Frank Donovan, W3LPL is one of the top contesters in the USA and is a key forecaster of upcoming conditions on the HF amateur radio bands.
In a recent forecast (April 2021), he made a forecast about how good upcoming Solar Cycle 25 would be based on the Solar Flux numbers at the end of December 2021. I have put together a graphic which shows this above. The starting point for April 2021 is a solar flux index of 77 which is roughly the average for the month.
W3LPL wrote... “If the SFI persists below 90 through December 2021, then propagation should improve gradually until a solar maximum weaker than Cycle 24’s arrives in 2024.
If the SFI persists above 110 through December 2021, then propagation should improve rapidly until a solar maximum similar to Cycle 24’s arrives in 2024.
“If the SFI persists above 125 through December 2021, then propagation is likely to improve more rapidly until a solar maximum stronger than Cycle 24’s arrives in 2024.”
(SFI = Solar Flux Index)
As with everything on the sun, there is always an amount of uncertainty in solar forecasts but it will be interesting to revisit this at the end of the year and see where we're at.
1) K7RA Solar Update - 23rd April 2021