To me, in layman’s terms, it seemed like things lined up just right with the sudden amplification of the pattern and the position of the ULL offshore to our SW to basically pull a “lobe” of the 4CH directly over our region with very little time for the airmass to moderate. And thanks to the ULL positioning dragging the center hottest air far to the NW, offshore flow became possible in the low levels which allowed an already bonkers airmass to become fully realized in the lowlands in a way that would usually be mitigated to some degree by flat or weakly onshore gradients.
Of course, down south, around or just after the summer solstice is when the 4CH can often be at its most robust, before it starts being eroded by monsoonal influence later in the summer. Which is why June is the hottest month of the year for some parts of the SW US.
To simplify it, the poleward transport of westerly momentum (culminating as a stronger/poleward northern jet) can “abandon” components of middle latitude waves. Which under certain wavenumbers (and other seasonal boundary conditions) can guide those waves into a state of quasi-resonance and/or amplifying constructive interference.
The timing of all elements has to be absolutely perfect for an outcome like June 2021. The probability of that happening is so low that it almost never happens.
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