NOAA’s Space Weather Prediction Center (SWPC) has issued a strong (G3) geomagnetic storm watch for September 7 through September 9. The SWPC said the watch for September 7 remains in effect due to the arrival of a coronal mass ejection (CME) and the effects of a CME on September 4.

“Additionally, a G3 watch is now in effect for the 8 and 9 September UTC days in anticipation of the arrival of another CME associated with the X9.3 flare (R3 — strong radio blackout) on 6 September at 1202 UTC (0802 ET),” the SWPC said early on September 7. “Analysis indicates likely CME arrival late on 8 September into early 9 September.” The September 6 flare is being called the strongest in more than a decade.

Its effect on HF radio propagation has adversely affected the Hurricane Watch Net (HWN), currently operating on 20 and 40 meters as Hurricane Irma sweeps through the Caribbean.

As of September 7 at 1400 UTC, the solar flux index stood at 127, the sunspot number at 27, the A index at 11, and the K index at 4. All HF conditions are being deemed as no better than fair. The possibility of extended auroral displays could work to the benefit of VHF and UHF operators who aim their antennas north to take advantage of “buzz” mode. SWPC posts a 30-minute forecast of visible aurora.

The SWPC said the September 4 CME arrived at the DSCOVR spacecraft on September 6 at 2308 UTC, and “a sudden impulse was observed at several of Earth's magnetometers” at 2348 UTC due to the effects from the shock arrival at Earth.

“While solar wind speed initially reached speeds of around 600 km/s and total interplanetary magnetic field strength (IMF) increased to around 14 nT, the Bz component [of the Sun’s magnetic field] was directed northward in the early stages of the event — an unfavorable orientation for escalating geomagnetic responses,” the center said. “However, the potential for geomagnetic storming still exists as the CME continues passing over Earth.

An S1 (minor) radiation storm warning has been extended to September 8 at 0600 UTC. This could have a minor impact on polar HF propagation. The SWPC has observed S2 conditions over the past day or two.

The September 6 X9.3 flare followed an X2.2 flare (also R3) at 0910 UTC. The source region for both flares was Region 2673 on the Sun.