The 100th was the traditional shorthand for where the Great Plains began:
credit: Earth Magazine
Over the years the division between wet and dry has been moving east but now we have the year-long drought moving out of what used to be known as the Great American Desert i.e. not something to be too concerned with, unless you live in Las Vegas or Phoenix - but it's stupid to build cities in the desert anyway - we see this in the latest change map from the University of Nebraska-Lincoln Drought Monitor:
Drought is increasing in agricultural areas from Minnesota to Texas and across the Florida panhandle with some relief in the Texas-Oklahoma panhandle.
Here's the drought map itself and the link to UN-L's discussion:
