The next 27 nights of Kp index
Loading NOAA SWPC's official 27-day Space Weather Outlook…
What the 27-day outlook actually is
Every day, NOAA's Space Weather Prediction Center publishes a 27-day Space Weather Outlook: a table of the largest Kp index expected each day for the next four weeks, plus the 10.7 cm radio flux and the daily A index. It is the only official long-range aurora planning tool that exists — and it works for a specific reason.
The Sun rotates on its axis roughly every 27 days. Long-lived features on its surface — coronal holes and active sunspot regions — fire streams of faster solar wind at Earth every time they rotate back into view. So if a coronal hole produced Kp 5 nights on the 8th–10th of this rotation, the outlook flags the same window next rotation. That periodicity is the whole trick: the outlook is climatology with a calendar attached.
How to use it to plan a trip
Step 1 — find the hot windows. In the grid above, look for clusters of Kp 4+ nights. A three-night run of Kp 4–5 is a far better trip target than one isolated Kp 6 spike between quiet nights, because single-day forecasts four weeks out are the least reliable cell in the whole table.
Step 2 — cross-check the moon. A Kp 5 window under a full moon serves strong displays but wastes faint ones. New-moon hot windows are the premium product; the moon explainer covers exactly how much illumination matters.
Step 3 — match the window to your latitude. The grid is planetary: Kp 4 is a spectacular night in Tromsø and nothing in London. Use the per-city thresholds in the table below, or go straight to your city's live page.
What "good" Kp means where you are
| Your latitude band | Examples | Plan around |
|---|---|---|
| Aurora-zone (65°+ magnetic) | Tromsø, Fairbanks, Yellowknife, Rovaniemi | Kp 2–3 — most clear dark nights deliver |
| Just south of the oval | Edinburgh, Anchorage, Stockholm, Edmonton | Kp 4–5 — a handful of storm nights per month |
| Mid-latitude | Seattle, Dublin, Copenhagen, Toronto | Kp 6–7 — G2–G3 storm events |
| Far south of the oval | London, New York, Berlin, Tokyo | Kp 8+ — rare G4–G5 events; consider flying to the oval instead |
How accurate is it, honestly?
Declining sharply with range. The first 3 days overlap with NOAA's short-term forecast (good); days 4–14 lean on recurring-structure logic (reasonable); days 15–27 are the previous rotation repeated (indicative only). The A index column is similarly directional. Our honest advice: use the 27-day grid to pick which week to travel, then use each city page's live 7-night strip — cloud, moon and darkness included — to pick which night to actually stand outside. Never book a one-night aurora trip around a single far-out Kp number; that is the most common planning mistake in the hobby.
Background on why the Sun does this at all: solar cycles, explained. The raw vocabulary: field glossary.
27-day outlook — quick answers
How accurate is a 27-day aurora forecast?
It's a climatology-based indicator, not a weather forecast. It works because active regions on the Sun persist across one or two solar rotations (~27 days each), so dates that were stormy this rotation often repeat next rotation. Day-to-day accuracy is limited — treat it as a trip-planning tool, not a promise.
What Kp should I look for in the outlook?
It depends on your city's magnetic latitude: aurora-zone cities like Tromsø or Fairbanks need Kp 2–3, mid-latitude cities like Edinburgh or Seattle need Kp 5–6, and far-south cities like London or New York need Kp 7+. Every city page states its exact threshold in the factor breakdown.
Why does the pattern repeat every 27 days?
The Sun rotates on its axis roughly every 27 days. Long-lived features — coronal holes and active regions — fire at Earth again each time they rotate back into view, which is why the same calendar dates tend to stay magnetically active across consecutive rotations.
The outlook looks quiet — should I cancel a trip?
No. Quiet rotations still deliver Kp 2–3 nights in the aurora zone, which is all Tromsø, Fairbanks or Yellowknife need. What a quiet outlook does mean: mid-latitude cities should not expect a storm night, and aurora-zone travellers should lean even harder on each city's live cloud and darkness factors.