Broadly speaking, the atmosphere moves west-to-east.
The mid-west US and east coast benefit from all of the evaporation west of them that eventually maximizes the amount of water that air holds. Then a low-pressure front (lower pressure means air can’t hold as much water) passes through and squeezes the air like a sponge.
California is a sponge anti-squeezer. It is hot sun-baked land that heats up the air and allows the air to absorb more water.
There’s much more going on like the effects of mountains causing a shadow effect in the western US which is why Kansas is still sort of dry. But this is ELI5.
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