Most powerful solar storm in 6 years caused auroras all over the US. And nobody saw it coming.

The strongest solar storm to hit Earth for six years sparked stunning auroras across the U.S., with intense light displays appearing as far south as Alabama and Northern California. Yet no one saw it coming.

The March 24 solar storm was the result of a "stealth" coronal mass ejection (CME) – or a gargantuan, fast-moving blob of plasma and magnetic field released from the sun –  Live Science’s sister site Space.com reports. The CME came from a coronal hole wider than 20 Earths that was spewing out solar winds at speeds over 1.3 million mph (2.1 million km/h).

Tamitha Skov, a U.S. space weather forecaster, told Space.com that no one saw the G4 storm coming because it was "nearly invisible." She said these stealth storms launch far slower than a typical CMEs that erupts, so are more difficult to observe.

The blob of particles smashed into Earth’s atmosphere early on March 24, compressing the planet’s magnetic field and triggering a geomagnetic storm. It was classified as a G4, or "severe" storm on the National Oceanic and Atmospheric Administration’s five-level scale. Impacts of a G4 storm include radio blackouts for several hours and navigation outages.

Storms can also push auroras being pushed to much lower latitudes than usual. Auroras, or Northern Lights, normally only appear at high latitudes near the North and South Poles, because Earth's magnetic field deflects electrons in solar wind toward the poles. But when a solar storm strikes, more charged particles collide with gases in the upper atmosphere, meaning the night light shows can appear closer to the equator.

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That was the case for last night's stealth storm.

"Forecasters completely missed this one," a post on SpaceWeather.com said. "Auroras spread into the United States as far south as Colorado and New Mexico during a severe (category G4) geomagnetic storm — the most intense in nearly 6 years."