The weather radar (Nagano) shows where it is currently raining or snowing. The radar map is updated every 5 minutes with a new radar observation. The different colours indicate the intensity of rainfall or snowfall. Light blue indicates drizzle, blue a medium intensity, and red and yellow indicate very strong precipitation, usually associated with thunderstorms. Current lightning strikes are marked with small orange dots on the map (Europe only). Note that lightning is not shown on the forecast, as it cannot be predicted. Moreover, some countries do not operate a weather radar network, and in those countries satellite data is used to estimate rainfall, which is less accurate than a realtime weather radar.
Nagano: How accurate is the radar based forecast?
The rain/snow forecast is computed by estimating the movement of precipitation cells observed by radar and extrapolating this movement into the future. This so called precipitation nowcast is the most accurate precipitation forecast possible but the forecast horizon is limited to about an hour.
Longer forecasts are not possible, as new precipitation cells are developing or existing ones are disappearing within a short time.
Real weather is more complex than just the displacement of existing precipitation cells.
The forecast works very well when weather fronts or large organized precipitation structures are moving regularly, without disappearing or being created. If the radar animation of the last hours shows local thunderstorms or precipitation cells forming and disappearing in an irregular manner, then the forecast is not vey accurate.