The limits of forecasting
What can be predicted, how far ahead, and why aurora forecasts are sometimes wrong.
Aurora forecasting has improved a lot, but it still can't promise a display. Knowing the limits helps you decide when to go out.
How far ahead
| Range | Based on | How reliable |
|---|---|---|
| Next 30 to 90 minutes | Solar wind measured at L1, OVATION | The most reliable: the wind that will drive the aurora has already been measured. |
| Next 1 to 3 days | NOAA's Kp forecast, CME models | Good for timing, uncertain for strength. |
| Next 27 days | Recurring coronal holes | A guide to likely active periods, not specific nights. |
Why CMEs are hard
Forecasters can see a CME leave the Sun and estimate when it will arrive, typically to within several hours either way. What they can't see in advance is the direction of its magnetic field. A CME whose field points south can bring a big storm; one pointing north may bring very little. That's only known when it passes the spacecraft at L1, less than an hour before it reaches Earth.
Why short-term changes happen
Bz can swing between north and south within minutes, and substorms release energy in sudden bursts. A quiet sky can light up in minutes, and a bright display can fade just as fast. That's what the short-range alerts are for.
Local conditions
Even a perfect space forecast can be spoiled by cloud, moonlight, light pollution or twilight. Polar Lights combines these with the aurora forecast in the Tonight score, but cloud forecasts carry their own errors; the satellite check in Your sky helps catch them.
Making the most of it
- Check the Tonight card for the best window, and look again before you go.
- Turn on alerts so you hear when things change. See Set up aurora alerts.
- Use Best nights ahead for planning trips, not for promises.
- When in doubt, go out: OVATION tends to be cautious, and aurora is often seen somewhat further from the pole than its line suggests.