Space Weather Forecast

How to Read the Space Weather Numbers

Solar wind speed is usually 300–400 km/s. Above 500 km/s is fast and favourable; above 700 km/s usually means a strong stream or the arrival of a coronal mass ejection.

Bz is the north–south direction of the magnetic field carried by the solar wind, in nanotesla (nT). This is the key number. Negative (southward) Bz lets solar wind energy into Earth’s magnetic field. Bz below −5 nT for 30 minutes or more is good; below −10 nT can trigger strong displays.

Bt is the total strength of that field. A high Bt (above 10 nT) means a lot of potential energy. If Bz turns south, things can happen fast.

Storm level is NOAA’s G-scale, from G1 (minor) to G5 (extreme), derived from Kp.

Geomagnetic Storms and the Aurora

A geomagnetic storm happens when a burst of solar wind energy couples into Earth’s magnetic field. The two main drivers are coronal mass ejections (CMEs), huge clouds of plasma thrown off by solar eruptions, and high-speed streams from coronal holes.

When a CME arrives, the solar wind speed and Bt jump suddenly. Forecasters call this a shock arrival. If Bz then turns strongly south, the storm can reach G3–G5 and the aurora can be seen far from the poles, as in May and October 2024, when it was seen as far south as Florida and Mexico.

Even without a big storm, short substorms can brighten the aurora for 15–30 minutes at a time. They often follow a period of southward Bz, which is why aurora hunters watch Bz closely.

Space Weather Tips

If Bz has been strongly negative for 20–30 minutes and you are under the aurora oval, go outside now and look north.

A storm watch is not a guarantee. CMEs can arrive late, arrive weak, or arrive with northward Bz and do little. Keep an eye on this page and the 3-day forecast.

Strong storms can be visible across most of the northern US and Europe. When a G3 or higher storm is under way we show a banner on every page.

The Solar Cycle and Aurora

The Sun follows an activity cycle of about 11 years. Solar Cycle 25 reached its maximum around late 2024, and the years just after a maximum are often among the best for big geomagnetic storms, because large sunspot groups and powerful eruptions remain common while coronal holes become more frequent.

That means strong storms are still likely in the 2026–2027 season. Even as the cycle declines, recurring high-speed streams from coronal holes keep producing regular Kp 4–5 nights that light up the Arctic and sub-Arctic.

Space weather can also affect radio communication, GPS accuracy, and power grids during severe storms. NOAA’s scales describe those effects; for aurora watchers, a higher G level means the aurora reaches further from the poles.

The best habit is to check three numbers together: Kp for the big picture, Bz for what is happening right now, and cloud cover for whether you will actually see anything. When all three line up after dark, go outside.