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. 

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," 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.

A geomagnetic storm also means aurora borealis, otherwise known as the northern lights, could be seen as far south as Alabama and in Northern California. 

Earlier, NOAA had issued its first watch for a potential G4-level geomagnetic storm in almost 20 years. "If geomagnetic storms were hurricanes, 'severe' would be category 4," SpaceWeather.com says. 

In a press release on Thursday, NOAA said the most recent series of solar events started on May 8, when a large cluster of sunspots produced "several moderate to strong solar flares." Solar flares are bursts of radiation known to be the solar system's largest explosive events, according to NASA. The area where the flares are occurring is 16 times the diameter of Earth, the NOAA said, and more solar activity is expected. 

That sunspot is so big you may be able to see it with your own eyes — with your solar eclipse glasses. The spot is known as AR3664, and it was responsible for most of the geomagnetic activity Friday, the NOAA reported. According to Space.com, it measures about 124,000 miles across and is one of the "largest and most active sunspots seen this solar cycle." 

The NOAA reported that a strong solar flare was observed peaking from AR3664 at 9:23 p.m. Eastern Time Friday. 

"Flares of this magnitude are not frequent," the prediction center said. 

Still have your solar eclipse glasses? There's currently a sunspot so large you will be able to "spot" it while wearing them

15x wider than the earth! pic.twitter.com/XpQJEd4Qk0

— Eric Fisher (@ericfisher) May 9, 2024

There has also been a series of coronal mass ejections (CMEs), which are explosions of plasma and magnetic fields that come out of the sun's corona, the outermost part of the sun's atmosphere. At least five CMEs appear directed toward Earth and could arrive as early as midday on Friday and persist through Sunday, the agency said. 

"This is an unusual event," NOAA said.

In a call with reporters on Friday, Shawn Dahl, service coordinator at the Space Weather Prediction Center, said that some CMEs "are catching up with other ones." He said officials are expecting a "big shock arrival" when they hit Earth. Dahl said at the time that while officials weren't predicting a G5 storm — the strongest of geomagnetic storms — they couldn't discount a "low-end G5 event."

"We're really buckling down here," Brent Gordon, chief of the space weather services branch, also said on the call.

G4 conditions were detected by Friday afternoon, marking a "major disturbance in Earth's magnetic field," NOAA said, adding that "the public should stay properly informed of storm progression."  

In a forecast discussion at 12:30 p.m. on Friday, NOAA's Space Weather Prediction Center said that solar activity is expected to continue at "high to very high levels" through the weekend, with additional solar flares expected, including X-class flares, the most powerful class of solar flares.

As of Friday afternoon, NOAA said it had observed a moderate solar radiation storm that could expose people in high-flying aircraft to "elevated radiation risk" and cause infrequent issues with satellite operations. 

Radio blackouts have also been detected with an R3 designation, meaning that the blackouts were "strong" on a scale from R1 (minor) to R5 (extreme). At this level, wide blackouts of HF radio communication is expected, as well as loss of radio contact, for about an hour on the sunlit side of Earth, as low-frequency navigation signals decline for roughly an hour. 

"Geomagnetic storms can impact infrastructure in near-Earth orbit and on Earth's surface, potentially disrupting communications, the electric power grid, navigation, radio and satellite operations," NOAA said. "[The Space Weather Prediction Center] has notified the operators of these systems so they can take protective action."

Dahl agreed Friday that the event is "pretty extraordinary" and said that it could impact infrastructure, including high-voltage transmission lines of the power grid. Dahl said that infrastructure operators have been notified to adequately prepare. 

This is the first time a storm watch has been issued for a G4 since January 2005. There is an average of 100 severe geomagnetic storms every solar cycle, but so far, there have only been three observed in the most recent cycle that began in December 2019. The most recent occurred on March 23. 

In:
• National Oceanic and Atmospheric Administration
• Space

Li Cohen

Li Cohen is a social media producer and trending content writer for CBS News.