An “extreme” G5 geomagnetic storm reached Earth on Friday, NOAA’s Space Weather Prediction Center said, after issuing a watch earlier in the day warning of the potential for a severe impact.

According to CBS News, the watch followed days of solar activity that sent several explosions of plasma and magnetic fields toward Earth.

G5 is the strongest level of geomagnetic storm, on a scale from G1 to G5.

“Widespread voltage control problems and protective system problems can occur,” the National Oceanic and Atmospheric Administration (NOAA) warns. “Some grid systems may experience complete collapse or blackouts. Transformers may experience damage.”

Radio transmissions and satellite navigation may also be disrupted.

The last G5 geomagnetic storm, in October 2003, caused power outages in Sweden and damaged transformers in South Africa.