Related to soil temperatures and temperature in general, the application of post-emergent herbicides late in the fall are temperature dependent.  A post-emergent product can be applied as long as the broadleaf weeds are photosynthesizing and appear green.  These products work faster when the plants are actively growing, thus this time of the year the activity is much slower. The chances are that you may not even see any effects on the weeds this year but over the winter they will die.  If applying a post-emergent herbicide try applying on dry sunny day when temperatures are warm (> 50 F).