Great shot!
It does indeed look like the disapating stage of a CB but as if all of the upper 4/5ths has mysteriously been removed. My guess is it something to do with a localised low-level inversion (ground cooler than upper air as you say it was early morning) and the 'invisible' top part of the 'cloud' consists of warmer air holding the water vapour in suspension. That flat terrain would also tend to inhibit orographic mixing giving a temperature-only type of cooling gradient. As the water vapour circulates at the lower part of the air parcel it hits the dew point and condenses, in this case looking like it freezes fairly rapidly and then falls as very light snow.
I don't know if there is a specific name for this type of cloud, perhaps you could name it after yourself? as long as you're not called "Nigel Scroggins" or something similar (F/O: Oh my God! there's a "NS" ahead! Captain: Good grief! they ain't paying me enough for this caper! I'm turning this crate around and heading for that last airport with the known windshear)
OK that's enough of that, having combed my "Jack Hargreaves" beard, I am off to smoke my pipe and listen to a bit of quality Jethro Tull whilst wearing my brown polyester flares.
Flug
