Predicting how much water will flow through rivers and streams is crucial for flood safety, water supply, and ecosystem health. Scientists have built a powerful new computer model called “NextGen” to make these forecasts, but its results can be hard to explore and check. We created the NextGen In A Box Data Visualizer—an easy-to-use website that lets anyone see model predictions on a map and as simple graphs, then compare them with real-world measurements from U.S. Geological Survey gauges. The Visualizer runs inside portable software “containers,” so it works the same on a laptop, in the cloud, or on a supercomputer. With just a few clicks, users can watch the model’s hydrograph trace rise and fall next to observed river flows and instantly see summary scores that show where the forecasts are strong or need improvement. In a test on Utah’s Provo River, the tool quickly highlighted that the model captured seasonal flow changes well but missed the exact timing of peak floods. By lowering technical barriers, the Visualizer helps researchers, water managers, and the public explore and improve river-flow forecasts together.