas, which can lead to inconsistencies in the forecast. This is where the value of the NWS’s NBM comes into play. The NBM is a nationally consistent suite of calibrated forecast guidance leveraging both NWS and non-NWS model output. The goal of the NBM is to create a highly accurate, calibrated, skillful, and consistent starting point for NWS forecasters - a foundational component in evolving NWS capabilities to achieve a Weather-Ready Nation. By utilizing the NBM to initialize forecasts, it also gives forecasters more time to use their expertise and focus on high-impact weather/periods and ensure that NWS forecasts are more consistent across office borders. This shift in focus of responsibilities allows forecasters to more routinely message the impacts of various weather hazards to emergency management officials and partners and bring us closer toward the NWS goal of “1 event, 1 forecast” which will better prepare partners and communities for hazardous weather, water, and climate events and build a Weather-Ready Nation.

NBM Background:
Numerical Weather Prediction (NWP) is a complicated process with millions of observations and calculations needed to create a single forecast. At the top of each hour of the day, the NBM processes NWP output from both the National Center of Environmental Prediction’s (NCEPs) operational job stream and several non-American NWP weather sources. The NBM ingests and bias corrects a host of weather elements and determines the optimum weights to be assigned to each model when generating a final blended forecast. Simply put, the NBM takes whatever models are available from 31 different model systems (for rainfall forecasts a total of 171 inputs) and combines the information into a single forecast. Intense research into model performance over time has given today’s meteorologists a baseline of known model biases.  Verification of the NBM output has shown that this methodology consistently outperforms individual model runs over time leading to a more accurate and consistent dataset of available guidance.

NBM 3.2 and Beyond:
NBM 3.2 is publicly available in GRIB2 format, ASCII text file format and can be found here. Forecast images can  be viewed online here. Several enhancements were made to the NBM v3.2 including updating the blend to include additional model inputs and updating codes to improve compute performance in calculating wind and wave products. As the NWS evolves, so too will the NBM with improvements constantly being considered and implemented. NBM v4.0, which is scheduled to be implemented in the fall of 2020, will feature probabilistic guidance for daytime maximum temperature and nighttime minimum temperature - similar to the probabilistic products currently being provided in NBM v3.2 for winter weather type and snowfall amounts.

To date, the NWS NBM has performed exceptionally well, providing similar or improved forecasts in the medium-range when compared to NWS official forecasts.  Feedback from public users has been positive, especially its usefulness for marine forecasting. The NWS looks forward to leveraging this new suite of weather guidance nationwide as it is developed and to continue to bring you the very best in weather forecasts, warnings, and IDSS!