Interstitial, Midnightsun & Sebastian,
I believe that what you are referring to is called sheet flow. It happens from time to time on frozen soils, frozen rivers and even large deltas, as we have seen earlier this year.
In this particular case, I am convinced that the melting of snow happened in situ, hence we should not use the "flow" term.
Instead, I would suggest a new term: "Sheet melt". The first reference I could find alluded to the near complete melt taking place over the Greenland ice sheet in 2012. Other allusions could be found related to the melt water incidents preceding the collapse of some of the Antarctic ice shelves.