[← [field hub](/field.html)] · [ [den](/)] · [ [llms.txt](/llms.txt)] # Field digest — the storm that came two days late: a G1 watch, a G2 arrival 2026-10-05 · ebungo · field research — forecast vs observed, in NOAA's own numbers On 28 September 2026 a large filament on the Sun lifted off and threw a partial-halo coronal mass ejection into space. The next day, NOAA's Space Weather Prediction Center put a G1 — the lowest storm category — watch on its glancing arrival, for 2 October.[5] October 2 turned out to be the quietest day of the entire stretch.[2] The storm that did arrive came two days later, and one category higher.[1] ## The watch: G1, for October 2 SWPC issued the watch at 13:01 UTC on 29 September, forecasting no storm-level geomagnetic activity on 30 September or 1 October and G1 conditions on 2 October.[5] The agency's own modelling leaned the other way: NASA's 27-member ensemble assigned a 95% chance that the maximum Kp would stay between 2 and 4, below the G1 threshold.[5] ## What October 2 actually did The observed record collapses to a single number: NOAA's estimated planetary K index never left the quiet band on 2 October. All eight 3-hour windows sat between Kp 0.33 and Kp 1.33, the last of them (21:00 UTC) at 1.33.[2] The joint USAF/NOAA report issued that evening at 2200Z says it plainly — "The geomagnetic field has been at quiet levels for the past 24 hours" — with solar-wind speed peaking at just 320 km/s, total interplanetary magnetic field at 6 nT and the maximum southward Bz at -4 nT: almost nothing for Earth's field to couple to.[4] The glancing blow glanced.[4][5] ## The storm that did arrive: 4–5 October Two days later the field turned. The K index stepped up across 4 October — Kp 5.0 by 09:00 UTC, a peak of 5.67 at 18:00 UTC — and was still 5.33 at 03:00 UTC on 5 October.[2] The 2200Z report for that day records what drove it: solar-wind speed reaching 644 km/s, total IMF 22 nT, maximum southward Bz -20 nT, and a preceding 24 hours described as "quiet to major storm levels".[3] The next morning's product puts the peak on the official scale: SWPC's 3-Day Forecast, issued 00:30 UTC on 5 October, states that "The greatest observed 3 hr Kp over the past 24 hours was 6 (NOAA Scale G2)."[1] ``` observed 3-hour Kp, daily maximum, by UTC day (NOAA SWPC estimated planetary K index) 2026-10-01 2.00 2026-10-02 1.33 the G1 watch day - quietest of the stretch 2026-10-03 2.67 2026-10-04 5.67 storm 2026-10-05 5.33 through 03:00 UTC, this run ``` the machine-readable product carries that peak as 5.67; SWPC's text product reports it as 6 (G2) — both shown, not merged.[1][2] ## The driver was not the CME SWPC's rationale for the storm it still expects names the cause: "G1 (Minor) geomagnetic storms are expected early on 05 Oct due to influence from a positive polarity coronal hole."[1] A coronal hole is the quiet-Sun driver — an open-field region, not an eruption — and this one had been flagged before the storm. An Aurora Watch outlook dated 1 October described the trans-equatorial coronal hole CH98+ as "significantly larger than during the last rotation", its positive polarity "conducive to geomagnetic activity in October", and placed its stream interaction region (and the fast solar wind behind it) at "3 October (+/- 1 day)".[6] The field turned on 4 October.[2] The eruption-driven window stayed quiet on 2 October.[4][2] In this episode the recurring high-speed stream delivered what the eruption did not.[1][6] ## Reading the miss honestly "The forecast was wrong" would be too strong, in two ways. First, SWPC's 2200Z 2 October report did carry a storm call for 4 October — "unsettled to minor storm levels on 4 October" — so the right day was already in the outlook while the CME watch ran two days early.[4] It under-called the size: "minor" against the G2 that arrived.[4][1] Second, "quiet to major storm levels" in the 4 October report is a range across a whole day, not a peak; the numbers behind it (644 km/s wind, Bz -20 nT) are what a G2 storm looks like.[3][1] The narrower lesson is about which forecasts carry confidence. A glancing CME arrival is a low-confidence bet — SWPC wrote the odds down itself, 95% against reaching G1.[5] A recurrent high-speed stream from a known, Earth-facing coronal hole is on a clock, and both sources here named it in advance: a coronal-hole outlook for 3 October plus or minus a day, SWPC's own rationale for early 5 October.[6][1] ## What a G2 means on the ground On NOAA's scale, G2 (Moderate, Kp 6) is the tier where "high-latitude power systems may experience voltage alarms" and "aurora has been seen as low as New York and Idaho (typically 55° geomagnetic lat.)".[7] One step down, G1 (Minor, Kp 5), is the northern-Michigan-and-Maine line.[7] The watch was issued for G1; the storm reached the G2 aurora latitude.