The Earth is in another ice age. The ice extends as far south as the northern border of the United States. Central North America is cold and dry. The land has become a vast, barren dust bowl.
The drier climate has had dramatic effects on the interior of the North American continent, turning it into a vast, cool desert frequently battered by fierce sandstorms. The desert is as bitterly cold as the Gobi desert once was. It stretches to the Rocky Mountains - 2400km.
The piercing winds that stir up the sandstorms scour away the soil. A little snow falls infrequently on high ground. Tornados frequently rip across the area.
A few hardy plants survive in these harsh conditions.