apostalism:

Yellowstone erupts like clockwork with a periodicity of approximately 600,000 years – in case you were curious, it last erupted 630,000 years ago. The initial blast would probably instantly kill everyone and everything from about Salt Lake City, Utah to Billings, Montana and Salmon, Idaho to Casper, Wyoming (the purple), though everything between San Francisco and Chicago would basically be unlivable due to ash fall, and ash would rain as far away as Mexico and southern Florida.

The ash would be pretty devastating for the United States The fallout would include short-term destruction of Midwest agriculture, and rivers and streams would be clogged by gray muck. On top of that, global cooling would wreak havoc on global agriculture for more than a decade likely spawning famine and severe winters.