Wednesday, December 15, 2021
NOAA has posted G2 to G3 geomagnetic storm watches for July 29 to 31 as merging CMEs reach Earth, bringing weak power grid fluctuations, GPS drift, and aurora across the northern United States.
A coronal mass ejection that left the sun on July 24 reached Earth on July 27. It was not the last one inbound. The NOAA Space Weather Prediction Center has geomagnetic storm watches posted for July 29 through 31, when several outbursts merging into a single front, a stack forecasters call a cannibal CME, are expected to bring the strongest conditions of this stretch.
The first wave delivered G1 to G2 conditions, minor to moderate on NOAA's five level scale, with the planetary K index running between 4 and 6 as of the July 28 dateline. Those readings are expected to climb rather than settle as the combined ejecta arrive. The active watch for July 29 through 31 calls for G2 to G3 storming, moderate to strong.
At this level a geomagnetic storm is mostly a grid and signal story rather than a direct danger to people. Utilities can see weak voltage swings, GPS accuracy can drift during the disturbance, and satellite operators plan for extra atmospheric drag. High latitude power systems carry the most exposure, which is why grid operators across Canada, Alaska, and the northern states track the forecast closely. For everyone else the visible payoff is a strong chance of aurora across the northern United States on clear nights.
A space storm earns a place on a preparedness feed because the same force that lights up the sky can also stress the power grid. The response is ordinary readiness you can handle today. Learning to prepare for a power outage covers the first day in the dark. Setting up backup power for your home carries a household through a longer one. The background on why these storms happen, and what a larger one could do, lives in our guide to preparing for a solar flare.
For now the sun remains active and the SWPC watch runs through the end of the month. A minor to moderate storm like this one rarely does more than flicker a few lights, but it is a low stakes cue to top off your outage kit and charge your devices before a stronger storm cycle arrives.
