Scenario
Publish lag matters: SHMÚ stamps a frame with its observation time but publishes it minutes later, so at the chosen moment the app could not yet see that frame. 10 min ≈ what a live device sees.
Where — click either map
FORECAST engine's advection
ACTUAL radar, what happened
Both panes are the SHMÚ CAPPI‑2km grid over the same bounds, at the same lead time, so they are directly comparable pixel for pixel. Pan, zoom and the clicked point are shared — click either one to move the sampled location. Blue rectangle = radar coverage (outside it there is no nowcast at all).
Step the slider and watch the two together: if the predicted blob drifts short of where the real one went, the motion field is under‑reading speed — visible in seconds, and much clearer than a table of numbers.