The air masses that have the greatest influence on weather conditions in the midwestern united states are

The air masses that have the greatest influence on weather conditions in the Midwestern United States are continental polar.
This air masses come from the north, they are heavy, low, cold and predominantly dry air masses, and are most intense in the colder part of the year. But they are not the only one, even though they are the dominant ones, the tropical warm and wet climate also has a big influence, especially in the warmer part of the year, and it's influence is gradually increasing.

Ithink this is an opinion question, it's based on your interest.