The cloud formations within the near perfect circle don’t indicate a thermal inversion to me. Observers reported that the cloudcover was fairly low altitude, too.
Read what I said again. I said that it’s a relatively common occurence when there is an inversion above a very cold airmass.
The most likely cause is cooler air above falling through the cloudcover causing a hole because it warms (lowers humidity) as it falls.
Consider it a reverse thermal upwelling. It warms as it descends thus causing a hole.
I’ve got a long description of the phenomena in one of my met notebooks but I don’t have them here at work.