everything you said (snowcap, valery, jane) is correct.
maybe i should explain my caution. the bird indeed looks long legged, with bright yellow bare parts and a nice red orbital, ergo: it
looks like a YLG.
my point was that i wouldn't want to be 100% firm in the ID considering the huge variation in large gulls even concerning structure.
and for snowcap: sure YLG regularly wanders in large numbers northwards and reaches southern sweden after breeding. sure vagrant YLGs have been recorded in north america. but, most YLGs breed in the mediterranean and are more or less sedentary there, no real southbound migration like the more northern taxa. now, tanzania is really very far, at the equator. and i'd expect a real migrant to be more likely there, like steppe gull for instance. we are used to see smallish, short legged and finer billed steppe gulls from pics in oman etc but on breeding grounds in central russia there are many birds that give a rather robust impression. actually we don't know very much about that taxon. of course even armenian and steppe would be off their normal wintering range, which are in much more east (
barabensis, rel. long distance migrant) or in the levant for
armenicus, as a short distant migrant. armenicus also is not very numerous and little recorded outside its normal range.
an open wing would allow to see how far down black reaches: p5, p4 or even p3? considering variation within the single taxons even that would not be conclusive. the bird has shed the outer 2 primaries so there would be no mirror visible, a crucial feature to separate YLG from armenian and steppe.
so, it might well be a
michahellis but i'd like to hear how people conclusively exclude steppe gull, armenian or even
taimyrensis heuglin's.
these are really hard and a single vagrant bird should be examined cautiously. there's no head striation on this bird for instance.
http://www.elisanet.fi/antero.lindholm/Linnut/lokit/kazakstan/korgalzyn.html
http://www.elisanet.fi/antero.lindholm/Linnut/lokit/murals/ad.html
http://www.uaebirding.com/uae-gulls/gullblog/gullblog.html
http://www.pbase.com/hans_larsson/barabensis_type_gulls_oman_2611412_2009_
http://chrisgibbins-gullsbirds.blogspot.com/2010/05/steppe-gulls-in-uae.html
links for 'mich' would fill pages...
end of the sermon,
all the best and hope we can get further