I usually use 50 x 559 tyres on my Raven Tour with the P65 mudguards, but 26" x 2.15" (55 x 559) Marathon Mondials will just fit.
So it seems reasonable be able to fit at least as big on the Nomad, which has greater frame and fork clearance than the Raven Tour, although, as Mickeg says, the mudguards may be the limiting factor.
But be careful, the advertised sizes of tyres do not always correspond to their true dimensions. So the true size can vary between makes and models of tyre even if they have the same advertised size.
My Nokian Hakkapeliitta W240 spike tyres, advertised as being 26" x 1.9 (47 x 559) are actually slightly bigger than the Schwalbe Supreme 26" x 2.0 (50 x 559) that I use on most of my 26" wheel bikes.