NOAA forecasts Kp 4 aurora window

NOAA’s Space Weather Prediction Center said Sunday that minor geomagnetic storm conditions remained possible as aurora watchers across the northern United States and Canada tracked another weekend window for the northern lights. The agency’s aurora dashboard showed a G1, or minor, geomagnetic storm in its forecast for May 17 UTC and again for May 18 UTC. (swpc.noaa.gov) NOAA’s latest observed conditions, however, were below storm level at the time the dashboard snapshot was captured. The agency said its products update continuously as solar wind conditions change. The forecast followed a weekend of elevated interest in aurora viewing after media reports said parts of North America could get a faint display overnight. (swpc.noaa.gov) NOAA’s public-facing aurora products do not guarantee visibility at a given location, and the agency says the display depends on both geomagnetic activity and local observing conditions such as darkness and clear skies. ### What exactly did NOAA forecast for Sunday night? NOAA’s aurora dashboard said the greatest expected three-hour Kp value for May 17 through May 19 was 5.00, which corresponds to a G1 minor geomagnetic storm on the agency’s scale. The same dashboard also showed several forecast intervals below that threshold, including values in the 3 to 4.67 range across parts of May 17 and May 18 UTC. (swpc.noaa.gov) The Space Weather Prediction Center said on its homepage that G1 minor geomagnetic storm conditions were predicted for both May 17 UTC and May 18 UTC. NOAA’s scale defines geomagnetic storms beginning at Kp 5, while lower Kp values indicate unsettled but non-storm conditions. (swpc.noaa.gov) ### How does Kp fit into whether people can see the aurora? NOAA says the planetary K-index, or Kp, measures disturbances in Earth’s magnetic field on a scale from 0 to 9. The agency says Kp is used to characterize the magnitude of geomagnetic storms and to decide whether geomagnetic alerts and warnings should be issued. A Kp value of 5 or more indicates a geomagnetic storm, according to NOAA. (swpc.noaa.gov) The agency also says K-index warnings can be issued when estimated Kp values of 4, 5, 6 or 7 and higher are expected, which means aurora interest can rise even before storm-level conditions are reached. ### Why can northern states sometimes see the lights without a major storm? (spaceweather.gov) NOAA’s aurora forecast page says the display does not need to be directly overhead to be visible and can sometimes be observed from as far as 1,000 kilometers away if the aurora is bright and conditions are favorable. That is why people in northern U.S. states may occasionally see a low glow on the northern horizon during modest space weather events. (swpc.noaa.gov) The agency’s “aurora tonight and tomorrow night” product says its viewline represents the southernmost locations from which people may see the aurora on the northern horizon. NOAA says that map is based on the OVATION model and uses the maximum forecast geomagnetic activity between 6 p.m. and 6 a.m. (swpc.noaa.gov) U.S. Central Time. ### What causes the aurora in the first place? NPR reported on May 16 that the aurora forms when energized particles from space collide with Earth’s atmosphere, producing the familiar green, purple and pink light. NOAA’s forecast materials say the aurora is tied to solar wind conditions and the interplanetary magnetic field measured upstream from Earth. (swpc.noaa.gov) NOAA’s 30-minute forecast page says the OVATION model uses solar wind velocity and magnetic field data measured at the L1 orbit position, about 1.6 million kilometers upstream from Earth, to estimate the location and intensity of the aurora. The agency says those maps provide a 30- to 90-minute forecast window. (swpc.noaa.gov) ### Where should people check before going outside? NOAA’s aurora dashboard said Sunday that it provides predictions for visibility “tonight and tomorrow night” and links users to both the nightly viewline and the short-term aurora forecast. The agency says the maps are updated continuously, with the 30-minute forecast designed for near-real-time tracking as the solar wind reaches Earth. (wrvo.org) Sunday night’s next step is on NOAA’s own pages: the aurora dashboard, the “tonight and tomorrow night” viewline, and the 30-minute forecast, all of which the agency updates as conditions evolve. (swpc.noaa.gov 1) (swpc.noaa.gov 2)