The Northern Lights could be visible again Sunday night for millions of people in the United States, forecasters predicted, as Earth continues to experience the effects of a “historic” geomagnetic storm.

The Northern Lights will likely be viewable in most of Canada and Alaska, the National Oceanic and Atmospheric Administration predicted on Sunday.

The lights might also be visible in the continental U.S., potentially reaching New York, Chicago, and as far south as Alabama, NOAA’s Space Weather Prediction Center said.

The lights are caused by an ongoing geomagnetic storm, which are expected to again bombard the Earth with coronal mass ejections—plasma released from the Sun—during a “secondary peak” of the storm early Sunday into Sunday night at speeds of 1,800 kilometers per second, the National Weather Service said.

The National Weather Service issued a G4+ watch for a “Severe to Extreme” geomagnetic storm for Sunday, cautioning the storm could cause “power grid irregularities, degradation to high-frequency communication, GPS, and possible satellite navigation.”

The storm will still affect Earth through Monday, until a large and “magnetically complex” sunspot—known as NOAA Region 3664—rotates out of view on Tuesday, according to forecasters.

WHERE AND WHEN IS BEST TO VIEW THE NORTHERN LIGHTS?

Predicting the Northern Lights is notoriously difficult, but forecasters communicate the strength of geomagnetic storms using the K-index, a scale from zero to nine indicating how bright and visible the Northern Lights will be on a given day. The forecasted strength for Sunday as of now is Kp 6—meaning the lights will “move even further from the poles and will become quite bright and active,” according to NOAA. Although it is difficult for forecasters to predict exactly where the lights will be visible on a given night, they do offer suggestions for prospective Aurora Borealis viewers. Stargazers should travel as close to the magnetic poles as possible, where even low level activity Kp 3 or Kp 4 is visible, according to NOAA. Viewers should also try to find a viewing location away from city lights and other sources of light pollution. The best time of day to view the lights is typically between 10 p.m. and 2 a.m., the agency said.

KEY BACKGROUND

The Northern Lights are caused by solar wind—streams of ionized gas originating from the Sun that interact with the Earth’s magnetic field, according to NOAA. The current storm began on Friday, causing the National Weather Service to issue its first severe geomagnetic storm watch since January 2005. Friday night’s aurora display was historic, with sightings of the Northern Lights reported across essentially the entirety of the United States, even down to the Florida Keys, and the storm was strong enough that viewers in many urban settings with significant light pollution were also able to take in the rare spectacle. Severity of geomagnetic storms is measured by NOAA’s G-scale, which ranks them from 1 through five based on their severity. The current storm was ranked a G3 or G4 over the weekend, and briefly reached G5 or “extreme” activity Friday night.

WHAT TO WATCH FOR

The geomagnetic storm is expected to continue through Monday, forecasters predicted. However, the storm is expected to begin to weaken, with conditions likely only reaching a G3 or a “strong” storm on the NOAA scale. The Northern Lights could still be viewable Monday, but they will likely appear dimmer with a predicted Kp 4. The storms will likely dissipate on Tuesday after Region 3664 rotates away from the Earth.