Our current (classic-era) campaign features a Yellowstone eruption in 2020. There are several meters of ash deposited on Wyoming, Colorado, etc. ... my referee map has notations on ash depth here and there.
Except for boltholes and facilities within ~100 km of Yellowstone, there wasn't any direct effect on Project bases. However, there might be a few meters of ash on top of your bunker!
Roads and other remains of 20th Century civilization are pretty hard to find in the ash-covered areas.
http://tywkiwdbi.blogspot.com/2009/0...fall-maps.html
A few weeks later, possibly in sympathy, we had the Long Valley Eruption.
http://volcano.oregonstate.edu/vwdoc...ng_valley.html
http://vulcan.wr.usgs.gov/Volcanoes/...framework.html
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Michael B.