“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.
“Water infrastructure in the West is built around snowpack,” said Noah Diffenbaugh, a climate scientist at Stanford University. “It doesn’t need to be stored in a reservoir if it’s being stored in the snowpack.” Reservoirs have limited capacity, Diffenbaugh pointed out, so if an extreme rainstorm filled a reservoir beyond capacity, that water — which otherwise might have fallen as snow, or over a longer period of time — would have to be released.
Instead, we see a vicious cycle: As the soil and vegetation in drought-prone regions dry up, they become prone to wildfires and less able to retain water, so when extreme rainstorms roll in, they trigger floods and erosion. The heat makes the water dry up before it has any particular impact on the drought, and the erosion makes the soil even less able to retain water, so the next flood becomes ever so slightly worse. We saw this kind of mid-drought flooding just a week before the floods in the Midwest, when monsoon rains swept through Flagstaff, Arizona.”
https://www.vox.com/science-and-health/23297362/kentucky-flood-lake-mead-california-arizona-drought