Tokuyama's Identity for Factorial Schur $P$ and $Q$ Functions

A recent paper of Bump, McNamara and Nakasuji introduced a factorial version of Tokuyama’s identity, expressing the partition function of six vertex model as the product of a t-deformed Vandermonde and a Schur function. Here we provide an extension of their result by exploiting the language of primed shifted tableaux, with its proof based on the use of non-intersecting lattice paths.