A nowcast is three steps
No weather model. Just the latest radar maps, and the fact that rain drifts with the wind.
DMI radar · 15 September 2026, 15:00 UTC
Radar nowcasting · Denmark
Will it rain?
Rain apps predict the next hour by moving the radar's rain along. We tested how simple that can be, on six months of Danish radar: a simple method gets within 2% of the state of the art.
1 · The radar sees the rain
Five DMI radars, one map every 10 minutes, on 3.2 km cells.
2 · Measure how it moved
Find the rain again in the next map; the shift is its motion.
3 · Move it forward
Carry the rain on along that motion, ten minutes at a time.
Simple motion is nearly as good as the state of the art
We replayed 1,649 past moments in 157 rainy periods, gave every method only the radar up to that moment, and checked where it put the rain an hour later.
+36%
our simple block matching places rain better than "nothing moves"
+39%
pysteps, the state-of-the-art research library
2%
the gap between them, one hour ahead; gone by three hours
Over Denmark, one arrow is almost enough
Rain rides the wind about 3 km up. Over flat Denmark that wind is nearly the same everywhere within about 50 km, so one motion for the whole map does almost as well as one per block. Typical rain here moves at 40 km/h, toward the east or north-east.
Real motion of each 26 km block between two scans, 4 September 2026, merging into one arrow for the whole map.
The limit is rain that forms or dies
Accuracy halves within three hours for every method. Moving rain cannot see a shower being born, or one fading away.
One forecast can also give an honest chance of rain
Copy the forecast 40 times and shift each copy a little, more the further ahead: about a kilometre for every three minutes. The chance of rain at a spot is the share of copies that bring rain there. Drag the slider.
0%
chance of rain at the spot (illustration)
Scored on real rain, this simple model reaches 96% of what the STEPS ensemble achieves for "rain starts here within the hour", and 99% within two hours.
At your door
At five Danish cities, moving the rain catches 66% to 69% of the hours that bring rain in the next hour, against 50% if you assume nothing moves.
Want the details?
The full report has every method, test and robustness check behind these numbers, from how the motion is measured to how much history to use.
Read the full report →