High-Resolution Ensemble Forecast (HREF) systems are the next generation of ensemble prediction systems (EPS). HREFs use convection-allowing models (CAMs) to provide high-resolution probabilistic forecast information, including the location of mesoscale precipitation bands, and the probability of exceeding important temperature, precipitation, and wind thresholds at the scale of the HREF.
Many of you are familiar with using the Environmental Modleling Center (EMC) HREF from the National Centers for Environmental Prediction (NCEP) for severe weather guidance. In the cold season, however, different forecast problems arise. Snowfall and ice accretion rates depend on Quantitative Precipitation Forecasting (QPF) and the vertical structure of temperature and moisture. Frontogenetic forcing and conditional symmetric instability can result in heavy mesoscale precipitation bands. Precipitation type (ptype) forecasts are critical for assessing social and economic impacts of winter weather. Timing of ptype changeovers, precipitation onset, and precipitation conclusion are all important aspects of potentially high-impact winter weather.
For Impact-based Decision Support Services (IDSS), many emergency managers and other NWS core partners base decisions on specific thresholds for snow accumulation, temperature, and other weather elements. Probabilistic forecasts can help inform the likelihood that these thresholds are met; more so when the data is bias corrected. However, the best results are obtained when the probabilities are all calibrated to the verification probabilities. Presently, the forecast data in the HREF are only bias corrected, so the probability of exceedance should be used with caution.
You might ask, ”Since CAMs are so realistic, why not just use a single CAM forecast?” Just as in coarser-scale NWP, uncertainty comes into play. Model and initial condition imperfections increase forecast uncertainty in the timing, intensity, and location of winter weather features. HREFs allow you to quantify this inevitable forecast uncertainty and to assess the amount of confidence you should place in the forecasts for a potential winter weather event. In this lesson, you will see how to use and interpret HREF winter weather products to address these winter forecast problems.
This lesson includes the following sections:
We will also provide web links to additional information on statistical methods used to produce the weather elements available from the HREF, including: