CAPE CANAVERAL, Fla. — A strong solar storm headed toward Earth could produce northern lights in California and potentially disrupt communications this weekend.

The National Oceanic and Atmospheric Administration issued a rare severe geomagnetic storm warning when a solar outburst reached Earth on Friday afternoon, hours sooner than anticipated. The effects were expected to last through the weekend and possibly into next week.

The storm’s severity is currently categorized as G4, the second strongest. During a call with reporters Friday, Shawn Dahl of NOAA’s Space Weather Prediction Center said that the agency can’t rule out a “low-end” G5 event.

NOAA has alerted operators of power plants and spacecraft in orbit to take precautions.

Even when the storm is over, signals between GPS satellites and ground receivers could be scrambled or lost, according to NOAA —  but there are so many navigation satellites that any outages should not last long.

Increased radiation also could threaten some of NASA’s science satellites. Extremely sensitive instruments will be turned off, if necessary, to avoid damage, said Antti Pulkkinen, director of the space agency’s heliophysics science division. NASA said the storm posed no serious threat to the seven astronauts aboard the International Space Station.

“As far as the worst situation expected here at Earth, that’s tough to say and I wouldn’t want to speculate on that,” Dahl said. “However, ‘severe’ level is pretty extraordinary, it’s a very rare event to happen.”

An extreme geomagnetic storm in 2003 took out power in Sweden and damaged power transformers in South Africa.

NOAA said the sun produced strong solar flares beginning Wednesday, resulting in five outbursts of plasma capable of disrupting satellites in orbit and power grids here on Earth. Each eruption — known as a coronal mass ejection — can contain billions of tons of solar plasma.

NOAA said the flares seem to be associated with a sunspot that’s 124,000 miles across — 16 times the diameter of Earth.

People who’ve saved their solar eclipse glasses will be able to see that sunspot without magnification, in the sun’s lower right quadrant.

The storm could produce aurora borealis — northern lights — as far south in the U.S. as Alabama and Northern California, according to NOAA.

Experts stressed it would not be the dramatic curtains of color normally associated with the northern lights, but more like splashes of greenish hues.

“That’s really the gift from space weather — the aurora,” said Rob Steenburgh, a scientist with the Space Weather Prediction Center. He and his colleagues said the best aurora views may come from phone cameras, which are better at capturing light than the naked eye.

Snap a picture of the sky and “there might be actually a nice little treat there for you,” said Mike Bettwy, operations chief for the prediction center.

The most intense solar storm in recorded history, in 1859, prompted auroras in central America and possibly even Hawaii. “We are not anticipating that” but it could come close, Dahl said.