The MUF's are back, now as a magic T index

Maximum Useable Frequency, or MUF, has recently seen a resurgence in DX circles.
In the past, when the interest in shortwave was at its strongest, the thing with MUF was a hot thing. Not least during the Cold War.
Which wasn't that strange.
With a number of stations in a limited space, it was a matter of finding the best possible opening. And if the propagation conditions on shortwave for a certain time also favored higher frequencies, so much the better. With more frequencies available, the limited space for the shortwave transmitters became less crowded.
Did this affect reception on the lowest frequencies such as medium wave, long wave and the VLF of the submarine world?
No.

Nevertheless, interest in MUF, the Maximum Useable Frequency, has recently gone into self-oscillation.
Not as MUF.
But as a magical T-index.
Now, we are told, we can predict which signals we will hear, and which we will not hear. A constantly updated T-index map will give you a magical world map showing where "the ionospheric absorption is high and reception over those areas is degraded". You may see the areas and frequencies with exceptional DX signals on medium wave. And all those without.
Fantastic!
Real-time T index difference map

No, of course you won't.
The T-index, to quote the researchers in Australia, is "an index derived from observed values of maximum ionospheric frequencies and has the same scale as the number of sunspots."
From this it is possible to obtain a relationship between frequency and a number of sunspots. Which can then be used to create better, faster forecasts for possible shortwave communications. In short, to find the highest maximum usable frequencies.
Good old MUF.
Which, of course, does not change previously known propagation conditions:
Low T index values still mean use of the lower HF frequencies such as around solar minimum or during a strong ionospheric storm.
Higher T indices and therefore higher frequencies will usually be the norm near solar maximum.

All of this has little to do with the propagation ratio at the lowest frequencies. Here, too, it may be wise to study long-term research findings, which show that aurora effects are more important at low frequencies. Or in short, if a large number of signals are knocked out by polar cap absorption, it opens up signals coming from areas closer to the equator, because those signals are not as affected.
And now for the holy grail:
How do I predict these signal openings on the lower broadcast bands?
You can't.
At least not very reliable.
But with all the research into solar weather (thanks to satellites, GPS, space travel and robotic shields), we are getting step by step closer to utopia. But for the foreseeable time, you will still have to rely on well-known, proven techniques to predict propagation conditions, i.e. K-index, magnetometers, protons, sunspots, solar winds and polar cap absorption.