Final tally: 18.4kWh for the day.
Although the day was "sunny" all day, I think that the system was still capable of another couple of kWh, based on recent generation.
My wife commented that it was slightly hazy in places as she drove home this afternoon, so I assume that there must have been an ever-so-slight haze which I couldn't see, but which slightly reduced the sun's strength.
We've certainly had a lot more-than-normal mist and fog recently, with the cold, wet, saturated ground in the area boling off the wetness when the sun comes out.