Geomagnetic watches, warnings and alerts explained

SWPC's planetary K-index page defines all three in terms of Kp. The words sound alike, so the thresholds and the timing are what tell them apart.

Watch

A watch covers a day. SWPC issues one when the highest Kp predicted for that day is 5, 6, 7, or 8 and above, and reports it on the NOAA G-scale, G1 to G4 and up. A day-scale forecast is possible because some storm drivers travel slowly: SWPC says coronal mass ejections typically take several days to arrive.

Warning

A warning says a Kp level of 4, 5, 6, or 7 and greater is expected. It sits between the day-scale watch and the real-time alert. Warnings start one step lower than watches, at Kp 4, which is still below storm level on SWPC's scale.

Alert

An alert says the estimated Kp has reached 4, 5, 6, 7, 8 or 9. It reports something happening now. For aurora watchers it is the most direct trigger: if it is dark and clear where you are, go out.

The table lists Kp 4 to 9, the products that can fire at each, the matching G level, and the SWPC oval-edge latitude. The last column names the Glow towns whose dipole latitude sits at or poleward of that edge.

At Kp 4 the average edge sits at 58 degrees, which already covers the five towns in Alaska, Canada, Iceland and Norway. The lower 48 towns join at Kp 6 (Duluth) and Kp 7 (the rest). Remember SWPC's 1000 km viewing distance: aurora can be seen before the edge reaches you, so an alert for one or two Kp below your threshold is still worth a look north.

Every product is built on Kp, a planetary index. A Kp 7 alert means the field is disturbed worldwide, not that the oval is bright over your town. The storm might peak while your side of Earth is in daylight. It might favor one longitude over another. SWPC itself calls the link between Kp and auroral latitude approximate.

Alerts also ignore your sky. Cloud and moonlight are local and change hour by hour. That is why Glow reads the OVATION cell over your coordinates and puts darkness and cloud beside it, rather than repeating the Kp alert.

Alerts and the map also run on different clocks. An alert reports the estimated Kp, which SWPC updates every minute from 8 magnetometers. The OVATION map runs on solar wind data from the L1 point, 30 to 90 minutes ahead. When a storm is building, check both.

• A watch for tonight or tomorrow: check dark hours and the moon for those nights.
• A warning: check NWS cloud for your spot and a backup.
• An alert at or near your town's Kp: open the oval map and your live cell, then go out.

Find your town's Kp on the Kp needed tool, or on the storm levels hub, which lists each G level with the towns it typically reaches.