Modeling results of sprite streamer formation from large ionospheric inhomogeneities or patches (several tens to hundreds of meter wide) indicate that positive streamers can be initiated at subbreakdown conditions from the inhomogeneities with a density comparable to sprite halo densities. For spherical patches with a given radius, the minimum density required for streamer initiation decreases with increasing ambient field. For a given density, the minimum size of the inhomogeneity decreases with increasing ambient field. The modeling results on the associated optical emissions show that a luminous spherical-like cap appears around the lower tip of the ionization patch before streamer initiation, and the streamer is initiated from the bottom of this cap, which appears to be similar to streamer initiation from luminous structures in the lower ionosphere recorded by recent high-speed videos. Our study suggests that if the sprite halo front is unstable, inhomogeneities developing from it can initiate sprite streamers at subbreakdown conditions.