I'm a meteorologist in Florida trying to encourage people to evacuate barrier island areas of south Florida earlier rather than later because of the very real potential of a landfall from a strong hurricane Matthew in under 48 hours, while there are currently no evacuation orders in place in the three big counties of the Miami metropolitan area, and remembering the stories of massive gridlock during Andrew and Floyd evacuations. I looked for any population estimates of those areas or tools to quickly make one, but found nothing. I see that Miami Beach itself is 88,000 and looked at a few other cities, but most are split between the mainland and islands. I have done minor GIS before, including some geocoding with QGIS [though unfortunately don't have the databases installed now or I'd try slogging something together :-(] and have various other limited interaction with GIS overlays, such as utilizing them in GrADS or working with OSM slicers. But suffice to say my abilities with core GIS programs isn't the best.
I believe the National Weather Service itself must use some similar tool to my needs, as with each warning they can indicate the number of people affected. But I have no idea of websites or tools available to the public that could produce such products quickly. And it isn't the first time I've had interest of a quick rough estimate for a certain area.
A county-by-county or city by city estimate for the Florida Atlantic coastline could be very useful information to the public tonight. I'm certain some have that information, such as emergency managers. But I am convinced the public is the one needing it in bulk tonight.
I'm not sure it's a perfect SE question, especially on GIS, but I can think of no better place where experts may be able to point me to a site or produce a count in moments when time may be of the essence.