A 'severe' geomagnetic storm is forecast for Earth for the first time in years — here's what to know about these storms

• NOAA issued a G4 geomagnetic storm watch for the first time in nearly 20 years.
• This type of storm generates dazzling aurora, but can also cause radio blackouts and outages.
• Experts say we should look out for aurora, but shouldn't worry about major issues. 

For the first time in nearly 20 years, the National Oceanic and Atmospheric Administration has forecasted a "severe" G4 geomagnetic storm to hit this Friday, dazzling states across the northern US with aurora.

G4 storms are the second-strongest type of geomagnetic storm. "If geomagnetic storms were hurricanes, 'severe' would be category 4," according to SpaceWeather.com. 

In the past, powerful geomagnetic storms have also been known to mess with electronics on satellites causing communication blackouts and disrupt the grid — triggering voltage control problems that can result in power outages.

For example, in October 2003, a G5 solar storm — the most severe type of g-storm — caused power outages in Sweden and damaged power transformers in South Africa.

In preparation for Friday's G4 event, NOAA's Space Weather Prediction Center has notified satellite and grid operators of the oncoming storm "so they can take protective action," the center announced Thursday.

For example, grid operators will likely withhold maintenance on Friday and Saturday to reduce the amount of stress they're putting on the grid, Matt Owens, a professor of space physics at the University of Reading, told Business Insider.

All that is to say, experts told Business Insider they don't expect to see any major issues this Friday.

"It's possible," Owens said. "If I was a betting man, I'd say there won't be serious effects. But I imagine there will be some impressive aurora."

What causes a geomagnetic storm

Geomagnetic storms occur when high-energy particles from the sun reach Earth and interact with our magnetic field.

But the sun is 93 million miles away, so to reach us these particles have to get a major boost. That boost comes from solar storms.

Solar storms happen when the sun shoots powerful explosions of highly-energized and magnetic plasma called coronal mass ejections toward Earth.

To be clear, the sun is constantly erupting and hurling particles into space. Most of them are directed away from Earth and never reach us. Occasionally one will come our way, sparking a minor G1 or G2 storm.

However, a G4 storm, like the one forecast through this weekend, is rare. The last time NOAA issued a G4 storm watch was in 2005.

Forecasts like these better help the folks managing satellites and power grids prepare. However, ultimately, we won't know how intense the storm will be until it's already here.

Forecasting solar storms is tricky

Forecasting solar storms is tricky. "Sometimes we find that the storms can be stronger than we thought, or they can be somewhat of a dud," Alex Young, the associate director for science at NASA Goddard's Heliophysics Science Division, told BI.

This particular storm is especially difficult to track because it's made up of five separate coronal mass ejections, all hurtling towards Earth at roughly 560 miles per second, Owens said.

In case the storm is severe and triggers outages, it's always best to respond how you would for any power outage: keep your fridge closed, disconnect appliances to avoid damage from an electrical surge, and check with local officials about heating and cooling locations. Also, a severe storm might disrupt GPS, so it's best to have a written record and directions to important locations, like hospitals.

Also all signs point to an incredible light show for northern US states on Friday night, potentially reaching as far south as New York and Pennsylvania, Young said.