Some forecast models generate both rainfall rate but also rainfall probability. Certainly UK Met Office forecasts generate a rainfall probability though I don’t know if that’s included in the source they provide to FlowX.
It would be great is there is some means to display that probability. Easiest way would be to add it as a graph option eg user could create a graph with two lines: rainfall (mm/hr) and 2nd line probability. But on the map best I can think of is a separate layer maybe a bit like the cloud layer where the shade/transparency indicates the probability … but that doesn’t work is also displaying the expected rainfall but maybe that’s just a constraint that you can’t show everything at the same time or maybe there are better ideas I can’t think of.