The HREF is a “multi-model” ensemble that combines several different 3km models (like the HRRR and the NAM-Nest) into one product. In 2026, it is the primary tool for “Severe Weather Outlooks,” as it provides “neighborhood probabilities”—the likelihood of a hazard occurring within a 25-mile radius of any point.
Most ensembles use the same model with different starting points. The HREF uses different models entirely. This “Ensemble of Opportunity” is better at capturing “structural uncertainty”—when one model’s math is simply better at a certain storm type than another. Pros search for “HREF v3.0 membership” to see which specific models are currently “weighted” most heavily in the final average.
Instead of a “yes/no” forecast, the HREF produces PoE maps. A professional might search for “HREF 1-inch-per-hour rain probability” to see if there is a 70% chance of extreme rain. This is the “gold standard” for flash-flood forecasting, as it allows hydrologists to see if the entire ensemble agrees on a heavy rain core over a vulnerable watershed.
UH is a measure of “spinning air” within a storm, a key indicator of a tornado. The HREF provides “UH tracks”—maps that show where the ensemble members “think” the most intense rotating storms will travel. Pros search for “HREF Max-UH plumes” to identify the “threat corridor” for a given afternoon, allowing them to pre-position storm spotters and emergency teams hours before the first cloud forms.