How much warning do aurora forecasts give?

Aurora forecasting works in layers, each with its own horizon. The longest layer starts at the sun, with coronal mass ejections. SWPC says CMEs typically take several days to arrive, though some of the most intense storms have arrived in as little as 18 hours.

That transit time is what allows a watch. SWPC issues K-index watches when the highest Kp predicted for a day reaches 5, 6, 7, or 8 and above, and reports them on the G-scale. A watch says a storm is possible on a date. It cannot say which hours, or how far south the oval will reach.

The middle layer is warnings: SWPC issues them when a Kp of 4 or more is expected. The shortest layer is the live map and alerts. An alert fires when Kp actually reaches a threshold, and the OVATION map shows the oval for the next 30 to 90 minutes.

Spacecraft at the L1 point, 1.6 million km upstream of Earth toward the sun, measure the solar wind before it arrives. OVATION takes those measurements and models the aurora they will cause. The lead is the travel time from L1 to Earth, which depends on how fast the wind is moving.

The arithmetic is simple: divide 1.6 million km by the speed. The table runs it for sample speeds. SWPC's 30 to 90 minute range corresponds to speeds between roughly 300 and 890 km per second. At 400 km per second the trip takes about 67 minutes; at 800 km per second, about 33.

Faster wind gives less warning. That is awkward, because fast wind is part of what drives strong storms, according to SWPC's storm page. The nights with the most dramatic aurora can come with the shortest lead on the map.

If the L1 data are missing or contaminated, SWPC runs OVATION from the current Kp instead. SWPC says there is then no forecast lead time: the map becomes a nowcast. Glow reads both timestamps in the file and labels a map with less than 20 minutes between observation and forecast as a Kp-based fallback.

Glow also rejects any file more than 2 hours old. Feeds can stall, and an old map looks the same as a fresh quiet one. The status strip at the top of every Glow page shows the feed's age and mode. If it reads fallback, treat the map as a picture of now, not of the next hour.

• Days ahead: watch SWPC's forecasts and watches. Use the best nights tool to see which of those nights are dark and moon-free where you are.
• The afternoon of: check the cloud timeline and dark hours.
• Once dark: open the oval map and your live cell. Refresh every few minutes; the map only sees 30 to 90 minutes ahead.

No layer can promise a sighting. The early layers are forecasts of solar wind that has not yet reached L1. The last layer is a model of aurora from wind already measured. The alerts guide lists the thresholds behind each SWPC product.