Have soil temps really reached 50 for a long enough period already? This site suggests we're still in the 45-50 range because of all the cold weather ...
http://www.greencastonline.com/tools/soiltempmaps.aspx
Also, isn't 55 the real cutoff for the bulk of germination? I've read that 50 is when the 'precocious' weeds sprout - and only after a ~5 day consecutive period - but 55 is when the masses really start going.
You might ask a garden store if they'd advise applying a pre-emergent (which would be needed ASAP if not too late). Otherwise, just prepare to pull crabgrass sproutlings as they appear ... get to them early and do it every week and you can control them to a large degree ... once they're big clumps and located everywhere you're in trouble.