Forecast tools · live SWPC data
Kp index: live values, the forecast, and the Kp your town needs
One chart for the planet, one number for your place. Green bars are the blocks that reach your town.
Kp you need at Minneapolis, MN
5.9
A G2 moderate storm or stronger. Overhead from Kp 5.9.
| Kp forecast during the dark hours | not yet forecast |
|---|---|
| Kp needed here | 5.9 (G2 moderate) |
| Cloud cover in the dark hours | loading |
| Dark sky (Sun 18° down) | 8:08 PM to 5:50 AM CDT |
| Moon | 13% lit |
| Right now, OVATION (30 to 90 min) | loading or unavailable |
| Magnetic latitude (AACGM) | 54.2° |
Hour by hour, CDT
7 PM
Kp –
– ☁
No
8 PM
Kp –
– ☁
No
9 PM
Kp –
– ☁
No
10 PM
Kp –
– ☁
No
11 PM
Kp –
– ☁
No
12 AM
Kp –
– ☁
No
1 AM
Kp –
– ☁
No
2 AM
Kp –
– ☁
No
3 AM
Kp –
– ☁
No
4 AM
Kp –
– ☁
No
5 AM
Kp –
– ☁
No
6 AM
Kp –
– ☁
No
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The planetary K-index, Kp, measures how disturbed Earth's magnetic field is over each three-hour block, on a scale from 0 to 9 written in thirds: 4.67 is 5-, 5.00 is 5 and 5.33 is 5+. The official index is computed by the GFZ in Potsdam from 13 observatories; SWPC publishes a real-time estimate from a smaller set of magnetometers and a three-day forecast, which this page draws in your time zone. What makes the number useful is the line it is compared with. The tool turns the magnetic latitude of your town into the Kp at which aurora rises above your northern horizon: about Kp 5.0 in Duluth, 7.4 in Denver. Any bar that reaches that line is drawn in green. Kp 5 and above are geomagnetic storms on the NOAA scale, from G1 at Kp 5 to G5 at Kp 9.
What the Kp index measures
A magnetometer records the horizontal component of the magnetic field all day. Its K-index is the largest swing over three hours, sorted into ten classes whose limits are scaled to the station, so that a K of 5 means the same level of disturbance in Sitka and in Fredericksburg. The planetary index averages the standardized K of observatories between 44 and 60 degrees magnetic latitude. SWPC's Planetary K-index page lists the stations of its real-time estimate, from Alaska to Australia, and the GFZ keeps the official series going back to 1932.
The link to the aurora is physical: the currents that flow along the auroral oval are the same currents that shake the magnetometers. When Kp rises, the oval brightens, thickens and slides toward the equator.
From a Kp value to your town
SWPC gives a simple relationship on its aurora viewing tips: at Kp 0 the equatorward edge of the oval sits near 66 degrees magnetic latitude and it moves about 2 degrees per Kp step, reaching about 48 degrees at Kp 9. For the storm levels, the NOAA Space Weather Scales give the latitudes where aurora has been seen: 55 degrees at G2, 50 at G3, 45 at G4, 40 at G5. The tool joins the two and keeps, at each Kp, the more southern line: the oval rule up to about Kp 6.3, then the storm-scale latitudes, because during big storms the aurora is seen from much farther than the edge of the oval, rising high enough above the horizon to be noticed.
Magnetic latitude comes from the AACGM-v2 model at 110 km. For towns of the site it is computed exactly; for a ZIP code or coordinates it is interpolated in a one-degree grid, which the site's tests check to within 0.3 degree of the exact value for every town.
Reading the chart under the tool
Each bar is one three-hour block, drawn in your local time. The darker bars on the left are the observed and estimated values of the last 24 hours; the lighter ones are the SWPC forecast. A bar turns green when it reaches the Kp the place you chose needs, so the chart answers at a glance whether any block of the coming nights is worth staying up for. Hover or tap a bar to read its value and type.
Kp compared with the other signals
Kp is a three-hour average and a global one. It lags: a sudden substorm can light up Montana for twenty minutes inside a block that ends at Kp 4. And it says nothing about the solar wind that will arrive in the next hour. That is why the forecast on the tonight page also reads OVATION for the current hour, and why the Bz guide is worth reading before a storm night. For a single number to plan around, though, Kp remains the one SWPC forecasts days ahead, and the one its alerts, watches and warnings are expressed in.