  We give a complete list of smooth and rationally smooth normalized Schubert
varieties in the twisted affine Grassmannian associated with a tamely ramified
group and a special vertex of its Bruhat-Tits building. The particular case of
the quasi-minuscule Schubert variety in the quasi-split but non-split form of
${\rm Spin}_8$ ("ramified triality") provides an input needed in the article by
He-Pappas-Rapoport classifying Shimura varieties with good or semi-stable
reduction.
