<p>Once shear fractures begin to propagate in relatively thin weak layers underneath dry snow slabs, avalanche release is imminent. The evolution of the dynamic weak layer fracture likely determines the size or destructive potential once release takes place. In this paper, 54 values of mode II fracture speed collected from field tests are analyzed. In addition, values of speed collected with connection with avalanche release which involves both mode II and mode III shear fracture are analyzed. The Rayleigh wave speed is the normal maximum speed limit for a mode II fracture based on continuum formulations and the shear wave speed is the companion speed limit for mode III fracture. The 54 values of speed data from field tests are shown to have a median value of 12% of the predicted Rayleigh wave speed with a range between 2 and 56%. The data collected during avalanche release do not exceed 59% of the Rayleigh wave speed. The test data were collected from weak layers composed of highly porous crystal forms which are known to exhibit dynamic porosity loss during fracture propagation. Such events imply viscous dissipation and high friction which help to explain the low speeds. For snow slab avalanche release, which occurs for slope angles greater than <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(25^{0}\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mn>25</mn> <mn>0</mn> </msup> </math></EquationSource> </InlineEquation>, field observations show that the mixed mode (II, III) weak layer fracture terminates with mode I fracture through the slab following dynamic propagation. Analysis using the avalanche speed data with respect to terminal mode I fracture suggests that the shear stress drop behind the fracture process zone can contribute to produce the mode I fracture to promote avalanche release but dynamic contraction of the fracture process zone is a minor effect at best.</p>