A Tuesday-afternoon watch went up when NOAA’s Space Weather Prediction Center posted product WATA30 at 16:33 UTC on Oct. 6, 12:33 p.m. Eastern. Its one-line message reads: “G1 to G2 (Minor-Moderate) geomagnetic storms are likely on 09 Oct with the arrival of a CME that left the Sun on 06 Oct.” A coronal mass ejection, in other words, is already crossing the roughly 93 million miles toward Earth.
The watch is a forecast of a range, and the range matters. G1 is the bottom rung of the five-step NOAA storm scale; G2 is the next one up. The center expects the eruption’s arrival to land somewhere in that band on Oct. 9, and the exact strength will not be known until the cloud’s magnetic field is measured upstream of Earth.
The Oct. 6 radio signature behind the watch
The earliest sign of the eruption came in the center’s machine-readable alert feed, hours before the watch. An alert issued at 06:15 UTC on Oct. 6 reported Type II radio emission beginning at 05:59 UTC, with an estimated shock velocity of 947 km/s. Type II bursts are produced when a shock wave plows outward through the Sun’s corona, and they are one of the standard markers forecasters use to link a flare to a coronal mass ejection rather than a flare alone.
A speed near 947 km/s sits in the middle of the range SWPC describes for these eruptions, which run from under 250 km/s to about 3,000 km/s. The fastest Earth-directed ones can arrive in 15 to 18 hours; slower ones take several days. A Type II velocity is a measurement of the shock in the corona, not of the cloud’s cruise speed through interplanetary space, which is why the center times arrival from its modelling and not from that single number. The watch points to Oct. 9, roughly three days after launch.
G2 on the NOAA scale: what Kp 6 means
NOAA’s space weather scales tie a G2 storm to a planetary Kp index of 6 and count about 600 such storms per 11-year solar cycle, or about 360 days of them. The listed effects are modest compared with the top of the scale: voltage alarms on high-latitude power systems, with transformer damage possible only in long-duration storms; satellite operators adjusting orientation and refreshing orbital-decay predictions because of extra atmospheric drag; fading of high-frequency radio at higher latitudes.
Aurora is the visible effect. At G2 the scale places the northern lights as far south as New York and Idaho, and the watch itself is framed around activity poleward of 55 degrees geomagnetic latitude. Cloud cover and moonlight will decide how much of that is seen, and neither is part of the NOAA product.
A magnetic field that must point south
Size alone does not set the grade. SWPC’s explanation of geomagnetic storms says energy moves into Earth’s magnetosphere most efficiently when the solar wind’s magnetic field points southward, opposite Earth’s own field on the dayside. A cloud carrying a northward field can pass with little more than a ripple, and the same cloud with a southward field can produce a stronger storm. That orientation is only known once the front reaches the DSCOVR spacecraft, which typically gives 15 to 60 minutes of warning.
The same page lists the practical risks of a stronger disturbance: ionospheric density changes that “modify the path of radio signals and create errors in the positioning information provided by GPS,” and induced currents in power grids and pipelines. At G2, those are the edge of the envelope, not the expectation.
An already unsettled week
The Oct. 6 launch follows a stretch of agitated conditions. SWPC’s alert log for Oct. 5 and 6 records a G1 (Minor) alert at a K-index of 5, then K-index 4 warnings that were extended through 06:00 UTC and again through 12:00 UTC on Oct. 6. Separately, relativistic electrons at geostationary orbit crossed the 1,000 particle flux unit threshold at 2 MeV on GOES-19, with a peak of 2,358 pfu at 20:10 UTC on Oct. 5; the center issued a continuation on Oct. 6 at 05:07 UTC.
None of that is the cause of the Oct. 9 watch, and the electron alerts concern satellite hardware, not people on the ground. They do show that the field around Earth was already restless before the new cloud was even launched.
The center’s three-day forecast issued at 00:30 UTC on Oct. 6, before the eruption was logged, had expected no G1 or greater storms through Oct. 8 and a 15 percent daily chance of minor radio blackouts. That forecast covers the days before the cloud’s expected arrival and was written without it, so the Oct. 9 watch is the product that carries the new information.
The watch was the newest entry in the alert feed when it was last read for this article, with nothing yet issued to upgrade it to a G3 or cancel it. The first measurement that will settle the question is the field orientation at DSCOVR on Oct. 9, and SWPC’s range of G1 to G2 stays in force until then.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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