The climate of Koblenz is classified as warm and temperate. Weather can be cool at times, but will not be freezing even during the peak winter times. There is no specific rainy season and so, you can expect showers any moment.
Koblenz is on the trip planners of vacationers all year long. However, the best time to visit the city is during the summer months (June through August) when the Rhine-Valley is at its best.