Well that depends on where you are. Places where they have “winter” like Minnesota the weather is much much less volatile. Places where they don’t have a “winter” like California. I believe their weather is more raining in the winter because cold air collides with warm ocean air more often. I’m sure some will have a better answer

Some of the fluctuations are from the arctic jet stream in the north wobbling and the gulf trade winds from the warm caribbean sea and warm winds of off the pacific coast from the continued elninia weather patterns. The jet stream is fluctuating due to the warming of the Atlantic Ocean and the melting glaciers around the poles mixed with the solar radiation from the sun. It really is climate change, whatever the cause I'm not gonna argue that, but it is climate changing due to warming surface temperatures. We don't notice the changes in the summer time as much unless a cold front creates thunder storms and potentially tornadoes.

The midwest is huge, but I'm going to assume you live in a humid subtropical/continental climate (most of the midwest). This means that the weather is subject to extremely cold air masses from the far north, and warm, humid air masses from the Gulf of Mexico. The fact that summers are more humid than winter means that temperatures are more stable as well (water in the air does not allow volatile changes), winter is cold and dry, giving room for drastic changes.

By the way, the midwest has gotten warmer in the past years and the trend suggests that temps will continue to rise.

TLDR: humidity = stable temps

That's an interesting observation! It could be because the midwest is closer to the equator, so the sun is stronger in the winter and more likely to bring warm days.

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Midwest is very unstable weather-wise because it’s wide open to both wet and hot air from the Caribbean and cold and dry air from the arctic. In the winter the jet stream drops down to lower altitude, and due to climate change there are signs that it’s actually weakening which allows for more frequent/deeper blobs of arctic air dropping far south; those fronts collide with warm air, push it up, drain it of moisture and once stabilized bring cold but sunny days.

Instability in weather patterns and more extreme weather events are one of the consequences of global warming, often refered to as global weirding.