Fair weather or "sunny day" assumes failure of the dam due to piping, earthquake or other means which doesn't occur during a flood.  Reservoir is assumed at some normal operating level.  Flows in downstream spillway channel or inundation area are also assumed to be at normal levels.

PMF or severe storm assumes failure due to overtopping, inadequate spillway, erosion caused by extreme flows through the spillway or outlet works, structural failure due to high water levels etc.  Flows in downstream spillway channel or inundation area are assumed to be quite high due to the presence of the severe storm.

Generally, the depth of the flood flow during a pmf breach is higher than a sunny day, however, the sunny day failure often causes more loss of life and property, because it happens with very little warning.  The PMF breach may only cause an incrementally small increase in the downstream water levels if the downstream area is already inundated due to the extremely heavy rainfall.