A space weather storm that was forecast to be the strongest in five years has fizzled out and ended up causing no impact to power grids or modern navigation systems, US experts said on Thursday.

A series of eruptions on the Sun this week sent radiation and solar plasma hurtling toward Earth at high speeds but in the end, the geomagnetic storm registered the lowest level, G1, on a five-step scale.

"Our forecasters really struggled with this," said National Oceanic and Atmospheric Administration scientist Joseph Kunches.

He compared the bungled prediction to watching a pitcher toss a baseball but knowing nothing else until it reaches the catcher.

"We missed the spin on the ball," said Kunches.

NOAA had forecast the storm would be a level three, or "strong," and said it would be the worst since 2006. NASA had said it might even be "severe."

In this case, the "spin" that forecasters missed was contained in the orientation of the magnetic field inside the coronal mass ejection from the Sun that raced toward Earth and arrived early Thursday after a 34-hour journey.

"It is very difficult for forecasters, literally almost impossible, as you watch the coronal mass ejection come off the Sun to be able to predict the orientation of that embedded magnetic field," he said.

"It's like if you were a hurricane forecaster and you didn't know the barometric pressure at the eye of the storm."

Kunches said there were no reports of GPS disruption, no reports of problems in terms of electric power, and that any displays of the northern lights, or aurora borealis, would be visible further north than NOAA initially said.

However, the impacts could worsen over the next 24 hours as the storm continues, he said.

NOAA and NASA had warned on Wednesday that the storm could garble global positioning systems, satellites and power grids, and had already caused some air carriers to change their planes' polar flight paths.

Astronauts aboard the International Space Station were not affected by the radiation storm, NASA said.

Geomagnetic and radiation storms are growing more frequent as the Sun leaves its solar minimum period and moves into a solar maximum over the coming years, but people are generally protected by Earth's magnetic field.

However, some experts are concerned that because the world is more reliant on GPS and satellite technology now than it was during the last solar maximum, more disruptions to modern life are likely.

The fuss began late Sunday at an active region on the Sun known as 1429, with a big solar flare that was associated with a coronal mass ejection that thrust toward the Earth at some four million miles (6.4 million kilometers) per hour).

A pair of solar flares and a CME followed overnight Tuesday-Wednesday.

NASA said the first of the two flares on March 6-7 -- classified in the potent X class and facing directly at the Earth -- was the biggest this year and one of the largest of this cycle known as the solar minimum, which began in early 2007.

In fact, it was second only to a stronger one that erupted in August.

The solar flares alone caused brief high frequency radio blackouts that have already passed, according to NOAA.

But while solar radiation storm registered a level 3, the geomagnetic storm ended up being the same minor level as a similar event in January, Kunches said, adding that forecasters were doing their best to balance the need to alert people with the realization that their forecasts may set off false alarms.

"How do you give good information to users to safeguard them and allow them to take precautionary actions, knowing in fact that it might not turn out to be as bad as one might think?" he asked.

"We really worry about crying wolf."(AFP)