The widespread power outage that affected over 100,000 residents and businesses across western South Dakota and central Wyoming Thursday remains under investigation. The Western Area Power Administration (WAPA) attributed the disruption to an “abnormal voltage event” originating outside its transmission system in central Wyoming, which triggered cascading failures across multiple power lines, including some of WAPA’s.
The outage began around 12:45 p.m., knocking out traffic lights in Rapid City and prompting law enforcement to manage 31 intersections. The Rapid City Public Library closed early, and the Pennington County Courthouse was evacuated due to an electrical issue and remained closed through Friday.
WAPA coordinated with regional utilities to restore service, and by 9 p.m., all systems were back online. Rocky Mountain Power, which oversees the affected external system, said the root cause remains under investigation. Black Hills Energy confirmed the issue was not related to solar activity or cybersecurity threats and emphasized it was likely an isolated operational fault.
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