We present a numerical study and analytical model of the optical near field diffracted in the vicinity of subwavelength grooves milled in silver surfaces. The Green's tensor approach permits the computation of the phase and amplitude dependence of the diffracted wave as a function of the groove geometry. It is shown that the field diffracted along the interface by the groove is equivalent to replacing the groove by an oscillating dipolar line source. An analytic expression is derived from the Green's function formalism, which reproduces well the asymptotic surface plasmon polariton (SPP) wave as well as the transient surface wave in the near zone close to the groove. The agreement between this model and the full simulation is very good, showing that the transient "near-zone" regime does not depend on the precise shape of the groove. Finally, it is shown that a composite diffractive evanescent wave model that includes the asymptotic SPP can describe the wavelength evolution in this transient near zone. Such a semianalytical model may be useful for the design and optimization of more elaborate photonic circuits, whose behavior in a large part will be controlled by surface waves.