Nor am I a climatologist nor meteorologist, but this looks like it's related to the dry line > A dry line (also called a dew point line, or Marfa front, after

Nor am I a climatologist nor meteorologist, but this looks like it's related to the dry line > A dry line (also called a dew point line, or Marfa front, after Marfa, Texas)[1] is a line across a continent that separates moist air and dry air. One of the most prominent examples of such a separation occurs in central North America, especially Texas, Oklahoma, and Kansas, where the moist air from the Gulf of Mexico meets dry air from the desert south-western states. The dry line is an important factor in severe weather frequency in the Great Plains of North America. It typically lies north-south across the High Plains states in the warm sector of an extratropical cyclone and stretches into the Canadian Prairies during the spring and early summer - https://en.wikipedia.org/wiki/Dry_line https://en.wikipedia.org/wiki/Dry_line at what is culturally referred to as the 100th meridian. > In the central Great Plains, the meridian roughly marks the western boundary of the normal reach of moist air from the Gulf of Mexico, and the approximate boundary (although some areas do push the boundary slightly farther east) between the semi-arid climate to the west and the humid continental (north of about 37°N) and humid subtropical (south of about 37°N) climates to the east. The type of agriculture west of the meridian typically relies heavily on irrigation. Historically the meridian has often been taken as a rough boundary between the eastern and western United States. - https://en.wikipedia.org/wiki/100th_meridian_west https://en.wikipedia.org/wiki/100th_meridian_west The 125° is for heat index, which combines temperature and humidity. The humidity in that region is more variable than the humidity in the desert south-west.