The strongest geomagnetic storm in more than 20 years slammed Earth on Friday, with explosions of plasma and magnetic fields causing some radio blackouts and the northern lights to extend to the southern U.S. On Monday, officials warned the storms aren’t yet over. 

NOAA’s Space Weather Prediction Center said early Monday that a G3, or “strong,” geomagnetic storm warning was in effect until 2 a.m. ET. While stronger storms are no longer likely and conditions are expected to “gradually wane” throughout the day, the center said in its forecast that moderate to strong geomagnetic storms are “likely” on Monday, as are minor storms on Tuesday. 

The center also said “solar activity is expected to be at high levels” with a possibility of more solar flares, or bursts of electromagnetic radiation from the sun.

The update came as another X-class solar flare was recorded. X-class flares are the strongest class of these solar bursts, and the latest was recorded as “moderate.” 

“Flares of this magnitude are not frequent,” the center said. “…Users of high frequency (HF) radio signals may experience temporary degradation or complete loss of signal on much of the sunlit side of Earth.” 

A separate and stronger X-class flare was recorded on Sunday and may have caused roughly hour-long high-frequency radio blackouts across wide areas on the sunlit side of the Earth. 

The flares came from Sunspot Region 3664, a massive area of the sun responsible for much of the flares and coronal mass ejections (CMEs) that led to the weekend’s extreme geomagnetic storm. That spot remains “the most complex and active on the disk,” NOAA said. That spot is so large that people wearing eclipse glasses can see it from Earth, as it measures roughly 124,000 miles across, according to Space.com.

That spot has been active alongside Regio 3663, which combined with 3664, is considered “magnetically complex and much larger than Earth,” NOAA said. 

CMEs, or large bursts from the sun’s atmosphere filled with plasma and magnetic fields that lead to geomagnetic storms, are expected to continue throughout Monday and fuel G3 activity. 

“Continuing, but weaker CME influences are anticipated to decrease responses down to unsettled to G1 (Minor) levels on 14 May,” NOAA forecasts. 

Solar radiation storms, though minor, are also expected in the same timeframe, as are more radio blackouts, although some of those could be considered “strong” events depending on the solar flares that erupt.