  The formation of planetesimals in protoplanetary disks is not
well-understood. Streaming instability is a promising mechanism to directly
form planetesimals from pebble-sized particles, provided a high enough
solids-to-gas ratio. However, local enhancements of the solids-to-gas ratio are
difficult to realize in a smooth disk, which motivates the consideration of
special disk locations such as the snowline. In this article we investigate the
viability of planetesimal formation by streaming instability near the snowline
due to water diffusion and condensation. We adopt a viscous disk model, and
numerically solve the transport equations for vapor and solids on a
cylindrical, 1D grid. We take into account radial drift of solids, gas
accretion on to the central star, and turbulent diffusion. We study the
importance of the back-reaction of solids on the gas and of the radial
variation of the mean molecular weight of the gas. We find that water diffusion
and condensation can locally enhance the ice surface density by a factor 3--5
outside the snowline. Assuming that icy pebbles contain many micron-sized
silicate grains that are released during evaporation, the enhancement is
increased by another factor $\sim$$2$. In this `many-seeds' model, the
solids-to-gas ratio interior to the snowline is enhanced as well, but not as
much as just outside the snowline. In the context of a viscous disk, the
diffusion-condensation mechanism is most effective for high values of the
turbulence parameter $\alpha$ ($10^{-3}$--$10^{-2}$). Therefore, assuming young
disks are more vigorously turbulent than older disks, planetesimals near the
snowline can form in an early stage of the disk. In highly turbulent disks,
tens of Earth masses can be stored in an annulus outside the snowline, which
can be identified with recent ALMA observations.
