NOAA’s Space Weather Prediction Center recorded a minor, G1-level geomagnetic storm at 23:59 UTC on Oct. 21, which was 7:59 p.m. Eastern time that evening. The center had been signalling the possibility for days, issuing G1 watches that covered Oct. 19, Oct. 20 and Oct. 21 before the K-index finally reached 5.
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The record is short and dated, which makes it easy to follow. Three watches, one warning and one alert tell the sequence, and each was released by the same federal forecast office with its own serial number and timestamp. What follows is the order in which they appeared and what a G1 storm is officially expected to do.
Three watches, three overlapping forecast windows
The first G1 watch in the sequence, serial 1130, went out at 20:14 UTC on Oct. 17. Its three-day table listed no storm for Oct. 18 or Oct. 19 and a G1 (Minor) level for Oct. 20, according to the SWPC alert feed. It looked furthest ahead, and the two watches that followed shifted the dates it named.
Serial 1131 followed at 17:24 UTC on Oct. 18 and moved the expected storm earlier, listing G1 (Minor) for both Oct. 19 and Oct. 20, with Oct. 21 below G1. Serial 1132, issued at 19:12 UTC on Oct. 20, covered the remaining day: G1 (Minor) on Oct. 21, with Oct. 22 and Oct. 23 below G1. Between them, the three products account for the watch days named above, and each one replaced the last rather than adding to it. The feed therefore shows a forecast that moved with the data: a single G1 day on Oct. 20 became Oct. 19 and 20, and the last product moved the storm to Oct. 21.
The warning and the alert on Oct. 21
Closer to the event, the center changed product types. Serial 2271, a warning, was issued at 22:28 UTC on Oct. 21 with the message “WARNING: Geomagnetic K-index of 5 expected,” valid until 09:00 UTC on Oct. 22. A watch looks days ahead, while a warning says conditions are expected within hours. SWPC describes its alerts, watches and warnings as bulletins similar to those from local National Weather Service offices, conveying levels of severity.
The alert itself, serial 2058, was issued at 00:03 UTC on Oct. 22 and read “ALERT: Geomagnetic K-index of 5.” Its threshold-reached field gives 2026 Oct. 21 at 23:59 UTC, the moment that anchors the storm date. The warning had come 91 minutes before the threshold time, and the alert went out four minutes after it, so the observation and the notice are nearly simultaneous. Because the alert was stamped after midnight UTC, it carries an Oct. 22 issue date even though the storm threshold was an Oct. 21 event, a distinction that matters when sources cite different days. In the part of the feed reviewed, covering Oct. 17 through Oct. 22, this was the only K-index 5 alert.
What G1 means on NOAA’s scale
The Kp value of 5 is the line that defines G1. On NOAA’s space weather scales, a G1 (Minor) storm corresponds to Kp 5, and the listed effects are modest: weak power grid fluctuations can occur, and there can be minor impact on satellite operations. The page adds that aurora is commonly visible at high latitudes, naming northern Michigan and Maine as examples.
G1 is also the most common storm category. NOAA’s scale puts the average at about 1,700 G1 storms per 11-year solar cycle, or roughly 900 days per cycle, because a single storm can span several days. That frequency explains why watches at this level are routine products rather than rare events, and why the center issues them with a day-by-day table. Readers following aurora forecasts will recognize the pattern: the center predicts a level for each of three days, and the G1 label applies at Kp 5, with higher storm levels reserved for higher index values.
The listed effects are official expectations, not measured outcomes from this storm. The scale describes what can occur at a given level, and the alert feed records that the threshold was reached; neither establishes that any grid or spacecraft was affected on Oct. 21.
What the forecast said afterward
The center’s own outlook after the storm was calm. The three-day forecast issued at 12:30 UTC on Oct. 22 stated that no G1 (Minor) or greater geomagnetic storms were expected through Oct. 24. It did note that a negative-polarity coronal hole high-speed stream was expected to disturb the near-Earth environment on Oct. 24, with the highest expected Kp of 4.33 in the 18-21 UT window, below storm level.
The forecast is a snapshot and is replaced as new data arrive. Anyone tracking conditions can compare it against the one-minute Kp estimate feed, which records the index as it evolves, though the Oct. 21 alert remains the official marker of the event. For the purposes of this report, the storm is a single threshold crossing at 23:59 UTC, bracketed by three watches before it and a calmer outlook after it.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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