Yup. I was right. It was AR3415, now AR3536. Batten down the hatches!
Spaceweather:
MAJOR X-CLASS SOLAR FLARE: Mere hours after emerging over the sun's eastern limb on Dec. 31st, big sunspot AR3536 erupted, producing a major X5-class solar flare. NASA's Solar Dynamics Observatory recorded the extreme ultraviolet flash.
Radiation from the flare has caused a deep shortwave radio blackout over the Pacific Ocean: blackout map. #HamRadio operators may notice loss of signal at all frequencies below 30 MHz for more than 60 minutes after the flare's peak (2155 UT).
It's too soon to know for sure, but this explosion probably launched a coronal mass ejection (CME) into space. Because the blast site is located near the extreme eastern edge of the sun, the CME is unlikely to have an Earth-directed component, but stay tuned. Solar flare alerts: SMS Text
Update: This explosion caused a #SolarTsunami. You can see the shock wave in this animation from NASA's Solar Dynamics Observatory.
This is the strongest flare of #SolarCycle25 (so far) and the most powerful eruption the sun has produced since the great storms of Sept. 2017.