How is space weather forecast, and what do the G1 to G5 storm scales mean?

Space weather is forecast in two steps: watch the Sun for eruptions, then watch the solar wind just before it arrives. In the United States, the official source of space weather alerts and warnings is the Space Weather Prediction Center (SWPC), a part of the National Weather Service and the National Oceanic and Atmospheric Administration (NOAA), in Boulder, Colorado.

The first step gives days of notice. A coronal mass ejection typically reaches Earth one to five days after leaving the Sun, so forecasters who see one leave can say a storm may be coming. The second step gives minutes. NOAA keeps a lookout at the L1 point, about 1.5 million kilometres from Earth toward the Sun, because the solar wind reaches L1 about 60 minutes before it reaches Earth. From there its DSCOVR satellite could usually give a 15 to 60 minute warning before a surge from a CME hit and created a geomagnetic storm, and since June 2026 a newer NOAA spacecraft, SOLAR-1, has been fully operational at the same spot.

To explain all this to the public, NOAA uses numbered scales for geomagnetic storms, solar radiation storms and radio blackouts, analogous to hurricanes, tornadoes and earthquakes. Geomagnetic storms run from G1 (minor) to G5 (extreme). At G5, some grid systems may experience complete collapse or blackouts, and aurora has been seen as low as Florida and southern Texas. NOAA lists G5 at about 4 per 11-year cycle.

DSCOVR itself is a rescue story: it began in 1998 as an Earth-viewing idea from Al Gore, was put into storage in 2001, was revived and launched in 2015, and went offline after a software fault in July 2025.