How far ahead can an aurora forecast be trusted?
From the 30 to 90 minute OVATION nowcast to the 3-day Kp forecast and the 27-day outlook: what each NOAA product sees and when to check it.
Tämä sivu on kirjoitettu englanniksi.
Päivitetty 17 October 2026
Four horizons
Aurora forecasts come in four lengths. Each uses a different input, so each has a different reliability and a different use.
- Minutes to an hour: the OVATION map, driven by the solar wind measured at L1.
- Hours to three days: NOAA's Kp forecast, built by forecasters from solar observations, coronal holes and eruptions that are on their way.
- Up to 27 days: the 27-day outlook of geomagnetic indices, which relies on structures that return as the Sun rotates.
- Beyond that: only statistics, such as the season and the solar cycle.
The nowcast: 30 to 90 minutes
The solar wind passes a satellite at L1, about 1.6 million km from Earth, before it reaches us. At 400 km/s it needs about 67 minutes to cover that distance; at 800 km/s about 33. NOAA gives the range as 30 to 90 minutes. That is the best warning time there is, and it is also why a sudden storm cannot be predicted much earlier from the data in the OVATION model.
On the Aurorix home page, the chance tile shows the OVATION value and the time it is valid for. Check it when you are already outside or about to leave.
The 3-day forecast
NOAA's 3-day forecast gives the expected Kp for each three-hour block of the next three UTC days and a short rationale. It covers the arrival of coronal mass ejections, which take roughly one to three days to reach Earth, and of fast solar wind from coronal holes.
Skill is lower than for the nowcast. A forecast of Kp 3 can verify as Kp 1 or 5. Use it to pick which evening to prioritise, not to book a once-only trip. Aurorix shows NOAA's forecast values as hatched purple bars and quotes NOAA's own rationale on the home page.
The 27-day outlook
Coronal holes can persist for several solar rotations. As seen from Earth the Sun rotates in about 27 days, so a high-speed stream that disturbed the field once often returns one rotation later. NOAA's 27-day outlook uses that pattern to forecast daily geomagnetic indices.
It is a rough guide and not a promise. It is more useful for judging whether a trip window contains any recurring activity than for any single night.
A practical routine
- A week before: look at the 27-day outlook and the season. Choose months from the darkness table.
- Two to three days before: read the 3-day forecast and the cloud outlook for each candidate evening.
- On the day: use the hourly strip to see which hours are dark, clear and above the needed Kp.
- In the field: check the OVATION chance and your own sky every hour or so.