The Northern lights, aka aurora borealis, were visible across Denver and Colorado on Tuesday night due to a severe geomagnetic storm that lit up the skies.

The Space Weather Prediction Center on Wednesday issued a Level 4 (severe) geomagnetic storm watch. Forecasters say the aurora may be visible “over much of the northern half of the country and may stretch as far south as Alabama and as far west as Northern California.”

The northern lights should be visible Wednesday night as well.

The Kp index tracks the magnitude of geomagnetic storms, and on Tuesday night it was at a Kp of 8.3-8.6.

Pink and red colored northern lights are caused by nitrogen molecules interacting with charged particles at altitudes of between 200-300 kilometers above Earth’s surface.

The most common color, green, is produced by solar particles colliding with oxygen atoms at altitudes of about 100–250 kilometers. 

The National Oceanic and Atmospheric Administration’s Space Weather Prediction Center released an alert for a severe geomagnetic storm just after 6 p.m.

Three solar flares, aka Coronal Mass Ejections, launched from a particularly active sunspot called AR4274, which is currently aimed toward Earth, according to NOAA. The first two CMEs erupted on Sunday and Monday, respectively, and were each triggered by powerful X-class solar flares.

These flares were categorized as class X1.7 and X1.2.

The third, and most powerful CME, was launched on Tuesday morning following the eruption of a huge X5.1 solar flare, which was almost five times the intensity of the two preceding flares, according to NOAA.

The monster CME accompanying this flare is predicted to hit Earth sometime on Wednesday, according to NOAA. Combined with the effects of the two prior outbursts, this could cause serious disturbances to Earth’s magnetic field.

NOAA predicts the resulting geomagnetic storm could reach ‘severe’ G4 levels — the second highest level on the agency’s scale.

A severe geomagnetic storm is “a major disturbance in Earth’s magnetic field,” with the potential to impact electricity and satellite operations, including GPS, NOAA officials wrote in the alert.

According to NOAA, the northern lights are expected to last into early Wednesday morning. People can check to see when the auroras will be at its peak on the Space Weather Prediction Center’s website.

NOAA suggests the best way to see the light show is to get as far away from city lights as possible.