When the whole sky takes a pause...

I sorta hate to interrupt the conversations by asking a question about
actual weather, but here we are...
Last fall, I posted a question on "where's the next thermal" that got
helpful responses. Somewhere in this group, a pilot made the
observation that on some days, the thermals vanish mid-afternoon --
and if this pause can be weathered, it's possible to stay up until
sunset.

This led me to think about my own northern-plains experiences. Often
I've gotten low, or thought the day had died, in midafternoon (3-4
pm). One flight in particular, this summer --
http://www.onlinecontest.org/olc-2.0...Id=-1396180689
-- I had flown ENE, and after making the first turnpoint at about 2:45
pm, I couldn't find any lift at all. At about 600' (175m) agl, I gave
up scratching and restarted my engine while aproaching a field. All
the clouds as far as I could see had turned raggedy and shrinking. I
assumed the day was for some reason done (upper-level dry front?).
After I'd been despondently climbing with engine for a few minutes, I
realized that all the bases look good again, shut down, and had an
easy thermalling flight back home.

I can recall other, similar times, usually with a 20-30 minute episode
of scratching around down low, when I blamed myself, or the small
upwind lake, or the unknowns. But this day and that prior post makes
me wonder whether the boundary layer might not "shift gears"
midafternoon more often than we realize.

In this respect, I have (as others, I'm sure) that early in the day
the thermal sources are single fields and the lift broken and narrow;
late in the day, the thermal sources are savannas and the lift broad
and smooth, whether or not strong.

1: Do you have any meterological knowledge or links to suggest
regarding this?

2: Have you seen this phenomenon?

Thanks,
Dan Johnson