Kennewick has a semi-arid climate , that closely borders on a desert climate due to its position east of the Cascade Mountains. The Cascades create an effective rain shadow, causing Kennewick to receive a fraction of the precipitation that cities west of the mountains like Portland and Seattle get annually, with values being more similar to that of Phoenix, Arizona. The mountains also insulate Kennewick from the moderating effects of the Pacific Ocean, allowing the city to experience more extreme temperatures.
Before McNary Dam was built on the Columbia River downstream of Kennewick, the river would periodically flood. The worst of these floods happened in 1948 and caused one death and $50 million ($533.6 million in 2019) worth of damage. The government responded by building the McNary Levee System to protect lower parts of town. Floods like this were the result of melting snow, and were most extreme when a heavy snowpack developed in the mountains over winter followed by a strong regional heatwave. The flood threat from the Columbia has significantly decreased since dams were built. Zintel Canyon Dam located near the Southridge Sports and Events Complex was built to protect parts of the city from a 100-year flood. While the creek that flows through Zintel Canyon typically runs dry, summer thunderstorms in the Horse Heaven Hills can generate destructive flash floods.
Laying at the bottom of a basin, temperature inversions can develop, creating dense fog and low clouds in Kennewick. This is particularly common in the winter and can last for several days. Inversions form during periods of high pressure. High pressure combining with the low angle of the sun in winter brings stability in the atmosphere, allowing denser cold air to sink to the floor of the Columbia Basin. Pollutants will also become trapped, lowering the air quality. When fog develops during an inversion, it will often limit diurnal temperature changes to just a few degrees. Temperatures in areas above the inversion will often be warmer despite being at a higher elevation. These inversions cause a major decrease in the amount of sunshine Kennewick receives annually. If a weather system drops precipitation but isn’t strong enough to clear the inversion, freezing rain or sleet can fall in Kennewick.
The average annual wind speed in Kennewick is 8 miles per hour (13 km/h), but strong winds are a common occurrence in Kennewick and can sometimes cause damage. Wind and the arid nature of the region can cause dust storms. These events can happen any time of the year but is most common in the spring and fall months when farms in the region have high amounts of exposed soil. Chinook winds can also be experienced during the winter. These are formed when moisture gets removed from air moving across the Cascade Mountains, allowing the air to warm significantly as it descends into the Yakima Valley and Columbia Basin. Many of the high temperature records set during the winter months result from Chinook events.
Summer brings extreme heat and low humidity, which are ideal conditions for wildfires in undeveloped areas adjacent to town. One such fire in 2018 started along Interstate 82 south of Kennewick and burned 5,000 acres (2,000 ha), destroying five homes on the edge of Kennewick. While rare, severe thunderstorms can also cause damage in Kennewick. Severe storms can produce damaging wind, hail, lightning, and weak tornadoes. No tornadoes were recorded in Kennewick between 1962 and 2011, but one did touch down in 2016.