"It's headed straight for us like a freight train," said John Kohl, a solar astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. "This is the real thing."
Predictions are it could strike Earth's magnetic field by midday Wednesday.
The explosion of gas and charged particles into space from the corona, the outermost layer of the sun's atmosphere, isn't harmful to people. But it can knock out satellite communications, which some emergency crews are relying on in battling California's wildfires.
Similar solar events in recent years have disrupted television transmissions, GPS navigation, oil pipeline controls and even the flow of electricity along power lines.
Space weather forecasters first warned of that possibility last week, when a previous solar flare erupted, and then they saw a new sunspot region developing in another region of the sun's face.
The cloud of charged particles from last week's eruption struck Earth "with only a glancing blow," Kohl said. It disrupted some airline communications.
But Kohl said scientists observed the biggest such explosion in 30 years shortly before 6 a.m. EST (1100 GMT) Tuesday. It produced a particle cloud 13 times larger than Earth and hurtled through the solar system at more than 1 million miles per hour.
The resulting geomagnetic storm could be ranked among the most powerful of its kind and last for 24 hours. It is expected to disrupt the communications satellites and high frequency radios.
In southern California, wildfires already have knocked out many microwave communication antennas on the ground, making satellite communications important to emergency efforts. Researchers said safety personnel might encounter communications interference.
Federal researchers said they already have turned off instruments and taken other precautions with science satellites.
A positive note: strong geomagnetic storms can produce colorful auroras in the night sky visible as far south as Texas and Florida beginning late Wednesday.
Sunspots and solar storms tend to occur in 11-year cycles; the current cycle peaked in late 2000.
Scientists compared the latest flare to the "Bastille Day storm" that occurred in July 2000.
"The Bastille Day storm produced considerable disruption to both ground and space high-tech systems," said Bill Murtagh, a space weather forecaster for the National Oceanic and Atmospheric Administration.