Complex sentence: Typical high temperatures in the warm months of June, July and August are around, but during heat waves, that can occur anytime from May to September, daytime temperature highs often top for sometimes one or two weeks.
Split into simpler sentences: Typical high temperatures in the warm months of June, July and August are around, but during heat waves, that can occur anytime from May to September. Daytime temperature highs often top for sometimes one or two weeks.