When you have a relatively shallow low-level moist layer (only up to about 850mb or a little above at best on a couple soundings I just cherry-picked off this morning's 12Z 3KM NAM), vertical mixing (helped along by sunshine) can bring down that drier air, lowering surface dewpoints, thus raising LCLs (cloud bases) and possibly increasing low-level capping. At least, that's how I understand it.