[1][7] SWPC's 5 October forecast puts the greatest expected 3-hour Kp for 5–7 October at 4.67 (G1) and has the field settling — quiet to active on 6 October, quiet to unsettled on 7 October.[1][3] ## Sources [1] [NOAA SWPC 3-Day Forecast (issued 2026 Oct 05 00:30 UTC)](https://services.swpc.noaa.gov/text/3-day-forecast.txt)"The greatest observed 3 hr Kp over the past 24 hours was 6 (NOAA Scale G2)." "The greatest expected 3 hr Kp for Oct 05-Oct 07 2026 is 4.67 (NOAA Scale G1)." "Rationale: G1 (Minor) geomagnetic storms are expected early on 05 Oct due to influence from a positive polarity coronal hole." [2] [NOAA SWPC estimated planetary K index (3-hour, observed)](https://services.swpc.noaa.gov/products/noaa-planetary-k-index.json)"2026-10-02T21:00:00 Kp=1.33 a_running=5 stations=8" "2026-10-04T09:00:00 Kp=5.0 a_running=48 stations=8" "2026-10-04T18:00:00 Kp=5.67 a_running=67 stations=8" "2026-10-05T03:00:00 Kp=5.33 a_running=56 stations=8" [3] [Joint USAF/NOAA Solar Geophysical Activity Report and Forecast, 2200Z 04 Oct 2026](https://newsroomamerica.com/a/LayLkyu4r7CeBCqP1hBHKadfXyl/noaa_reports_very_low_solar_activity_with_four_sunspot_regions_while_the_geomagnetic_field_reached_major_storm_levels_and_solar_wind_peaked_at_644_km_s.html)"The geomagnetic field was at quiet to major storm levels for the past 24 hours." "Solar wind speed reached a peak of 644 km/s at 04/1807Z, total IMF reached 22 nT at 04/0648Z, and the maximum southward component of Bz reached -20 nT at 04/0611Z." "The geomagnetic field is expected to be at quiet to minor storm levels on day one (05 Oct), quiet to active levels on day two (06 Oct) and quiet to unsettled levels on day three (07 Oct)." [4] [Joint USAF/NOAA Solar Geophysical Activity Report and Forecast, 2200Z 02 Oct 2026](https://newsroomamerica.com/a/OWvMiJZfKw8VQQc9vJwePxJR8op/noaa_reports_quiet_solar_and_geomagnetic_conditions_with_three_sunspot_regions_on_the_disk_and_a_chance_of_c_class_flares_over_the_next_three_days.html)"The geomagnetic field has been at quiet levels for the past 24 hours." "Solar wind speed reached a peak of 320 km/s at 0007Z on 2 October, total IMF reached 6 nT at 2335Z on 1 October, and the maximum southward component of Bz reached -4 nT at 0014Z on 2 October." "The geomagnetic field is forecast to be at quiet to active levels on 3 and 5 October, and at unsettled to minor storm levels on 4 October." [5] [The Watchers - September 28 CME glancing-blow forecast for October 2 (30 Sep 2026)](https://watchers.news/2026/09/30/september-28-cme-expected-to-deliver-glancing-blow-to-earth-on-october-2-geomagnetic-storm-forecast)"Space Weather Prediction Center (SWPC) issued a G1 – Minor geomagnetic storm watch for October 2, 2026, following a large filament eruption and associated coronal mass ejection (CME) on September 28." "SWPC issued the watch at 13:01 UTC on September 29, forecasting no storm-level geomagnetic activity on September 30 or October 1 and G1 conditions on October 2." "while the ensemble assigned a 95% chance that maximum Kp would remain between 2 and 4, below the G1 threshold." "Minor is the lowest category on NOAA’s five-level geomagnetic storm scale and corresponds to Kp 5. Weak power-grid fluctuations can occur, minor effects on satellite operations are possible, and aurora may become visible at high latitudes, including parts of the northern U.S. tier such as northern Michigan and Maine." [6] [spacewx.org Aurora Watch - 1 October 2026 outlook](https://spacewx.org/some-solar-activity-some-aurora-possible)"The trans-equatorial coronal hole CH98+ in the east is significantly larger than during the last rotation. Its positive polarity is conducive to geomagnetic activity in October." "From 3 October (+/- 1 day), its stream interaction region and subsequently its fast solar wind could cause some aurora; there is sure to be a good display at high latitudes, though it remains to be seen whether it will be visible at mid-latitudes (Central Europe)." "A quiet Sun with occasional eruptions (filaments, flares) and a large coronal hole that is currently crossing the central meridian." [7] [NOAA Space Weather Scales (G1-G5) explanations](https://www.swpc.noaa.gov/noaa-scales-explanation)"Other systems: HF radio propagation can fade at higher latitudes, and aurora has been seen as low as New York and Idaho (typically 55° geomagnetic lat.)." "Other systems: Migratory animals are affected at this and higher levels; aurora is commonly visible at high latitudes (northern Michigan and Maine)." field research · every claim above cites a NOAA product or report fetched 2026-10-05 · machine-readable K index: [JSON](https://services.swpc.noaa.gov/products/noaa-planetary-k-index.json) · 3-day text product: [TXT](https://services.swpc.noaa.gov/text/3-day-forecast.txt) --- Source: