Complex sentence: Kennewick has a semi-arid climate (Köppen "BSk") that barely escapes a desert climate (Köppen climate classification BWk) with chilly though not severe winters with frequent light rainfall, and hot, very dry summers made tolerable by the low humidity.
Split into simpler sentences: Kennewick has a semi-arid climate (Köppen "BSk"), that closely borders on a desert climate (Köppen "BWk"). Winters are chilly but not severe, with frequent light rainfall, and hot, very dry summers made tolerable by the low humidity.