A dramatic blast from the sun set off the highest-level geomagnetic storm in Earth’s atmosphere on Friday that is expected to make the northern lights visible as far south as Florida and Southern California and could interfere with power grids, communications and navigations system.

It is the strongest such storm to reach Earth since Halloween of 2003. That one was strong enough to create power outages in Sweden and damage transformers in South Africa.

The effects could continue through the weekend as a steady stream of emissions from the sun continues to bombard the planet’s magnetic field.

The solar activity is so powerful that the National Oceanic and Atmospheric Administration, which monitors space weather, issued an unusual storm watch for the first time in 19 years, which was then upgraded to a warning. The agency began observing outbursts on the sun’s surface on Wednesday, with at least five heading in the direction of Earth.

“What we’re expecting over the next couple of days should be more significant than what we’ve seen certainly so far,” Mike Bettwy, the operations chief at NOAA’s Space Weather Prediction Center, said at a news conference on Friday morning.

For people in many places, the most visible part of the storm will be the northern lights, known also as auroras. But authorities and companies will also be on the lookout for the event’s effects on infrastructure, like global positioning systems, radio communications and even electrical power.

While the northern lights are most often seen in higher latitudes closer to the North Pole, people in many more parts of the world are already getting a show this weekend that could last through the early part of next week.

As Friday turned to Saturday in Europe, people across the continent described skies hued in a mottling of colors.

Alfredo Carpineti, an astrophysicist, journalist and author in North London, saw them with his husband from the rooftop of their apartment building.

“It is incredible to be able to see the aurora directly from one’s own backyard,” he said. “I was hoping to maybe catch a glimpse of green on the horizon, but it was all across the sky in both green and purple.”

Here’s what you need to know about this weekend’s solar event.

How will the storm affect people on Earth?

A geomagnetic storm watch or warning indicates that space weather may affect critical infrastructure on or orbiting near Earth. It may introduce additional current into systems, which could damage pipelines, railroad tracks and power lines.

According to Joe Llama, an astronomer at Lowell Observatory, communications that rely on high frequency radio waves, such as ham radio and commercial aviation, are most likely to suffer. That means it is unlikely that your cellphone or car radio, which depend on much higher frequency radio waves, will conk out.

Still, it is possible for blackouts to occur. As with any power outage, you can prepare by keeping your devices charged and having access to backup batteries, generators and radio.

The most notable solar storm recorded in history occurred in 1859. Known as the Carrington Event, it lasted for nearly a week, creating aurora that stretched down to Hawaii and Central America and impacting hundreds of thousands of miles of telegraph lines.

But that was technology of the 19th century, used before scientists fully understood how solar activity disrupted Earth’s atmosphere and communication systems.

“That was an extreme level event,” said Shawn Dahl, a forecaster at NOAA’s Space Weather Prediction Center. “We are not anticipating that.”

Unlike tornado watches and warnings, the target audience for NOAA’s announcements is not the public.

“For most people here on planet Earth, they won’t have to do anything,” said Rob Steenburgh, a space scientist at NOAA’s Space Weather Prediction Center.

The goal of the announcements is to give agencies and companies that operate this infrastructure time to put protection measures in place to mitigate any effects.

“If everything is working like it should, the grid will be stable and they’ll be able to go about their daily lives,” Mr. Steenburgh said.

Will I be able to see the northern lights?

It is possible that the northern lights may grace the skies this week over places that don’t usually see them. The best visibility is outside the bright lights of cities.

Clouds or stormy weather could pose a problem in some places. But if the skies are clear, even well south of where the aurora is forecast to take place, snap a picture or record a video with your cellphone. The sensor on the camera is more sensitive to the wavelengths produced by the aurora and may produce an image you can’t see with the naked eye.

Another opportunity could be viewing sunspots during the daytime, if your skies are clear. As always, do not look directly at the sun without protection. But if you still have your eclipse glasses lying around from the April 8 event, you may try to use them to try to spot the cluster of sunspots causing the activity.

How strong is the current geomagnetic storm?

Giant explosions on the surface of the sun, known as coronal mass ejections, send streams of energetic particles into space. But the sun is large, and such outbursts may not cross our planet as it travels around the star. But when these particles create a disturbance in Earth’s magnetic field, it is known as a geomagnetic storm.

NOAA classifies these storms on a “G” scale of 1 to 5, with G1 being minor and G5 being extreme. The most extreme storms can cause widespread blackouts and damage to infrastructure on Earth. Satellites may also have trouble orienting themselves or sending or receiving information during these events.

The current storm is classified as G5, or “extreme.” It is caused by a cluster of sunspots — dark, cool regions on the solar surface — that is about 16 times the diameter of Earth. The cluster is flaring and ejecting material every six to 12 hours.

“We anticipate that we’re going to get one shock after another through the weekend,” said Brent Gordon, chief of the space weather services branch at NOAA’s Space Weather Prediction Center.

Why is this happening now?

The sun’s activity ebbs and flows on an 11-year cycle, and right now, it is approaching a solar maximum. Three other severe geomagnetic storms have been observed so far in the current activity cycle, which began in December 2019, but none were predicted to cause effects strong enough on Earth to warrant a watch or warning announcement.

The cluster of sunspots generating the current storm is the largest seen in this solar cycle, NOAA officials said. They added that the activity in this cycle has outperformed initial predictions.

More flares and expulsions from this cluster are expected, but because of the sun’s rotation the cluster will be oriented in a position less likely to affect Earth. In the coming weeks, the sunspots may appear again on the left side of the sun, but it is difficult for scientists to predict whether this will cause another bout of activity.

“Usually, these don’t come around packing as much of a punch as they did originally,” Mr. Dahl said. “But time will tell on that.”

Jonathan O’Callaghan contributed reporting from London.