SWPC Issues G3 Geomagnetic Storm Warning Through 2359 UTC as CME Hits Earth

Q: Despite the severe G3 warning, could a sudden magnetic shift completely erase the anticipated mid-latitude aurora displays tonight?

Yes. A G3 storm greatly improves the odds of mid-latitude aurora, but it does not guarantee them, and a sudden unfavorable magnetic shift could sharply reduce or even temporarily erase visible displays tonight. The key factor is not the G3 label by itself, but the interplanetary magnetic field’s north-south component, known as Bz. If Bz turns strongly southward, Earth’s magnetic field couples efficiently with the incoming CME and aurora can expand well southward. If Bz swings northward, that coupling weakens fast, and aurora intensity can fade even while overall geomagnetic storm conditions remain elevated. That is why forecasters have stressed that impacts from this CME could range from G1 to G3 depending on the ejecta’s magnetic orientation. The CME shock that arrived around 1434 UTC boosted solar wind and pushed conditions to G3 by 1758 UTC, but aurora visibility can still fluctuate on short timescales of minutes to hours. There is also only limited warning. Real-time measurements from spacecraft at the L1 point typically provide just 15 to 60 minutes of lead time, and current outages in some real-time solar-wind data streams can make nowcasting harder. So the expected mid-latitude aurora is still plausible under this warning, but it could underperform if Bz turns northward or weakens. Even with favorable space weather, local darkness, cloud cover, and light pollution will decide whether people actually see it.

3 articles · Updated · spaceweather.gov · Oct 10

G3 geomagnetic storm conditions began at 1758 UTC on Oct. 10, prompting SWPC to warn that strong or greater activity could last through 2359 UTC.
A coronal mass ejection launched on Oct. 8 arrived around 1434 UTC on Oct. 10, sharply boosting the solar wind environment and driving the escalation.
The geomagnetic field is expected to hold at or near G3 levels for the rest of the warning window, signaling sustained rather than brief storm conditions.