How to read an aurora forecast, from 27 days to the next hour

An aurora forecast is not one forecast but several layers with very different precision. The 27-day outlook hints at trends, the 3-day forecast tells you whether a storm is possible, and only measurements of the solar wind give a reliable picture of the next 30 to 90 minutes. The best strategy is to use the long-range forecasts for planning and the short-range ones for deciding when to head outside.

Four time horizons

Add the weather forecast to every layer. Without clear skies no Kp in the world helps; see clouds and clear skies.

The 27-day outlook: a rough trend

Every Monday NOAA publishes a table of expected activity for the next 27 days, including the largest daily K value. Why 27? That is about how long the Sun takes to turn once as seen from Earth. Coronal holes, dark regions in the Sun's outer atmosphere that release fast solar wind, can last for several solar rotations according to NOAA. A coronal hole that brought aurora today may therefore bring more about four weeks later.

The Norwegian Centre for Space Weather at UiT (the Arctic University of Norway) points out that such long-range forecasts work reasonably well when activity is low and dominated by recurring coronal holes, but perform poorly during high solar activity, when big eruptions arrive without warning. Use the 27-day outlook to pick a week for a trip, never to promise anyone aurora on a specific date.

The 3-day forecast: Kp in three-hour blocks

NOAA's 3-day forecast is a short text issued twice a day, at 00:30 and 12:30 UTC. The most useful part for aurora hunters is the table of expected Kp for every three-hour block over the next three days, plus a short rationale written by the duty forecaster.

An example from the forecast issued on 18 October 2026 at 12:30 UTC: the highest expected Kp over the next three days was 4.67 (G1), in the 03-06UT block on 20 October. The rationale was the effect of a high-speed stream from a coronal hole, abbreviated "CH HSS". How to read a forecast like that:

• Look for the highest numbers in the evening and at night. For Norway in winter, 18-21UT is 19:00–22:00 local time, 21-00UT is 22:00–01:00 and 00-03UT is 01:00–04:00.
• Read the rationale. "CME" means a cloud from an eruption on the Sun is expected. "CH HSS" means a high-speed stream from a coronal hole, which often brings moderate but long-lasting activity.
• Compare with what you need. A forecast of Kp 3 is plenty in Tromsø but not very promising in Oslo. See the table in the Kp index explained.

Remember that the forecaster is working from models. When a coronal mass ejection (CME) is expected, NOAA uses, among other tools, the WSA-Enlil model, which gives one to four days of warning about when the cloud may reach Earth. The arrival time is a model estimate, not a timetable, and the actual strength is only settled when the cloud sweeps past the spacecraft upstream of Earth. More in geomagnetic storms.

Watches, warnings and alerts

NOAA issues three kinds of Kp-related messages:

• Watch: issued when the highest predicted Kp for a day is 5, 6, 7 or 8 and above, expressed on the G scale.
• Warning: issued when Kp 4, 5, 6 or 7 and above is expected.
• Alert: issued when the estimated Kp has actually reached 4, 5, 6, 7, 8 or 9.

A "G3 watch" therefore means a strong storm is possible, not that it will happen. An "alert" means it is happening now, and that is the moment to look up, even far south in Norway.

The next 30–90 minutes: OVATION and the solar wind

Spacecraft orbiting the L1 point, about 1.6 million kilometres from Earth towards the Sun, measure the solar wind before it reaches us. That gives a warning equal to the travel time from L1 to Earth. NOAA's OVATION model uses these measurements to calculate where the auroral oval lies and how intense it is for the next 30 to 90 minutes. On the map the oval appears as a green band around the magnetic pole, turning red where the aurora is expected to be strongest.

Two values are worth watching in the solar wind data:

• Bz, the north–south direction of the magnetic field carried by the solar wind. When Bz is negative (pointing south) for a sustained period, the solar wind couples far better with Earth's magnetic field.
• Speed. The Norwegian Centre for Space Weather mentions more than roughly 450 km/s and a large, sustained negative Bz as good signs, while stressing that neither is strictly required.

More in solar wind, Bz and speed. Note that OVATION falls back on Kp when solar wind data are missing, and then the lead time disappears.

See for yourself: cameras and magnetometers

The final layer is observation. The Norwegian Centre for Space Weather shows all-sky cameras, which photograph the whole sky, and readings from magnetometers in Norway. When the magnetometer curves swing wildly, something is happening. An overview of these sources is in aurora apps and data sources.

An evening routine, step by step

1. Afternoon: check the hour-by-hour cloud forecast for where you are and nearby. Check the 3-day forecast for tonight's blocks and convert from UT.
2. At dusk: see whether it is still clear and where the Moon is. Check tonight's verdict for your location, which combines Kp, cloud cover and darkness.
3. Evening: watch the OVATION map and the solar wind. If Bz turns south and stays there, it is a good reason to put your boots on.
4. Outside: face north, give your eyes plenty of time to adjust to the dark, keep your phone screen down, and be patient. Aurora often comes in waves.
5. Do not blindly trust a poor forecast. The Norwegian Centre for Space Weather suggests peeking outside now and then even when the websites show little. Aurora can appear without any forecast catching it.

Timing through the night is covered in more detail in the best time of night.

Common misreadings

• "Kp 2, no point." In Northern Norway, Kp 2 can give a lovely display under clear skies.
• "The forecast said Kp 6, but nothing happened." Storms can arrive later or weaker than expected, and the Kp forecast says nothing about clouds.
• "The app says 90 percent." Percentages in aurora apps are rarely defined the same way. Find out what the number means before letting it run your evening.

In short

• Use the 27-day outlook to plan, the 3-day forecast to keep track, and solar wind plus OVATION to decide.
• Convert from UT to Norwegian time: add 1 hour in winter, 2 in summer.
• A watch is a possibility; an alert means it is happening now.
• The weather forecast matters as much as the space weather forecast.