We are talking about several Gt TNT equivalent. That would be sufficient to raise the temperature of the entire atmosphere within a 10 km radius around ground zero by some hundred K. The temperatures in the upper atmosphere, where most of the energy is initially released, would be much higher. A major part of the resulting heat radiation would pass the atmosphere and reach the ground, resulting in a second temperature peak at ground level. I don't believe that this would be harmless until I see a corresponding calculation.Light flash would be spectacular but won't be intense enough to do harm.
Even the Chelyabinsk meteor caused widespread window damage and moderate structural damage in some buildings in Chelyabinsk which is 40 km away from ground zero. A direct hit with several thousand times the energy would raze the city to the ground and let it go up in flames.Shock waves would cause widespread window damage and may cause moderate structural damage in some buildings.