Complex sentence: Daytime heating of the moistening boundary layer increased through the afternoon across Oklahoma and Texas and combined with cooling aloft in conjunction with the approaching storm system, the air mass began to destabilized, which resulted in conditions supportive of thunderstorm development.
Split into simpler sentences: Daytime heating of the moistening boundary layer increased through the afternoon across Oklahoma and Texas. Combined with cooling aloft in conjunction with the approaching storm system, the air mass began to destabilized, which resulted in conditions supportive of thunderstorm development.