The spatial stability of the near wake downstream of a low aspect ratio cantilevered cylinder, with and without the influence of synthetic jet forcing, was estimated using two-dimensional inviscid stability analysis, calculated by Chebyshev polynomials. Through the use of local inviscid methods, the general size of the three-dimensional vortex formation length was estimated, using a cusp method to map the size of a region of absolute instability downstream of the cylinder. The synthetic jet decreases the size of this vortex formation length, altering the spectral content of the near wake, in agreement with experimental measurements. Furthermore, the turbulence intensity within the near wake could be estimated from the eigenfunctions corresponding to the most amplified spatial instability.