A series of power outages across the continent that occurred on April 21 may have been caused by a geomagnetic storm. Coordinated cyberattacks were initially blamed for the events, although a fire caused by an overloaded circuit breaker in one of the major outages ruled out that theory. However, it is possible that a geomagnetic storm may have caused the near-simultaneous outages, as there had been an ongoing solar storm at the time.
While the Pacific Gas and Electric Company (PG&E) reported outages in Antioch, Brentwood, Hayward, Mountain View, Oakland, San Jose, San Leandro, Tracy, and Walnut Creek, the largest outages were experienced by New York City and San Francisco. Outages in New York began around 7:20 AM, interrupting the morning commute in the city’s subway system. San Francisco went down four and a half hours later, and was hit much harder, with 90,000 customers affected for over 7 hours, effectively paralyzing the city. While the cause of New York’s outage is still a mystery, San Francisco’s outage was due to a fire at a substation, caused by an overloaded circuit breaker.
At the time, a moderate (G2) geomagnetic storm watch was underway, according to the National Oceanic and Atmospheric Administration’s (NOAA) Space Weather Prediction Center. Despite the storm itself only being listed as "moderate", NOAA’s alert does warn that one of the effects can be "Induced Currents – Weak power grid fluctuations can occur." This means that the increase in geomagnetic activity brought about by the storm can cause a buildup of excess electricity in power grids: while most systems will be able to handle energy spikes from such an otherwise moderate storm, systems that have been weakened could be susceptible to failure, and overload a potentially worn-out circuit breaker, as had happened in San Francisco. On the same day, a town in Ontario, Canada, also experienced a community-wide power outage, caused by a "pole fire", according to the local utility.
Unfortunately, larger storms, such as 1859’s Carrington Event, have the potential to cause catastrophic, nation-wide power failures, resulting in power grid equipment failures that could take months to correct. Whitley Strieber outlines the dangers — and what needs to be done to avert them — in his 2012 ebook Solar Flares: What You Need to Know.