[Poohbah]

There has never been a nuclear ground burst in city before so we can only speculate. Anything shielded by a building would not catch fire and would receive a lot less blast damage. It would not be a firestorm.

Actually, the modern architectural style (glass facades that are very reflective in the SWIR and LWIR bands) would probably wind up igniting a bunch of fires--the initial thermal pulse would get reflected every which way (before those windows blew in/out) and hit just about every flammable surface available. Instant fires...the lack of a major blast wave would actually tend to keep those fires lit.

[Dan Evans]

Actually, the modern architectural style (glass facades that are very reflective in the SWIR and LWIR bands)

I would expect that to have a slight diffusing effect for a block or two, but mostly the effect would be to reflect the light upwards toward the sky.