Vox has an excellent explainer on how climate change is simultaneously causing drought and flooding:
How are floods in Kentucky and a drought at Lake Mead happening at the same time? - Vox:
The short answer for why these seemingly opposite things are happening at once is that climate change is making our atmosphere thirstier. Or, in more scientific terms, as the Earth warms, its atmosphere can hold more water vapor. This happens at an exponential rate: The back-of-the-napkin math is that the atmosphere can store about 7 percent more water per degree Celsius of warming, and we’re currently at about 1.2°C above pre-industrial temperatures. The result is an atmosphere that takes longer to get saturated with water, which means fewer rainstorms, but when they do occur, those storms dump more water at once, resulting in floods.
Paradoxically, our changing atmosphere is also a perfect recipe for drought. Higher temperatures mean water evaporates faster, and when it falls, it’s less likely to fall as the snow that has historically fed many of the American West’s rivers and streams. The rain isn’t very helpful either, since lifting a drought requires a combination of snowfall and long, sustained rainy seasons instead of short, extreme bursts.