Geomagnetic storming reached the G4 — severe — level at 0513 UTC on June 23. The storm began a day earlier. The NOAA Space Weather Prediction Center (SWPC) said more geomagnetic storming, although perhaps not quite as severe, could lie ahead.

“Solar wind conditions remain highly favorable for continued strong geomagnetic storming, with both fast solar wind and strong magnetic fields,” the SWPC said on its website. “Aurora watchers in North America, especially [in] northern tier states of the US, should stay alert.”

In a G4 event, electrical power systems can experience “widespread voltage control problems,” possibly tripping some protective systems. For radio amateurs, HF radio propagation can become sporadic, and satellite navigation degraded for hours at a time. Auroral displays may be visible as far south as Alabama and California (approximately 45° latitude).

According to the SWPC, the geomagnetic storm began as forecast, quickly ramping up to severe levels. This followed a G4 storm alert on June 22, after the storm slammed into Earth at 1839 UTC.

“This is the same intensity level reached in March 2015 during the St Patrick's Day storm,” the SWPC said. “This is the very early stages of an event that will play out over many hours, with SWPC forecasting continuing storm level intensities into June 23.”

The June 25 prediction is for G1 (minor) to G2 (moderate) geomagnetic activity over the next 24 hours. This means electrical power systems at high latitudes could experience voltage alarms, and long-duration storms may cause transformer damage. HF radio propagation can fade at higher latitudes, and aurora may be visible as far south as Idaho and New York.

A coronal mass ejection (CME) that erupted from the sun on June 22 in conjunction with a R2 (Moderate) Radio Blackout solar flare was expected to arrive on June 24 at 2300 UTC. “This timing bodes well for aurora watchers in North America,” the SWPC said.

At 1823 UTC on June 22, the active region that produced the event hitting Earth erupted again, and a long-duration x-ray flare reached R2 levels, “thus, users impacted by solar flares and users impacted by geomagnetic storms were each being affected,” the SWPC pointed out. A solar radiation storm continues unabated, briefly reaching S3 levels.

In an R2 (moderate) event, there is a limited blackout of HF radio communication on the sunlit side of Earth and a loss of radio contact “for tens of minutes.” Low-frequency navigation signals also can be degraded over a similar period.

The June 25 prediction is for a 75 percent chance of an R1 or R2 event and a 20 percent chance of an R3 to R5 event.