A severe geomagnetic storm has hit Earth. Here's what could happen.

A severe geomagnetic storm has hit Earth has the potential to knock out power and electronics this weekend, but it could also bring a spectacular light show from the aurora borealis as far south as Alabama and Northern California.

After days of heightened solar activity, NOAA's Space Weather Prediction Center issued Thursday its first watch for a G4 storm — the second-strongest rating on a scale from G1 to G5 — in nearly 20 years. It then upgraded to a G5 on Friday. 

The prediction center reported in a social media post that "extreme (G5) geomagnetic conditions" were "observed" for the first time since October 2003. Friday's G5 conditions were first observed at 6:54 p.m. Eastern time and could continue through the weekend, the agency said.