David Leavy Dear David, The effect an eruption will have on a nearby city could vary from none at all to catastrophic. For example, atmospheric conditions might carry ash away from the city or topography might direct lahars and pyroclastic flows to unpopulated areas. Your scenario brings up several interesting problems. I should point out that in most volcanic crises geologists advise local civil defense authorities. The civil defense authorities decide what to do concerning evacuations, etc.
Decade volcanoes are likely to erupt in the near future and are near large population centers.
Other Decade volcanoes include Santa Maria, Stromboli, Pinatubo, and Unzen. If you have a specific volcano in mind, send another question and I'll see what I can find.