IMO, as it relates to what most would consider regular brassicas, turnips and radishes, I think it is a tick early right now...but with some good rain in the forecast in the next 48 hours...it is tempting.
But I wouldn't do it just yet. Now then, I plan on planting rutabagas tomorrow...it is good to get them in about 2-3 weeks earlier than turnips.
If you plant turnips now they could get "woody" by the end of say September and I would also say that radishes planted now will be more likely to be mushy by say mid-October. All of these things are heavily dependent though on what we get for rains and frosts, etc. But in general, I would go with rutabagas now(mid July), turnips and radishes in about 2 weeks(late July to early August) and rye/clover/oats mix, with maybe a few brassicas mixed in to, right around Labor Day.