Propagation News – 18 May 2025

This week, you could be forgiven for thinking that the Solar Cycle 25 maximum is now over. With a reduced solar flux index, and an almost blank Sun, there was little to get excited about.

By Thursday 15 May, the SFI was 122 but previously, on the 12 May, it had been down to 116 which is the lowest it has been for some time. There is still a lot of solar activity going on, but most of it is not conducive to good HF propagation.

Over the past week we have had two X-class solar flares and five M-class events. The X-class events were associated with coronal mass ejections, but these appear to have been directed away from Earth. A fast solar wind has resulted in the Kp index standing at four for a lot of the time, namely on the 9 and 10 May and again on the 14 and 15 May.

Unfortunately, this is not uncommon at this point in the solar cycle and adds some evidence that we are now in the declining phase of Cycle 25.

Next week, NOAA predicts that things may improve with the SFI set to rise from the 19 May. It forecasts the SFI will start the week around 115, but then will climb steadily, reaching 145 by the 1 June. After a brief rise on the 19 May, with a Kp of four, things then become more settled with a Kp of two between the 22 and the 28 May.

A large elongated coronal hole became Earth-facing on 16 May which could cause raised K indices and lower MUFs from today, the 18 May, onwards.

It looks like the best HF F2-layer conditions may occur between the 22 and 28 May.

And don’t forget, despite the HF doldrums, we are now well into Sporadic-E season with short-skip occurring, predominately on 10m. But more of that shortly.

VHF and up

The prolonged spell of high pressure is likely to remain until near the end of this week so there is plenty of time to get some more Tropo in the log.

Cloudier and cooler air over eastern Britain at the end of last week may enhance the Tropo by adding moisture under the inversion. For most places the strongest Tropo conditions will be overnight and early morning, then they will degrade as the daytime heating destroys the surface temperature inversion.

Meteor scatter is declining a little, apart from a few stragglers from early May showers, so it’s probably better to keep to the early morning period as your go-to time for meteor scatter to focus on any random input.

Rain scatter is unlikely until next weekend, but it might be worth thinking about the GHz bands from this Friday, the 23 May, onwards.

Aurora is a different matter, and the solar activity is likely to keep things interesting as a large sunspot group rotates into an Earth-facing position.

Strong Es opened on Wednesday 14 May, so it’s definitely the season to be checking the upper HF and lower VHF bands for Sporadic-E. In this recent opening, the Dourbes ionosonde peaked at an Es critical frequency of 14MHz, which is a very high value, even for mid-season.

There were strong signals from the central Mediterranean on 6m CW, and some may have even got some 2m digital mode action from the brief opening. Check the Propquest NVIS tab for the day to see the details of this event.

Although weak jet streams were present, the opening may well have been helped by some very intense slow-moving thunderstorms over France. Remember to start checking on 10m and, if it’s open, look to the higher bands of 6m, 4m up to 2m.

EME path losses are still falling. Moon declination ended last week at minimum but goes positive again this Thursday 22 May. Moon windows and peak elevation are again rising. 144MHz sky noise is high this weekend, falling back to low as the week progresses.

Category: GB2RS Propagation News